Next-generation mucosal vaccines for respiratory viruses: Immunological correlates, platform design and clinical translation

Abstract

Influenza, respiratory syncytial virus (RSV), and severe acute respiratory syndrome coronavirus 2 continue to cause substantial morbidity and mortality. Currently licensed intramuscular (IM) vaccines effectively reduce severe disease and death but only partially suppress infection and transmission because they induce limited immunity in the respiratory mucosa. This minireview summarizes next-generation mucosal vaccines for respiratory viruses, focusing on the immunological correlates of protection, platform design, and clinical translation. The literature was identified through focused searches of PubMed and Scopus, prioritizing human studies and late-stage preclinical data published between 2000 and 2025. We outline the key mucosal immune correlates required to block viral entry at the airway epithelium, including secretory IgA and tissue-resident memory T cells, and review advances across major vaccine platforms. Current clinical experience with coronavirus disease 2019, influenza, and RSV mucosal vaccines is discussed, along with challenges related to immune measurement, delivery optimization, evaluation of transmission outcomes, and scalable global implementation, including heterologous systemic-mucosal prime-boost strategies. Overall, accumulating evidence positions mucosal vaccination as a promising complement to IM vaccines, with the potential to shift respiratory virus control from disease mitigation to prevention of infection and transmission.

Key Words: Mucosal vaccines; Secretory IgA; Tissue-resident memory T lymphocytes; Intranasal vaccination; Respiratory viruses; Vaccine platforms; Immune correlates

Core Tip: Respiratory viruses initiate infections at mucosal surfaces; however, most licensed vaccines induce predominantly systemic immunity. Next-generation mucosal vaccines aim to generate protective immunity directly at the point of viral entry by inducing secretory IgA, tissue-resident memory T lymphocytes, and rapid innate immune responses in the respiratory tract. This review integrates key mucosal immune correlates with vaccine platform design, delivery strategies, and the emerging clinical evidence. We highlight the translational challenges and practical approaches.

INTRODUCTION

Viral pathogenic agents that affect the respiratory system remain a significant global health burden. The morbidity and mortality associated with seasonal influenza are substantial, particularly among young and elderly individuals. Respiratory syncytial virus (RSV) is a leading cause of acute lower respiratory infections in infants and older adults. In 2019, lower respiratory tract infections caused by multiple pathogens (such as influenza and RSV) accounted for approximately 489 million infections and 2.5 million deaths[1]. Furthermore, novel coronaviruses have demonstrated pandemic potential, with over 760 million confirmed cases of coronavirus disease 2019 (COVID-19) and almost seven million deaths reported worldwide due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) alone as of 2023[1,2]. These viruses share common characteristics: They spread efficiently via respiratory droplets and aerosols, initially infect the mucosal surfaces of the airways, and replicate extensively in the epithelial cells. They are highly contagious and affect populations worldwide, underscoring the urgent need for improved preventive strategies to reduce their incidence. Existing vaccines have reduced disease severity; however, the persistent burden of infection and emergence of immune-evasive variants highlight the need for vaccines that act more effectively at the point of entry in the respiratory tract[2-4].

Most approved respiratory virus vaccines are administered intramuscular (IM) and induce robust systemic immunity, manifested by circulating IgG antibodies and T cells. Although these IM vaccines are highly effective in reducing the risk of severe illness and death, they are less effective in preventing upper airway infections[5]. Notably, during the COVID-19 pandemic, highly vaccinated populations experienced widespread SARS-CoV-2 transmission, particularly following the emergence of Omicron variants[6]. Even vaccinated individuals are prone to mild or asymptomatic infections and capable of viral shedding, as demonstrated by outbreaks in which nasal viral loads did not differ between vaccinated and unvaccinated individuals[5]. Similarly, IM influenza vaccines generally prevent severe outcomes but have a limited impact on infection acquisition or onward transmission, especially when viral strains undergo antigenic drift. A key limitation of IM vaccination is that it generates immunity primarily in the bloodstream, while resulting in low levels of virus-specific T cells and IgA in the respiratory mucosa[6-8]. Consequently, high serum antibody titers often fail to prevent pathogen entry into the nasal or bronchial epithelium. These observations, including repeated COVID-19 waves and annual RSV and influenza outbreaks, indicate that current vaccines are predominantly systemic and disease-modifying rather than truly infection-blocking.

To achieve sterilizing immunity that prevents infection at its onset and limits transmission, vaccines must induce strong local immunity at the mucosal surfaces of the respiratory tract. Most respiratory viruses enter the body through the airway mucosa, which serves as their initial site of replication. Therefore, vaccines that establish immune protection at this location may intercept pathogens before they proliferate[9]. Nasal and bronchial secretions contain secretory IgA (sIgA) antibodies that can bind to and neutralize viruses within the lumen, thereby preventing their attachment to epithelial cells[1]. In parallel, virus-specific tissue-resident memory (TRM) T cells localized in the respiratory epithelium can rapidly produce antiviral cytokines and eliminate infected cells upon re-exposure, providing a frontline barrier against infections. The concept of generating immunity at the appropriate anatomical sites, namely the nasal mucosa and lungs, forms the foundation of mucosal vaccination. Unlike systemic immunity, which primarily protects against severe disease, mucosal immunity can prevent infection and transmission. Moreover, mucosal vaccine delivery methods, including intranasal sprays and oral formulations, offer practical advantages such as needle-free administration, improved patient acceptance, and facilitated large-scale immunization[1,5]. Therefore, there is increasing interest in developing vaccines that specifically target the respiratory mucosa and elicit protective immunity at sites that are critical for preventing respiratory viral infections. This review examines the immunological basis, technological platforms, and translational progress of next-generation mucosal vaccines against respiratory viruses.

Literature search and study selection

A targeted narrative literature review was conducted using the PubMed and Scopus databases. Searches were performed using combinations of terms related to mucosal immunology and respiratory vaccination, including “mucosal vaccine”, “intranasal”, “oral”, “respiratory viruses”, “secretory immunoglobulin A”, “tissue-resident memory T cells”, “influenza”, “respiratory syncytial virus”, and “SARS-CoV-2”. Priority was given to peer-reviewed human studies, late-stage preclinical investigations, and clinical trial reports published primarily between 2000 and 2025 that evaluated the mucosal immune responses, vaccine platforms, or translational outcomes. Mechanistic animal studies were included if they provided direct insight into mucosal correlates of protection or informed platform design. Articles that were not relevant to respiratory mucosal immunity or vaccine delivery were excluded. This approach was intended to provide an integrative overview of immunological principles and emerging clinical evidence, rather than a systematic quantitative synthesis.

IMMUNOLOGICAL FOUNDATIONS OF MUCOSAL PROTECTION

Anatomy of the respiratory mucosal immune system

The respiratory tract possesses a specialized immune architecture that is adapted to monitor and defend against airborne pathogens. In the nasopharynx, organized lymphoid tissues collectively known as Waldeyer’s ring (including the adenoids and tonsils) function analogously to the nasal-associated lymphoid tissue (NALT) described in rodents[1,9,10].

Healthy adult humans generally lack permanent bronchus-associated lymphoid tissue (BALT) along the bronchi; however, infection or inflammation can induce tertiary lymphoid structures in the lung. These mucosa-associated lymphoid tissues (MALTs) consist of B cell follicles and T cell zones with antigen-presenting dendritic cells situated just beneath the epithelium. Specialized epithelial cells, known as microfold cells (M cells), are interspersed in the airway mucosa and efficiently sample luminal antigens and transport them to the underlying immune cells. The respiratory epithelium is ciliated and lined by a layer of mucus secreted by goblet cells that trap inhaled particles and pathogens. The lamina propria contains a web of immune cells, such as dendritic cells, macrophages, innate lymphoid cells, and memory lymphocytes, which are located below the epithelium. The purpose of this anatomical structure is to direct the rapid recognition of inhaled viruses by pattern recognition receptors on epithelial and immune cells[1] and to deliver antigens to inductive sites (such as NALT/BALT or lymph nodes) and induce adaptive responses.

