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

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.