Rethinking COVID-19 seasonality: A summer respiratory virus in the tropics, contrast to influenza

Abstract

This opinion challenges the conventional view that coronavirus disease 2019 behaves as a uniformly winter-dominant respiratory infection. Analysis of multi-year surveillance data across hemispheres reveals that severe acute respiratory syndrome coronavirus-2 exhibits seasonal divergence, with consistent summer surges in tropical regions, such as India, and winter peaks in temperate climates. We propose that this pattern arises primarily from human (host) behavioural responses to multi-animal tropism to climatic (environment) extremes, which recreate high-risk indoor transmission settings under both heat and cold. Unlike influenza, severe acute respiratory syndrome coronavirus-2 (agent) combines thermal resilience, broad tissue tropism, and efficient pre-symptomatic transmission, allowing persistence beyond classical winter bounds. Recognizing coronavirus disease 2019 as a behaviourally modulated (through agent-host-environment triad) seasonal virus may help tailor regional surveillance, ventilation, and vaccination strategies in an era of accelerating climatic change.

Keywords: Seasonality; SARS-CoV-2 transmission; Behavioral epidemiology; Tropical climate; Respiratory virus ecology

Core Tip: Coronavirus disease 2019 seasonality is not uniformly winter-driven but shaped by behavioural adaptation to climate and host. Severe acute respiratory syndrome coronavirus-2 exhibits a seasonal pattern, being predominantly summer-predominant in tropical regions and winter-predominant in temperate ones, due to its virological resilience and broad host adaptability. Recognizing this behaviourally modulated seasonality is essential for designing climate- and region-specific surveillance, vaccination timing, and ventilation strategies.