Secretory IgA-antibody on the frontlines

The characteristic antibody of the mucosal immune system is polymeric IgA. B cells in MALTs class switch to IgA and differentiate into plasma cells that reside in the submucosa upon mucosal immunization or infection[1,11]. These cells produce dimeric IgA, which is actively transported through the airway epithelium by the polymeric immunoglobulin receptor (pIgR). The released IgA dimer remains bound to a fragment of the pIgR, known as the secretory component, at the luminal surface, resulting in the formation of sIgA. The secretory component not only shields IgA from proteolytic degradation within mucosal secretions but also helps to anchor the antibody within the mucus layer.

sIgA plays a central role in neutralizing respiratory viruses before they infect epithelial cells by binding to viral surface proteins, as demonstrated for influenza virus, measles, rotavirus, and SARS-CoV-2, among others. Notably, viruses can also be neutralized by sIgA during transepithelial transport, as virions internalized by pIgR can be inactivated during transport by bound IgA. Moreover, sIgA possesses distinct properties, such as increased avidity (due to dimerization) and the ability to induce immune exclusion by agglutinating pathogens into clumps that are removed via mucus flow. In contrast to IgG, IgA does not strongly activate complement, thereby reducing collateral inflammation at fragile mucosal surfaces[1]. Importantly, a growing body of evidence links robust sIgA responses to protection against respiratory viral infections and shedding. Higher nasal IgA titers are correlated with a reduced risk of SARS-CoV-2 Omicron infection and faster viral clearance[12]. Likewise, studies on influenza and RSV have long shown that mucosal IgA is a better correlate of immunity against reinfection than serum antibody levels[1,13,14].

TRM cells

The cellular arm of mucosal immunity is crucial for comprehensive protection against pathogens. TRM T cells are a specialized subset of memory T lymphocytes that permanently reside in peripheral tissues (such as the lungs or nasal mucosa) without recirculating through the bloodstream[1,15]. These cells arise when effector T cells generated during infection migrate into tissues and differentiate in situ, upregulating characteristic markers such as CD69 (which prevents tissue egress) and CD103 (which facilitates epithelial adherence)[1,2].

In the respiratory tract, both CD8⁺ and CD4⁺ TRM cells can persist within the lung interstitial tissue and airway epithelium. Their presence is associated with rapid responses upon pathogen re-exposure, as TRM cells can immediately release antiviral cytokines (such as interferon-γ and tumor necrosis factor) and eliminate infected cells at the site of infection, thereby curtailing viral replication at an early stage. This “frontline” activity of TRM cells mediates protection even in the absence of circulating T cells. For example, murine studies have demonstrated that the density of influenza-specific TRM in the lungs is a better predictor of protection against reinfection than circulating memory T cell levels. In RSV infection models, lung-resident memory T cells have similarly been shown to accelerate viral clearance and reduce disease severity[1]. However, a notable challenge is that respiratory TRM cells may decline more rapidly than their counterparts in other tissues, with studies indicating that influenza-induced lung TRM can significantly wane after 3-4 months in mice[1,16].

Innate defenses at the mucosal surface

The innate immune system forms a critical backdrop for adaptive mucosal immunity, creating an antiviral state at mucosal surfaces even before specific antibodies or T cells are mobilized. Epithelial cells lining the respiratory tract serve as both a physical barrier and active participants in immune defense. Tight junctions between epithelial cells limit viral dissemination, and mucus secreted by goblet cells traps pathogens, which are then removed through coordinated ciliary activity. Epithelial cells and resident dendritic cells express a variety of pattern recognition receptors [such as toll-like receptors (TLR), retinoic acid-inducible gene I-like receptors, and nucleotide-binding oligomerization domain-like receptors] that detect viral components and trigger interferon and cytokine production[17].

Type I interferons (IFN-α/β), released early during infection, induce an antiviral state in neighboring cells, thereby inhibiting viral replication. This rapid interferon response is particularly important for controlling viruses with short incubation periods, such as influenza and RSV infections. Airway mucus is also enriched with antimicrobial factors, including defensins, lactoferrin, lysozyme, and complement proteins, which can directly inactivate viruses or limit their spread[1,18].

Beneath the epithelium, innate immune cells, such as macrophages, neutrophils, and natural killer cells, provide additional defense. Alveolar macrophages can phagocytose viral particles and coordinate inflammatory responses, while natural killer cells eliminate virus-infected cells exhibiting stress signals or reduced major histocompatibility complex class I expression. Innate lymphoid cells, particularly ILC2 and ILC3 subsets in the respiratory tract, contribute to barrier defense and shape adaptive immunity via cytokine production. Interleukin (IL)-17 and IL-22 produced by these cells recruit neutrophils and enhance epithelial integrity. A defining feature of mucosal innate immunity is its conditioned tolerogenicity; because mucosal surfaces are continually exposed to harmless antigens, innate immune cells often maintain a regulated response to prevent excessive inflammation[1,19].

Key correlates of protective mucosal immunity

When considering immunity against respiratory viruses, two factors consistently emerge as the most predictive correlates of infection blocking: SIgA and TRM T cells at the infection site. High titers of virus-specific sIgA in nasal secretions or saliva have been associated with reduced viral shedding and a lower likelihood of infection with influenza, RSV, and SARS-CoV-2[1,20].

Unlike circulating IgG, which may not reach the airway lumen in time to intercept incoming virions, sIgA is optimally positioned at the site of viral entry and, therefore, represents a superior correlate of sterilizing immunity. In parallel, the presence of virus-specific TRM cells in the respiratory tract correlates with rapid viral control and protection against disease recrudescence[1]. Several studies suggest that assessing lung TRM responses may provide a more informative benchmark for vaccine-induced protection than traditional systemic immune readouts[1,21]. Consequently, there is increasing support for evaluating novel respiratory vaccines based on their capacity to elicit both TRM cells and mucosal IgA responses, in addition to conventional serum-neutralizing antibodies[1].

WHY IM VACCINES ALONE ARE INSUFFICIENT

IM vaccination primarily induces systemic immunity characterized by circulating IgG antibodies and peripheral T cell responses, with limited immune protection at the respiratory mucosal surface. In contrast, mucosal vaccination promotes local immune responses in the airway epithelium, including secretory IgA production, TRM T lymphocytes, and rapid innate immune activation, which together contribute to infection and transmission prevention. The route of vaccine administration plays a critical role in shaping the nature and magnitude of the resulting immune response (Figure 1).

Figure 1 Comparison of immune outcomes after intramuscular vs mucosal vaccination. NALT: Nasal-associated lymphoid tissue.

Systemic-mucosal immunity mismatch

IM vaccination predominantly induces systemic immunity, with high levels of IgG in the circulation and activation of T and B cells in the lymph nodes and spleen. However, there is a fundamental compartmentalization in the immune system: Antibodies and lymphocytes concentrated in the blood do not automatically equate to robust immunity in the respiratory tract mucosa. Only a fraction of circulating IgG diffuses into airway secretions, and the respiratory epithelium has a separate immune inductive system directed toward IgA. Consequently, an individual can have strong serum neutralizing antibody titers (from IM vaccination) and still have low or negligible neutralizing activity in the nasal mucosa[5,6,22-25].

Immunological analyses have confirmed that standard IM vaccination induces minimal sIgA in the nasal passages of naïve individuals[26]. It is “very hard to protect the upper respiratory tract with (injected) systemic vaccines”, as one expert noted[5,27]. The same mismatch applies to T cells: IM vaccines generate circulating effector T cells that home to internal organs, but only a small number of these T cells take up long-term residence in the nasal or lung mucosa[6]. A recent study found that parenteral COVID vaccination induced essentially no spike-specific T_RM in the airway tissues, whereas individuals who had recovered from natural infection had significant airway-resident T cells[6,28].

Waning kinetics of mucosal vs systemic immunity

Another challenge is that mucosal immune responses tend to wane more rapidly than systemic responses in the absence of stimulation. After a mucosal infection or vaccination, local IgA and T_RM levels may peak and then decline relatively quickly over months unless boosted by re-exposure. Studies of convalescent COVID-19 patients showed that while serum IgG and IgA could persist for a year or more, the SARS-CoV-2-specific IgA in nasal secretions dropped back to baseline approximately 6-9 months post-infection[26,29-31]. In one report, nasal IgA against SARS-CoV-2 became undetectable in the majority of individuals nine months after infection, even though plasma IgG remained durable[31]. This relatively short-lived mucosal antibody response means that protection from infection can diminish faster than protection from severe disease. Indeed, susceptibility to breakthrough infections with SARS-CoV-2 has been correlated with the waning of mucosal IgA levels over time[32]. TRM T cells in the lungs can also be transient. Experiments in mice have shown that lung T_RM cells generated by influenza infection largely disappear after 100 days, resulting in the loss of heterosubtypic immunity[1].

