Seasonality and uncertainty in global COVID-19 growth rates

Abstract: Seasonality and uncertainty in global COVID-19 growth rates Cory Merowa,b,c,1 and Mark C. Urbanb,c a Eversource Energy Center, University of Connecticut, Storrs, CT 06268; bCenter of Biological Risk, University of Connecticut, Storrs, CT 06268; and cDepartment of Ecology & Evolutionary Biology, University of Connecticut, Storrs, CT 06268 Edited by Nils Chr. Stenseth, University of Oslo, Oslo, Norway, and approved September 16, 2020 (received for review May 1, 2020) The virus causing COVID-19 has spread rapidly worldwide and threatens millions of lives. It remains unknown, as of April 2020, whether summer weather will reduce its spread, thereby alleviating strains on hospitals and providing time for vaccine development. Early insights from laboratory studies and research on related viruses predicted that COVID-19 would decline with higher temperatures, humidity, and ultraviolet (UV) light. Using current, fine-scaled weather data and global reports of infections, we develop a model that explains 36% of the variation in maximum COVID-19 growth rates based on weather and demography (17%) and country-specific effects (19%). UV light is most strongly associated with lower COVID-19 growth. Projections suggest that, without intervention, COVID-19 will decrease temporarily during summer, rebound by autumn, and peak next winter. Validation based on data from May and June 2020 confirms the generality of the climate signal detected. However, uncertainty remains high, and the probability of weekly doubling rates remains >20% throughout summer in the absence of social interventions. Consequently, aggressive interventions will likely be needed despite seasonal trends. SARS-CoV-2 | climate | pandemic | UV light | humidity C OVID-19 is causing widespread morbidity and mortality globally (1, 2). The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) responsible for this disease infected more than 17 million people by August 2020 (3). Much of the world has implemented nonpharmaceutical interventions, including preventing large gatherings, voluntary or enforced social distancing, and contact tracing and quarantining, in order to prevent infections from overwhelming health care systems and exacerbating mortality rates (2, 4). However, these interventions risk substantial economic damage, and thus decision makers are currently developing or implementing plans for lifting these restrictions. Consequently, improved forecasts of COVID-19 risks are needed to inform decisions that weigh the risks to both human health and economy (2). One of the greatest uncertainties for projecting future COVID19 risk is how weather will affect its future transmission dynamics. SARS-CoV-2 might be particularly sensitive to weather, because preliminary laboratory trials suggest that it survives longer outside the human body than other viruses (5). Rising temperatures and humidity in the Northern Hemisphere summer could reduce SARS-CoV-2 transmission rates (6–8), providing a temporary reprieve. Simultaneously, the Southern Hemisphere has entered winter, and we do not know whether winter weather will increase COVID-19 risks, especially in countries with reduced health care capacity. Early analyses of COVID-19 cases predicted that high temperatures would reduce summer transmission (9–11). These predictions have been widely reported and are informing decisions about..