Ultraviolet radiation decreases COVID-19 growth rates: Global causal estimates and seasonal implications

With nearly every country now combating the 2019 novel coronavirus (COVID-19), there is a growing need to understand how local environmental conditions may modify transmission. To date, quantifying seasonality of the disease has been limited by scarce data and the difficulty of isolating climatological variables from other drivers of transmission in observational studies. We combine a spatially-resolved dataset of confirmed COVID-19 cases, composed of 3,235 regions across 173 countries, with local environmental conditions and a statistical approach developed to quantify causal effects of environmental conditions in observational data settings. We find that ultraviolet (UV) radiation has a statistically significant effect on daily COVID-19 growth rates: a standard deviation increase in UV lowers the daily growth rate of COVID-19 cases by approximately 1 percentage point over the subsequent 2.5 weeks, relative to an average in-sample growth rate of 13.2 percent. The time pattern of lagged effects peaks 9-11 days after UV exposure, consistent with the combined timescale of incubation, testing, and reporting. Cumulative effects of temperature and humidity are not statistically significant. Simulations illustrate how seasonal changes in UV have influenced regional patterns of COVID-19 growth rates from January to June. However, total COVID-19 seasonality had indeterminate sign for most regions during this period due to uncertain effects of other environmental variables. Thus, while our findings indicate UV exposure influences COVID-19 cases, a comprehensive understanding of seasonality awaits further analysis.