In humans, it is difficult to longitudinally sample lung T_RM, but seasonal reinfections with respiratory viruses suggest that mucosal T cell memory may not be maintained for long without periodic antigen exposure. In contrast, systemic IgG responses (especially with repeated IM boosting) and central memory T cells can persist for years. This implies that solely relying on systemic immunity might protect against disease for longer periods, but protection against infection at the mucosa could be fleeting. Therefore, mucosal vaccination strategies may require periodic boosters or a prime-boost approach to sustain local immunity. For instance, administering an intranasal booster dose after an initial IM vaccine prime could periodically replenish nasal IgA and T_RM pools[33-35].

Real-world outcomes: Infection and transmission persist

The practical consequences of gaps in mucosal immunity with IM vaccines have been observed epidemiologically. COVID-19 provided a clear case study for this. Despite the rapid vaccination of billions of people, the virus continued to spread, with fewer hospitalizations. In mid-2021, an outbreak occurred in Provincetown, Massachusetts, in which 74% of cases were in fully vaccinated individuals; analysis revealed that nasopharyngeal viral loads were equally high in the vaccinated and unvaccinated cases[5,36].

This indicates that vaccination did not substantially reduce the amount of virus in the upper airway, allowing onward transmission. Similar patterns were observed globally during the Omicron wave when vaccines showed > 80%-90% effectiveness against severe outcomes, yet population-level infection rates soared. For influenza, even in years of well-matched vaccines, we still see considerable circulation of the virus; vaccinated people may experience mild infections and can pass the virus to others, especially in household or close-contact settings. IM vaccine-induced immunity tends to manifest in a disconnect: It transforms severe pneumonia into a common cold-like illness, but the individual can still harbor and spread the pathogen[37]. From a public health perspective, this means that current vaccines alone cannot readily induce herd immunity against highly transmissible respiratory viruses. High vaccine coverage yields immense benefits in reducing deaths and intensive care unit admissions (as seen with COVID-19, where prior vaccination somewhat shortens the duration of viral shedding)[26], but it is often insufficient to halt outbreaks. These outcomes reinforce the fact that preventing infection at mucosal surfaces is a higher bar that existing systemic vaccines do not consistently meet.

PLATFORMS FOR MUCOSAL VACCINE DELIVERY

Multiple vaccine platform technologies are being investigated to elicit mucosal immunity against respiratory viruses.

Different mucosal vaccine platforms vary substantially in their ability to induce specific immune responses in the respiratory tract. Replicating vectors and live attenuated vaccines most closely mimic natural infections and are therefore particularly effective at inducing secretory IgA in airway secretions and establishing TRM T lymphocytes within the nasal and pulmonary epithelium. In contrast, non-replicating platforms, such as protein subunit or nucleic acid-based vaccines, generally require potent mucosal adjuvants or optimized delivery systems to overcome mucociliary clearance and achieve sufficient local immune activation. Oral vaccine platforms primarily engage gut-associated lymphoid tissues (GALT) but may indirectly contribute to respiratory mucosal immunity through the common mucosal immune system. Understanding how each platform aligns with key mucosal immune correlates is essential for rational vaccine design, platform selection, and the interpretation of clinical trial outcomes[38-42].

Table 1 summarizes the major mucosal vaccine platforms under clinical or advanced preclinical evaluation, highlighting their delivery routes, mechanistic features, predominant immune correlates elicited, and current developmental status.

Table 1 Comparison of mucosal vaccine platforms.

Platform	Example	Route	Mechanism/design	Noted immune features	Advantages	Key limitations	Status
LAIV[38]	FluMist® (MedImmune/AstraZeneca)	Intranasal spray	Cold-adapted live attenuated influenza virus replicates in nasal mucosa	Induces mucosal immunity; long track record in children	Licensed product; needle-free	Contraindicated in some groups; live virus	FDA-licensed; self/caregiver administration cleared September 20, 2024, with consumer rollout for 2025-2026 season
Adenoviral vector (inhaled)[39]	Convidecia Air™ (CanSinoBIO)	Inhaled aerosol	Ad5 vector; aerosolized booster formulation	Authorization news; peer-reviewed human mucosal immunogenicity not detailed in approval notice)	First approved inhaled COVID-19 booster; needle-free	Vector immunity; limited published efficacy data	Emergency use authorized in China (September 2022)
Adenoviral vector (intranasal)[40]	iNCOVACC™ (Bharat Biotech)	Intranasal spray	Non-replicating Ad5 vector expressing SARS-CoV-2 spike	Reported immunogenicity in Phase III; used as primary series or heterologous booster in India	Needle-free; deployable as booster	Vector immunity; comparative efficacy vs IM still being studied	India approval for primary 2-dose series and heterologous booster (restricted emergency use)
Live attenuated influenza-vectored[41]	dNS1-RBD (Beijing Wantai; “Pneucolin”)	Intranasal spray	ΔNS1 influenza backbone expressing SARS-CoV-2 RBD	Phase 3 overall VE 28.2% against symptomatic Omicron; safety acceptable	True mucosal delivery; some infection reduction	Modest efficacy in Omicron period	Phase 3 RCT completed; results published
Protein subunit + mucosal adjuvant[42]	Intranasal multivalent H5 (UMD/BlueWillow; nanoemulsion adjuvant)	Intranasal spray	Recombinant H5 antigen(s) + nanoemulsion (NanoVax® W805EC)	Phase 1 broad immune response incl. mucosal; strong priming	Stockpile-friendly concept; non-live	Needs adjuvant; data early-phase	Phase 1 RCT results published (nature communications, 2025) and institutional news

RCT: Randomized controlled trial; LAIV: Live attenuated influenza vaccine; Ad5: Adenovirus serotype 5; SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2; RBD: Receptor-binding domain; VE: Vaccine efficacy; IM: Intramuscular; FDA: United StatesFood and Drug Administration; ΔNS1: Deletion of nonstructural protein 1; H5: Influenza A(H5) hemagglutinin subtype.

Live attenuated and viral-vectored intranasal vaccines

Live attenuated viruses and viral vector vaccines administered intranasally are among the most established approaches to mucosal immunization. Using a replicating but harmless virus delivered to the respiratory tract, these vaccines can mimic natural infection and stimulate comprehensive local immunity. A prime example is the live attenuated influenza vaccine (LAIV), which is administered as an intranasal spray (FluMist^®)[43].

LAIV contains temperature-sensitive influenza strains that replicate in the cooler nasal passages but not in the lungs. It induces mucosal IgA in the nasopharynx and has shown efficacy in children, who often develop broad, local, and systemic responses. FluMist is the only United States Food and Drug Administration-approved intranasal vaccine to date[1], highlighting the potential of intranasal administration. Similarly, live attenuated RSV vaccines delivered intranasally are being developed for infants; these are engineered RSV strains [or chimeras, such as bovine/human parainfluenza virus (PIV) expressing RSV genes] designed to infect the nose and prompt mucosal immunity without causing disease[5].

Another strategy involves the use of recombinant viral vectors that can be administered intranasally. These vectors are typically non-pathogenic viruses modified to carry genes encoding antigens of the target pathogen. Adenoviruses are frequently used as vectors in gene therapy. For instance, a recombinant adenovirus type-5 expressing the SARS-CoV-2 spike protein has been formulated as an intranasal COVID-19 vaccine (iNCOVACC™) and was approved in India, whereas an inhaled aerosolized Ad5 vectored COVID-19 vaccine (Convidecia Air™) was approved in China[1,44]. These intranasal adenoviral vaccines can infect the cells of the nasal mucosa, leading to in situ expression of the antigen and induction of mucosal IgA and T cell responses, along with systemic immunity similar to that of an injected vaccine. Early phase trials of intranasal adenoviral COVID-19 vaccines have demonstrated that they are immunogenic and well tolerated, inducing nasal IgA as well as systemic antibodies[1,43,44].

