On the Environmental Determinants of COVID-19 Seasonality

Viral respiratory diseases (VRDs), such as influenza and COVID-19, are thought to spread faster over winter than during summer. It has been previously argued that cold and dry conditions were more conducive to the transmission of VRD than warm and humid climates, although this relationship appears restricted to temperate regions, and the causal relationship is not well understood. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing COVID-19 has emerged as a serious global public health problem after the first COVID-19 reports in Wuhan, China, in late 2019. It is still unclear whether this novel respiratory disease will ultimately prove to be a seasonal endemic disease. Here, we suggest that Air Drying Capacity (ADC; an atmospheric state-variable known to control the fate/evolution of the virus-laden droplets) and ultraviolet radiation (UV) are probable environmental determinants in shaping the transmission of COVID-19 at the seasonal time scale. These variables, unlike temperature and humidity, provide a physically-based framework consistent with the apparent seasonal variability in COVID-19 prevalence across a broad range of climates (e.g., Germany and India). Since this disease is known to be influenced by the compounding effect of social, biological, and environmental determinants, this study does not claim that these environmental determinants exclusively shape the seasonality of COVID-19. However, we argue that ADC and UV play a significant role in COVID-19 dynamics at the seasonal scale. These findings could help guide the development of a sound adaptation strategy against the pandemic over the coming seasons. Plain Language SummaryThere is growing scientific interest in the potential seasonality of COVID-19 and its links to climate variables. This study aims to determine whether four environmental variables, namely temperature, humidity, Air Drying Capacity (ADC), and ultraviolet radiation (UV), are probable environmental determinants for the observed seasonal dynamics of COVID-19 prevalence, based on extensive country-level data spanning the first year of the pandemic. Although the influence of socio-economic factors may be dominant, we here suggest that ADC and UV are key environmental determinants of COVID-19 and can potentially affect the transmission and seasonality of the disease across a wide range of climates. Key PointsO_LIThe seasonality of COVID-19 appears to follow seasonality of some environmental variables. C_LIO_LISeasonality of air drying capacity and ultraviolet radiation consistently match seasonality of COVID-19, across climatic zones. C_LIO_LISeasonality of air humidity and temperature, match seasonality of COVID-19 in temperate climates, but not in tropical monsoon climates. C_LI