Other vectors are also being explored. One is the PIV, an attenuated bovine/human parainfluenza type 3 (HPIV3) vector engineered to express the SARS-CoV-2 spike protein and tested as a pediatric intranasal COVID-19 vaccine candidate[35]. In animal models, a single intranasal dose of this HPIV3-based vaccine replicated in the upper airway and elicited robust mucosal IgA, serum neutralizing antibodies, and lung-resident T cells, providing protection against challenges with both ancestral and variant strains[35]. Similarly, a canine PIV5 vector is being studied for intranasal COVID-19 vaccination in adults. Parainfluenza vectors have the advantage of being naturally tropic to the respiratory epithelium and are unlikely to cause disease in humans (PIV5 is used in dog vaccines and has an excellent safety record in that context)[5].

Live replicating vaccines delivered intranasally tend to induce a broad immune response. In addition to strong sIgA responses, they can recruit CD4⁺ and CD8⁺ T_RM to the lung and nasal tissues and stimulate innate immunity via the infected epithelial cells. For example, intranasal immunization with a chimpanzee Ad-Oxford/AstraZeneca vaccine in animal studies generated systemic antibody levels comparable to those generated by IM immunization, but uniquely conferred protection in the upper airway (reduced viral load in nasal swabs) due to local immunity[5]. LAIV have also been shown to elicit cross-reactive T cells and IgA that provide heterosubtypic protection beyond the vaccine strains.

Of course, safety is a paramount consideration; attenuated vaccines must not cause significant disease, especially in populations such as infants or immunocompromised individuals. For instance, FluMist is not recommended for severely immunocompromised persons due to the theoretical risk of a live virus[45]. Nasal vaccines using replication-competent vectors must be carefully balanced in terms of attenuation; if too mild, they will not induce immunity, and if too virulent, they will pose risks. Thus far, the safety profile in clinical trials (for intranasal Ad5 SARS-CoV-2 or PIV5-vectored vaccines) has been acceptable, with mostly local reactions (transient nasal congestion or mild symptoms) reported[1]. The regulatory approval of the first intranasal COVID-19 vaccines in Asia provides proof of concept, but efficacy data on preventing infections in humans are still being gathered.

Protein subunit and nanoparticle vaccines (intranasal formulations)

Protein-based vaccines, including purified antigens, virus-like particles (VLPs), and self-assembling nanoparticles displaying antigens, have proven successful via injectable routes (recombinant protein influenza vaccines, nanoparticle COVID-19 vaccines). Adapting these subunit vaccines for mucosal delivery involves formulating them in a way that they can traverse the mucosal barrier and trigger a local immune response, often with the help of adjuvants or particulate carriers. One approach is to use VLPs, which are nanoscale particles that mimic the conformation of viruses but do not contain genetic material. VLPs can be administered intranasally and are taken up by antigen-presenting cells in the nasal mucosa, leading to immune activation[46]. VLPs based on norovirus or hepatitis E have been tested intranasally for influenza antigen delivery, demonstrating the induction of mucosal IgA and systemic responses in animal models[5].

Another promising strategy is the use of self-assembling protein nanoparticles that present viral antigens in a multivalent array. A notable example is the computationally designed I53-50 protein nanoparticle, which is composed of 120 subunits that spontaneously assemble into an icosahedral particle 40 nm in diameter[47]. This platform has been used to display the SARS-CoV-2 spike receptor-binding domain (RBD) in a highly immunogenic manner. When administered IM with an adjuvant, an RBD-I53-50 nanoparticle vaccine elicited potent neutralizing antibodies and protected non-human primates, including reducing viral loads in the upper and lower airways[48]. To use such nanoparticles intranasally, researchers have formulated them using mucoadhesive carriers or potent mucosal adjuvants. In mice, intranasal delivery of RBD-decorated nanoparticles (combined with an appropriate adjuvant) induced strong serum antibody responses, mucosal IgA in the respiratory tract, and lung T cell responses, conferring protection against viral challenge[49,50].

One challenge for intranasal protein vaccines is ensuring that the antigen persists long enough in the nasal mucosa to be picked up by antigen-presenting cells, given that mucociliary clearance can rapidly expel material. Nanoparticles and VLPs, owing to their size, may be retained for a slightly longer duration and can be taken up by M cells in the nasal epithelium. Additionally, these particulate forms often naturally engage pattern recognition receptors (via microbial or recombinant origin of VLPs), which provides an adjuvant effect. However, most protein antigens require a dedicated mucosal adjuvant to boost immunogenicity. Some intranasal subunit vaccines have employed adjuvants such as cholera toxin B-subunit (an ADP-ribosylating adjuvant) or bacterial endotoxin derivatives; however, toxicity issues have hampered their use in humans. Newer adjuvants, such as nanoemulsion-based adjuvants that can form a depot on the nasal mucosa, have shown the ability to enhance intranasal protein vaccines and are in clinical testing for influenza[51,52].

Stability is another consideration; unlike live vectors, purified proteins typically do not replicate or persist, so achieving long-lived mucosal immunity might require multiple doses. Nevertheless, intranasal protein vaccines offer a high level of safety (since they contain no live agents) and can be rapidly engineered as new variants emerge[30].

An interesting development is the idea of dry powder nasal vaccines, which are formulated with protein antigens or VLPs in a powder with stabilizers that can be inhaled. This could improve thermostability and eliminate the need for cold-chain storage, thereby facilitating global distribution. For example, a COVID-19 vaccine with a dry powder (a recombinant spike protein with a polysaccharide adjuvant) was recently reported to induce serum and salivary antibodies in animal models when delivered intranasally[53].

RNA vaccines adapted for mucosal delivery

The technology of mRNA vaccines became popular with COVID-19, and it proved capable of causing a robust systemic response through lipid nanoparticle (LNPs)-based mRNA injection into the muscle. The adaptation of mRNA vaccines to mucosal delivery is a fascinating area that has the potential to unite the immune capacity of mRNA systems with the ability of mucosal vaccines to prevent infection. Nonetheless, it is difficult to deliver mRNA to areas of the mucosal surface, such as the nasal epithelium. Free mRNA is easily degraded by mucus nucleases and is poor at overcoming the epithelial barrier. Moreover, standard LNPs as IM mRNA vaccines are capable of inducing inflammation in the respiratory tract and do not usually penetrate mucus[54,55].

One approach is to modify LNPs for better stability and mucosal penetration. For example, “muco-penetrating” nanoparticles have been engineered by altering the surface polyethylene glycol density or by adding mucolytic agents, allowing them to diffuse through the mucus layer[55]. Others have attached mucoadhesive polymers, such as chitosan, to nanoparticles, which can prolong the residence time of mRNA at the nasal epithelium and even inherently stimulate immune cells. A recent study showed that intranasally delivered chitosan-coated mRNA LNPs achieved higher uptake by lung cells and elicited both systemic and mucosal immune responses, including IgA and lung-resident T cells, whereas unmodified LNPs did not[55,56]. Another strategy is to use charge-altering releasable transporters or polyplexes, essentially polymer-based nanoparticles, to deliver mRNA to the lungs. In an animal model, an inhalable polymer nanoparticle was able to deliver mRNA into lung cells via aerosols, resulting in the expression of the encoded antigen throughout the airways[54].

The preclinical results of intranasal mRNA vaccines are promising. Mice immunized intranasally with an mRNA encoding a viral antigen (such as influenza hemagglutinin or a coronavirus spike) formulated in a specialized LNP developed antigen-specific sIgA in the bronchoalveolar fluid and IgA-secreting cells in the lungs, along with robust serum IgG responses[57,58]. Crucially, these immunized mice were protected against viral challenge in both the upper and lower respiratory tracts, which was not achieved by purely IM mRNA immunization. Moreover, intranasal mRNA-LNP vaccination generates lung TRM T cells, which are critical for rapid on-site immunity[55]. One example is an intranasal mRNA vaccine encoding a tuberculosis antigen with a cationic nanoemulsion as a carrier (“RNA vaccine droplet”), which induced lung T_RM and conferred better protection against Mycobacterium tuberculosis in mice than parenteral vaccination[59].

Despite this progress, it is important to note that no mucosal mRNA vaccine has been tested in advanced human trials as of 2025. Safety will be a key consideration, as introducing mRNA directly to the respiratory tract could provoke strong innate immune sensing (since unmethylated RNA and LNPs activate TLR and other sensors), potentially leading to local inflammation. Dose optimization and possibly new generations of LNPs (with biodegradable or less immunostimulatory components) are required. Delivering mRNA to the right location is critical; intranasal drops or sprays tend to stay in the upper nasal cavity, whereas inhaled aerosols can reach the lungs. Vaccines targeting upper-airway pathogens may focus on nasal delivery, whereas others may need to reach the lower airway. Some groups are investigating aerosolized mRNA vaccines using inhalers or nebulizers, which could deposit LNPs deep in the lungs; an example is an inhalable mRNA therapy for pulmonary diseases that showed promise in rodent models[60].

DNA vaccines and subunits with mucosal adjuvants

DNA and protein subunit vaccines can also be administered via mucosal routes; however, they usually require powerful adjuvants or delivery systems to be immunogenic in this context. DNA vaccines (plasmid DNA encoding a viral antigen) have typically been delivered by injection or gene gun, as naked DNA uptake by the mucosa is inefficient[61].

However, techniques such as electroporation and encapsulation of DNA in nanoparticles have been tested for intranasal delivery in animals. There have been small trials of intranasal DNA vaccines for influenza using a cationic lipid formulation, which showed the induction of some mucosal IgA, but the responses were modest. More recent research has focused on improving mucosal DNA delivery, for instance, using bacterial vectors (attenuated Salmonella carrying DNA plasmids) that can be administered orally or intranasally to introduce DNA into mucosal tissues. Although pure DNA vaccines are currently not front-runners for mucosal immunization, they remain an area of exploratory research, especially for pathogens where a combined systemic + mucosal response is needed (a DNA prime delivered IM followed by a mucosal boost with the same DNA might target both compartments)[8].

TLR agonists (pattern recognition receptor agonists)

Molecules that activate TLR can serve as intranasal adjuvants to enhance dendritic cell activation and drive IgA class switching. One example is CpG oligodeoxynucleotide, a TLR9 agonist that stimulates plasmacytoid dendritic and B cells. Intranasal administration of CpG together with an antigen has been shown to significantly boosts mucosal IgA and Th1-biased responses[6]. Some intranasal influenza vaccine formulations have tested CpG as an adjuvant (in mice and early trials), resulting in higher nasal IgA titers and improved protection against the disease. Another is poly (I:C), a TLR3 agonist (mimicking viral double-stranded RNA), which has been used experimentally as an intranasal adjuvant to induce strong interferon responses and enhance CTL generation[1]. Flagellin (TLR5 agonist) is another example; it has inherent mucoadhesive properties and has been fused to antigens to act as both a carrier and adjuvant intranasally[1].

STING agonists

The STING pathway can be triggered by cyclic dinucleotides (like cGAMP). STING agonists are emerging as potent adjuvants capable of inducing robust innate signaling. A liposomal STING agonist formulation (sometimes called “NanoSTING”) was tested intranasally with a protein vaccine and was found to elicit high levels of mucosal sIgA and a balanced Th1/Th17 response, affording protection in a mycobacterium tuberculosis challenge model[52]. Intranasal administration of cGAMP with an inactivated influenza vaccine significantly improved antibody responses and cross-protection in mice[62]. These results suggest STING agonists could be very effective for human mucosal vaccines as well, though safety (excess inflammation) must be monitored.

Emulsion and particle-based adjuvant

Oil-in-water emulsions, such as MF59 (used in IM flu vaccines), have mucosal analogs. An intranasal nanoemulsion adjuvant (NE01) composed of soy lecithin and cetylpyridinium has been shown to be safe and boost influenza antigen immunogenicity intranasally by facilitating antigen uptake and activating epithelial cells. These emulsions can create a depot effect in the nasal mucosa. Another particle-based adjuvant is bacterial outer membrane vesicles, which are immunostimulatory and have been used experimentally as intranasal adjuvants for pertussis and influenza vaccination. They carry natural TLR agonists and can induce strong IgA responses[63].

Cytokine adjuvants

Certain locally delivered cytokines can skew the immune response. For example, IL-1 or IL-12 administered intranasally with an antigen has been shown to enhance T_H1 and IgA responses, whereas chemokines such as CCL28 can be used to attract IgA-secreting cells to the mucosa. Although these are not in practical use, they inform the design of other adjuvants[64].

One success story in mucosal adjuvants is the development of an intranasal pertussis vaccine (BPZE1, a live attenuated Bordetella pertussis) that includes genetic adjuvanting by detoxifying pertussis toxin. This intranasal vaccine induced broad mucosal IgA in humans, whereas the conventional Tdap (injected) did not[1].

Oral vaccines and targeting of gut-respiratory immunity

Oral vaccination is a classic route for inducing mucosal immunity, primarily in the gastrointestinal tract; oral polio and rotavirus vaccines are notable examples. Interestingly, there is immunological crosstalk between the gut and respiratory mucosa, often termed the “gut-lung axis” or, more broadly, the common mucosal immune system. Immune activation in the intestines can disseminate lymphocytes that home to other mucosal sites, including the airways, owing to shared trafficking signals[65]. This raises the possibility that oral vaccines could confer some degree of respiratory protection by inducing IgA and T cells that migrate to the respiratory tract. Some studies have shown that oral immunization can elicit IgA not only in intestinal secretions but also in saliva and nasal fluids. Infants receiving oral poliovirus vaccines develop poliovirus-specific IgA in both stool and nasal secretions, suggesting that activated B cells in the GALT can seed distant mucosae[65].

Based on this concept, companies have pursued oral tablet vaccines for respiratory viruses. For example, Vaxart has developed an oral adenovirus vector tablet for influenza and SARS-CoV-2. In a phase 1 study, an oral adenoviral COVID-19 vaccine (expressing the spike protein) was found to induce serum antibodies and mucosal IgA in a proportion of subjects and some T cell responses in the airway[66]. In animal models, an oral adenovirus-based RSV vaccine in cotton rats generated respiratory mucosal immunity and protected against RSV challenge, validating this approach preclinically[66-69]. The proposed mechanism is that the adenoviral vector, when released in the gut, transfects intestinal cells and triggers an immune response in the Peyer’s patches and mesenteric lymph nodes (part of the GALT). Activated B and T cells from these sites express homing receptors that can lead a subset of them to the respiratory tract (for example, via upregulation of integrin α4β1 or CCR10, which can direct cells to the lungs and salivary glands)[1]. Moreover, some IgA plasma cells induced in the gut can migrate to the bronchial tissues, contributing to sIgA in the airways. Immunity induced in the gut tends to home preferentially to the gut and associated glands, whereas intranasal immunization is more direct for respiratory tract immunity[1]. The gut-lung connection exists but may be stronger for certain cell types (such as IgA B cells) than for others. In addition, high doses are often required for oral vaccines because of degradation in the digestive tract. Nonetheless, oral vaccine platforms remain an attractive area of research because of their ease of administration. If an oral tablet that induces significant protective IgA in the lungs can be developed, it would be a breakthrough for mass vaccination campaigns.

CLINICAL LANDSCAPE (2023-2025)

Table 2 summarizes recent human clinical trials assessing mucosal vaccine candidates, including pathogen targets, platforms and delivery routes, trial phases, key immunological or clinical findings, and regulatory or developmental outcomes.

Table 2 Recent clinical readouts for intranasal/inhaled respiratory vaccines (2022-2025).

Pathogen	Candidate and developer	Platform/route	Phase/setting	Key immune or clinical finding	Outcome/status
SARS-CoV-2[68]	ChAdOx1 nCoV-19 (Oxford/AZ) intranasal	Chimp adenoviral vector; intranasal	Phase 1, United Kingdom	Mucosal antibody responses detected in a minority; systemic responses weaker than IM; no safety concerns	Program not advanced further based on weak mucosal/systemic responses
SARS-CoV-2[41]	dNS1-RBD (“Pneucolin”, Wantai)	Live attenuated influenza vector; intranasal	Phase 3, multi-country, Omicron period	Overall VE 28.2% (95%CI: 3.4-46.6) against symptomatic infection (1.6% vaccine vs 2.3% placebo); acceptable safety	Demonstrated modest efficacy; first large RCT efficacy signal for an intranasal COVID-19 vaccine
SARS-CoV-2[39]	Convidecia Air™ (CanSinoBIO)	Ad5 vector; inhaled aerosol	Post-approval booster use (China)	Approved for booster use; first inhaled COVID-19 vaccine authorized	Emergency use authorization granted September 2022 (China NMPA)
SARS-CoV-2[40]	iNCOVACC™ (Bharat Biotech)	Ad5 vector; intranasal	India approvals	Approved for primary series and heterologous booster; phase III immunogenicity conducted in about 3100 subjects	Restricted emergency use authorization in India
Influenza A(H5)[38]	Intranasal H5 nanoemulsion-adjuvanted vaccine (UMD/BlueWillow)	Protein subunit + nanoemulsion; intranasal	Phase 1, United States adults	Broad immune response across H5 clades; robust priming for later IM boost (as reported)	Phase 1 RCT results published (nature communications, 2025)
RSV[69]	BLB201 (Blue Lake Biotechnology)	PIV5 vector; intranasal	Phase 1/2a (adults; pediatric expansion)	Interim reports: Immunogenicity and tolerability in adults; pediatric seropositive cohort showed mucosal/serologic responses (company communications)	Early-phase development; interim data communicated by sponsor

COVID-19: Coronavirus disease 2019; RSV: Respiratory syncytial virus; IM: Intramuscular; VE: Vaccine efficacy; RCT: Randomized controlled trial; NMPA: National Medical Products Administration (China); Ad5: Adenovirus serotype 5; PIV5: Parainfluenza virus type 5; RSV: Respiratory syncytial virus.

Current approved mucosal vaccines

As of 2025, only a handful of mucosal vaccines against respiratory viruses have been licensed. The longest-standing example is the LAIV FluMist, a cold-adapted intranasal flu vaccine approved for use in individuals aged 2-49 years[70]. FluMist has been deployed in seasonal influenza programs (especially for children) and has demonstrated that intranasal vaccination can safely induce protective immunity in humans. Beyond LAIV, virtually all other widely used vaccines for respiratory pathogens have been injected. However, recent regulatory approvals in a few countries have expanded the range of mucosal vaccines. Notably, in 2022-2023, China and India authorized the world’s first COVID-19 mucosal vaccines: One delivered as an inhaled aerosol of adenovirus-vectored vaccine (CanSino’s Convidecia Air) and another as an intranasal adenoviral-vectored spray (Bharat Biotech’s iNCOVACC)[71]. By early 2025, at least five mucosal COVID-19 vaccines (including those in Iran, Indonesia, and elsewhere) received limited approvals[72], although none attained widespread global use or World Health Organization prequalification. In addition, a few oral vaccines targeting enteric infections (such as oral polio and rotavirus vaccines) incidentally confer some respiratory mucosal immunity, but no oral vaccine is specifically licensed for a respiratory virus[65].

Table 3 summarizes the key regulatory milestones and deployment decisions for licensed or authorized mucosal vaccine products, including jurisdiction and approval context.

Table 3 Regulatory and deployment updates relevant to mucosal vaccines.

Item	Jurisdiction	Detail
First inhaled COVID-19 booster approval[39]	China	Convidecia Air™ (Ad5 aerosol) received emergency authorization as a booster (September 2022)
First intranasal COVID-19 vaccine approval[40]	India	iNCOVACC™ approved for primary series and heterologous booster under restricted emergency use
LAIV self-administration pathway[38]	United States (FDA)	FluMist® approved for self/caregiver administration (September 20, 2024); broad home-use rollout for 2025-26 season

COVID 19: Coronavirus disease 2019; LAIV: Live attenuated influenza vaccine; Ad5: Adenovirus serotype 5; FDA: United States Food and Drug Administration.

Intranasal and oral COVID-19 vaccine candidates

The COVID-19 pandemic catalyzed an intense push to develop intranasal or oral vaccines that provide sterilizing immunity. Dozens of candidates have entered clinical trials, employing strategies from live viral vectors to protein subunits and mRNA.

Intranasal adenoviral vectors

Several groups have tested non-replicating adenovirus-based intranasal COVID-19 vaccines in phase 1/2 trials. The University of Oxford/AstraZeneca evaluated an intranasal version of the ChAdOx1 nCoV-19 vaccine in a phase 1 study. The results showed that, while the nasal spray was safe, it induced only weak and inconsistent immunity; mucosal anti-spike IgA was detected in only a minority of participants, and systemic antibody/T cell responses were much lower than those from IM vaccination[73]. Essentially, the standard ChAdOx1 formulation delivered intranasally failed to generate robust local immunity; consequently, the program was halted after disappointing results[68]. In contrast, other adenoviral intranasal candidates performed better. In China, an intranasal two-dose regimen of a recombinant Ad5-vectored COVID-19 vaccine (encoding spike protein) elicited potent mucosal IgA responses: A phase 1 trial in 128 volunteers showed a > 50-fold increase in nasal sIgA titers after the second dose[12]. This vaccine (termed “NB” in one report) also induced moderate serum neutralizing antibody titers and was well tolerated, indicating that a properly formulated adenoviral spray can activate mucosal immunity in humans[12]. Another example is the intranasal Ad5-vectored SARS-CoV-2 vaccine (iNCOVACC) developed in India. In a phase 3 trial as a booster, intranasal iNCOVACC was immunogenic and comparable to injectable boosters in serum neutralization; it also showed a trend toward increased salivary IgA, although mucosal antibody differences between intranasal and IM boosters were modest in magnitude[74].

Crucially, these adenoviral vaccines have demonstrated an ability to recall mucosal memory in previously vaccinated individuals; for instance, nasal IgA levels increased appreciably when an intranasal Ad5 booster was administered to those with prior COVID immunization, whereas a third systemic dose alone usually yields little mucosal IgA[73,75].

Intranasal live attenuated virus

Taking a cue from FluMist, several live attenuated intranasal COVID-19 vaccines have entered trials. One of the most advanced vaccines is a live attenuated influenza virus expressing the SARS-CoV-2 spike RBD (dNS1-RBD, Beijing Wantai Biopharmaceuticals). This intranasal vaccine uses a deleted-NS1 influenza backbone and can infect the nasal mucosa without causing disease. Early trials showed that it induced both serum and mucosal antibodies in a proportion of subjects[76]. A Phase 3 trial with over 30000 participants was completed in 2022-2023, providing the first efficacy readout for any intranasal COVID vaccine. The two-dose dNS1-RBD vaccine was safe and significantly reduced symptomatic COVID-19 caused by Omicron, but with an efficacy of only about 28% (1.6% infection in vaccine vs 2.3% in placebo), it did not meet the predefined success criterion[77]. Nevertheless, the trial confirmed that intranasal vaccination can offer some protection in humans and provided valuable data for the field.

Oral vaccines

A few oral COVID-19 vaccines have been investigated with the aim of inducing immunity in the gut-associated lymphoid tissue, which could disseminate to the respiratory tract. One such candidate is Vaxart’s oral adenovirus-based vaccine (tablet encoding the spike protein). In Phase 1 studies, the Vaxart oral vaccine was found to be immunogenic, generating serum antibodies and, in some subjects, nasal IgA against SARS-CoV-2[78,79].

RSV and pan-influenza mucosal vaccine pipeline

RSV and influenza are two other major targets of next-generation mucosal vaccines. RSV is a leading cause of severe respiratory illness in infants; however, no traditional infant RSV vaccine exists (recently approved RSV vaccines are for maternal immunization or older adults via IM injection). However, intranasal live-attenuated RSV vaccines have been pursued for decades[80]. Several candidates have reached phase 1/2 trials in infants. While past candidates have suffered from issues (insufficient attenuation or immunogenicity), newer approaches are showing progress. For example, Meissa’s intranasal RSV vaccine uses codon deoptimization to weaken RSV without deleting the key antigens. Early clinical data in adults showed that it induced mucosal RSV-neutralizing antibodies, and pediatric trials are ongoing. Another program by Blue Lake Biotechnology uses a recombinant PIV5 vector to deliver the RSV F protein, intranasally. In 2024, Blue Lake reported interim results from a phase 1/2a trial of this PIV5-RSV vaccine (BLB201) in young children[78]. A single nasal dose was well tolerated, and notably, even RSV-seropositive toddlers showed boosted immunity; 80% of those given a higher dose had a ≥ 3.6-fold rise in neutralizing antibody titers, along with detectable RSV-specific mucosal IgA and T cell responses[78].

For influenza, mucosal vaccine research is driven by the quest for a “universal” flu vaccine that not only broadens strain coverage but also stops person-to-person transmissions. LAIV (FluMist) provides a baseline proof that nasal influenza vaccination works, but its effectiveness varies by season and age[81].

Therefore, researchers are developing next-generation intranasal flu vaccines with enhanced breadth and potency to overcome these limitations. One strategy involves using novel antigens and adjuvants to induce immunity against conserved viral targets. A recent milestone in this arena was a phase 1 trial of an adjuvanted intranasal H5N1 vaccine (an avian flu strain with pandemic potential). The vaccine consisted of a recombinant H5 antigen given with a potent nanoemulsion mucosal adjuvant (NanoVax^©) and was formulated to be shelf-stable for stockpiling[82]. In a trial of 40 adults, this intranasal H5N1 vaccine was safe and elicited broad immunity; it induced mucosal IgA and systemic IgG, and it “primed” recipients such that a later IM booster led to an anamnestic response far exceeding that of controls[82]. Only those who received the intranasal prime showed strong immune memory activation upon boosting, demonstrating successful priming. Moreover, the intranasal vaccine alone triggered cross-reactive responses against multiple H5N1 clades[82], suggesting that it could afford protection against drifted avian strains.

Another promising approach to influenza vaccination is the use of vector-based nasal vaccines. Vivaldi Biosciences is developing DeltaFLU, an intranasal universal influenza vaccine based on live ΔNS1 influenza viruses. By deleting the NS1 gene, DeltaFLU strains are replication-deficient but invoke a strong interferon response (acting as a built-in adjuvant)[83]. In preclinical ferret studies, intranasal DeltaFLU vaccination induced broadly protective immunity in the nasal passages and blocked viral transmission to unvaccinated animals. Notably, DeltaFLU aims to cover all groups A and B influenza viruses with a single formulation. As of 2025, DeltaFLU has completed multiple Phase 1 trials (demonstrating safety and immunogenicity) and is progressing in clinical development with support from the European Union[83]. Other groups are pursuing intranasal VLPs or protein subunits for influenza; for instance, a nasal influenza vaccine comprising the conserved M2e protein on a bacterium-like particle has shown high mucosal IgA in animals[84].

What works and what hasn’t: Insights from trials

Considering the diverse clinical outcomes to date, several patterns have emerged regarding the success or failure of mucosal vaccine strategies.

Effective induction of mucosal IgA and T cell

The clearest successes in trials have been candidates that have achieved strong local immune responses. Vaccines using replicating vectors or potent adjuvants tend to outperform simple protein or inactivated formulations. For example, the intranasal Ad5 vaccine in China that boosted nasal IgA > 50-fold stands out[12], as does the PIV5-based RSV vaccine that raised mucosal IgA and T cells in children[78]. These successes suggest that delivering sufficient antigens to the mucosa and engaging innate sensors (via vector replication or adjuvants) is critical. In contrast, candidates that failed often showed weak or absent mucosal immunity; the Oxford/AZ nasal vaccine is a case in point, where using an unadjuvanted, non-replicating ChAdOx1 vector led to almost no IgA in most recipients[73].

The importance of dose and formulation

Several trials have indicated that the dose and delivery format can make or break a mucosal vaccine. The Oxford study, for instance, used the same formulation as the injectable vaccine but delivered it as a spray; the investigators hypothesized that much of the dose was being swallowed and destroyed in the gut rather than remaining in the nasopharynx[68]. This highlights that nasal delivery needs to ensure that the vaccine contacts the nasal lymphoid tissue (using smaller volumes, mucoadhesive excipients, or devices that target the posterior nasopharynx). Similarly, the moderate about 28% efficacy of the dNS1-RBD intranasal COVID vaccine might be related to suboptimal dosing or the need for a stronger boost; its developers are exploring a third dose to further elevate immunity. In contrast, the successful intranasal H5N1 trial used an adjuvanted formulation and a two-dose prime-boost, which likely contributed to the strong immune priming observed[82].

Vector immunity and interference

Viral-vectored mucosal vaccines face the challenge of pre-existing immunity to the vector. Intranasal adenovirus vaccines using common human serotypes (such as Ad5) can be neutralized by serum IgG that transudates into the mucosa or by resident adenovirus-specific IgA from prior infections. This was a concern for Ad5-vectored COVID vaccines; some trials opted for less prevalent vectors (chimpanzee adenovirus or PIV5, to which humans have little immunity)[85]. Thus far, intranasal Ad5 COVID vaccines have induced immune responses even in populations with some Ad5 exposure, but efficacy could be undermined in regions where Ad5 seroprevalence is high. Vector interference is also observed in LAIV; if a person has recent immunity to a similar flu strain, the replication of LAIV can be blunted. Upcoming combination mucosal vaccines (such as COVID + flu nasal vaccines) will need to consider the interference between components. Novel vectors that the human immune system has not encountered (or replicating vectors that can initially outpace the immune response) may hold an edge in this regard[86].

Safety and tolerability

Repeated evidence has shown that mucosal vaccines are generally well tolerated, with mostly mild local reactions (transient nasal congestion, runny nose, or sore throat). None of the COVID-19 mucosal vaccine candidates in trials have reported serious vaccine-related adverse events[76]. This safety profile is encouraging and warrants further investigation. However, a few safety considerations are unique to the oral mucosal route. One is the theoretical risk of vaccine introduction to the central nervous system via the olfactory nerve; though rare, this was highlighted by an unfortunate signal in 2000, when an intranasal inactivated flu vaccine adjuvanted with Escherichia coli (E. coli) LT toxin was linked to an increased risk of Bell’s palsy (facial nerve paralysis) in Switzerland[87]. This vaccine was recalled, highlighting the fact that some mucosal adjuvants (especially neuroactive toxins) may be dangerous. Newer adjuvants are then developed to prevent such effects, and regulators insist on longer follow-up of trials of mucosal vaccine candidates to detect neuroimmune delayed events (3 months observation of Bell’s palsy)[87].

Minimal invasiveness is another safety consideration, as intranasal and oral vaccines prevent injection-site reactions and needlestick injuries, enhancing safety and acceptance. The most recent approval is for the self-administration of FluMist in the United States[70], which indicates a belief in its safety. In terms of tolerability, one of the realistic concerns is that mucosal vaccines can trigger transient nasal irritation or sneeze reflex, which, in theory, can spread the virus (in the case of live vaccines); however, experiments with LAIV demonstrated little shedding, and the secondary transmission risk is deemed insignificant[70].

The most disheartening thing is that proving efficacy against infection/transmission is more difficult than proving efficacy against disease. Numerous mucosal candidates that can generate acceptable systemic immunity have not been able to avert upper airway viral-induced infection. COVID-19 nasal vaccine trials have been forced to grapple with hypercontagious variants and high attack rates, and it is difficult to demonstrate an increment of protection over extant immunity. An example of this is the dNS1-RBD Phase 3 trial, which was carried out during the Omicron wave, and many of the participants had been vaccinated or exposed previously, which probably reduced the efficacy observed[76].

CHALLENGES IN MUCOSAL VACCINE TRANSLATION

Measuring mucosal immunity in trials

An empirical problem with the development of mucosal vaccines is the detection of mucosal immunity, which is our capacity to determine the quantity of local antibodies and T cells associated with protection. Mucosal immune assays are less developed than serum antibody titers, which can be easily determined using standardized methods. Nasal mucosa or lung sampling is challenging and fluctuates. Naresaline or swab-collected IgA and bronchoalveolar lavage (in investigations) of lung immune cells are among the common methods and can be inconsistent. The level of nasal secretions, such as nasal IgA, may also depend on recent infections and nasal secretion or even sample collection methods (saline volume, time, etc.)[88].

Interpersonal variation is also very high; some individuals inherently have higher secretions of base IgA, while others have virtually none, making comparisons difficult. Another problem is that there are no uniform reference ranges or universally acceptable correlates of protection regarding mucosal immunity. While serum neutralizing antibody titers have thresholds that predict protection for many viruses, no such benchmark yet exists for, say, “protective nasal IgA titer”. Researchers often have to correlate mucosal responses with outcomes post-hoc (for example, examining whether people with higher nasal IgA were less likely to become infected in a trial). In COVID-19 studies, this has suggested that mucosal IgA correlates with reduced infection risk[74]; however, regulatory agencies still primarily consider serum neutralization or systemic T cell markers when evaluating vaccines.

Stability and delivery obstacles

Delivering vaccines to mucosal surfaces is challenging because mucus and rapid clearance quickly remove formulations, requiring mucoadhesive systems or aerosol delivery to extend the contact time, while enzymes in mucosal secretions can degrade mRNA and protein vaccines. These hurdles drive the use of protective nanoparticles, higher doses, and interest in dry powder vaccines that offer better room-temperature stability and easier distribution. Delivery devices also affect how well the vaccine reaches the target tissues, as seen with the different intranasal and aerosol COVID-19 vaccines[68].

Safety considerations for mucosal administration

Safety is paramount in any vaccine deployment, and mucosal vaccines pose unique safety considerations that must be managed. The likelihood of unintentional impacts on the nervous system is one such issue. The upper nose has the olfactory epithelium, which is directly linked to the brain, and this is not a common issue with IM injection. As reported, an unexpected outbreak of Bell’s palsy (facial paralysis) months after vaccination occurred with an intranasal flu vaccine adjuvated with E. coli heat-labile enterotoxin in the early 2000s[87]. It was believed that the episode was associated with the potent adjuvant or perhaps the route of administration, which led to local nerve inflammation. The shadow cast over this incident is that mucosal adjuvants should be selected with care to prevent cases of neurotoxicity, and it also led to regulators asking that nasal vaccines be subject to rigorous neuro-safety testing. To avoid adjuvants such as cholera toxin or E. coli toxin subunits, modern mucosal vaccine candidates have used safer TLR agonists and nanoemulsions. To date, these agents seem to have manageable safety profiles in clinical studies. For example, neurological adverse events have not been reported in intranasal flu and COVID vaccinations with CpG DNA-based or chitosan-based adjuvants. However, one should be on guard: Participants of trials are usually observed for any neurological manifestations during a long period (6090 days) after vaccination to identify those who subsequently develop them[87].

Acceptability and administration errors also fall under safety. One advantage of mucosal vaccines is the elimination of needle-associated issues (injection site pain, local inflammation, and risk of infection from improper needle use). This can improve public acceptance and vaccination rates in the future. However, a new potential error arises: A nasal vaccine could be accidentally inhaled too deeply or swallowed, although neither is likely to be harmful (the main issue would be reduced efficacy, not classic “safety” harm). It should be trained and with clear instructions (particularly in self-administration) to take the vaccine correctly. The recent introduction of self-administered FluMist in the United States is good news; it shows that people are sure that they can safely apply a nasal spray at home[70].

Regulatory and policy hurdles

The transition of mucosal vaccines to the market will necessitate the modification of regulatory frameworks designed for traditional injectables. The efficacy endpoint must extend beyond preventing symptomatic disease to include infection and transmission prevention. Regulators may need to add efficacy measures, such as viral load, shedding, challenge studies, or mucosal immune correlates, such as IgA and tissue-resident T cells. There is also a lack of guidance on mixed mucosal-systemic schedules, updates on variants, and approval procedures for products that are used as boosters only. Important considerations for policymakers need to take include how people view and deploy it; emergency authorization might enable quick utilization in case they demonstrate excellent transmission-blocking consequences. Finally, manufacturing and quality control standards for sprays and aerosols differ from those for injectable formats, requiring scrutiny of devices, dosing consistency, and stability, as illustrated by regulatory assessments preceding the approval of self-administered FluMist[89].

FUTURE DIRECTIONS AND STRATEGIC RECOMMENDATIONS

An initial IM prime establishes a robust systemic immune memory, including circulating antibodies and T cells. A subsequent intranasal boost redirects immune responses toward the respiratory mucosa, enhancing secretory IgA production and TRM T lymphocyte formation at the viral entry site. Figure 2 schematically illustrates this approach, showing how an IM prime creates circulating immunity, which an intranasal boost then enriches at the site of infection (the respiratory mucosa).

Figure 2 Proposed prime → mucosal boost strategy illustrated along the respiratory immune axis.

Heterologous prime-boost (systemic + mucosal) strategies

One of the most promising strategies is the combination of systemic and mucosal vaccination in a prime-boost regimen. Rather than viewing IM and intranasal vaccines as competitors, this approach leverages their complementary strengths: An IM prime establishes a strong base of systemic immunity, and an IN boost focuses immune memory into the respiratory mucosa. Preclinical studies have demonstrated the value of heterologous schedules. For instance, mice primed with an mRNA COVID vaccine (IM) and later boosted intranasally with a protein vaccine plus adjuvant showed vastly improved mucosal responses compared to IM or intranasal alone[75]. The IM prime generated a pool of memory B and T cells, and the intranasal boost “rerouted” a portion of these cells to take up residence in the nasal and lung tissues, resulting in robust IgA production and lung-resident T cells. This hybrid regimen achieved sterilizing immunity in the upper airways of animals, whereas a purely IM regimen did not[75]. Early clinical data echo this: An analysis of people who received IM COVID vaccines and an intranasal booster (iNCOVACC) found that the boost could recall immune memory and possibly enhance mucosal IgA, an effect that would not be obtained from repeated IM doses alone[74].

Universal antigen design for mucosal vaccines

The pursuit of universal or broadly protective vaccines is naturally associated with mucosal delivery. If the goal is to achieve near-sterilizing immunity against a rapidly mutating virus, the vaccine must present conserved antigens that the virus cannot easily evade. Mucosal vaccines provide the added benefit of eliciting IgA and T_RM that target antigens at the site of viral entry. Several cutting-edge antigen design strategies have been explored in this context. For coronaviruses, researchers are creating pan-sarbecovirus vaccines that could neutralize not only SARS-CoV-2 variants but also related bat coronaviruses (to preempt the next SARS-like virus). One approach uses mosaic nanoparticles displaying RBD fragments from multiple coronavirus strains. Initial animal studies of such mosaics (from Caltech and others) showed a breadth of antibodies, and the next step could be intranasal delivery to induce mucosal IgA that blocks any sarbecovirus at the gate. Another candidate, already in trials, is the spike ferritin nanoparticle (SpFN) developed by the United States Army (WRAIR). SpFN presents multiple spike trimers on the ferritin scaffold. In a Phase 1 study, it elicited cross-neutralizing antibodies to multiple SARS-CoV-2 variants and even SARS-CoV-1[90].

Trials designed to assess transmission reduction

Trials must measure not only individual protection but also the impact on transmission. Emerging trial designs include household studies that track whether vaccinated people are less likely to infect their close contacts and controlled human infection models that directly test whether vaccination prevents infection or lowers the viral load after deliberate exposure. Larger approaches, such as cluster-randomized trials in schools or communities and ring-vaccination strategies during outbreaks, can be used to assess the real-world effects on spread. The designs are based on endpoints such as viral load, shedding time, and infection rates, which are not traditionally used in symptom-dependent measures. These studies will also be vital as regulators start to consider such data as a part of claims such as “reduces transmission” to demonstrate that mucosal vaccines can provide wider population health benefits and assistance in breaking chains of infection.

Manufacturing and global access considerations

Manufacturing and distribution should also be planned early. Although many platforms can be supported with the help of the available production systems, new formats, such as nasal powders or aerosolized nanoparticles, might require the utilization of specific equipment and cost-efficient delivery devices. It is particularly important to ensure that stability at room temperature can be achieved because needle-free vaccines that do not use cold chains are readily deployable in remote or low-resource regions. Combining the use of mucosal vaccines with existing immunization systems will also necessitate instructions on co-administration, storage, and methods of proper delivery. The work in global equity has already begun, with large producers in nations such as India and China ramping up the manufacturing process and global health organizations aiding in the creation of development to reach a large number of people. Developing robust regulatory channels, regional manufacturing locations, and post-licensure monitoring will further increase rapid deployment, acceptance, and coverage.

CONCLUSION

Mucosal vaccines represent a critical step in controlling respiratory viral infections by inducing immune protection directly at the site of pathogen entry. By eliciting local secretory IgA responses, TRM T lymphocytes, and rapid innate defenses, mucosal immunization strategies have the potential to block infection and transmission more effectively than conventional IM vaccines. Although scientific, regulatory, and implementation challenges remain, advances in vaccine platform design, delivery technologies, and clinical evaluation are rapidly accelerating progress in this area. As evidence continues to accumulate, mucosal vaccines are increasingly positioned not merely as complementary tools but as transformative interventions capable of shifting respiratory virus control from disease mitigation toward true infection and transmission prevention, with substantial implications for global public health.

ACKNOWLEDGEMENTS

The authors thank colleagues and collaborators for their valuable scientific discussions and support during the preparation of this manuscript.