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

Abstract

Nearly every country is combating the 2019 novel coronavirus (COVID-19). A central concern is whether COVID-19 transmission exhibits seasonality. If so, changing environmental conditions in coming months may shift COVID-19 infection patterns around the world. We estimate a relationship between growth of confirmed cases of COVID-19 and local environmental conditions by combining the most spatially-disaggregated global dataset of daily cases assembled to date, consisting of 3,235 administrative units across 173 countries, with a statistical model isolating random variation in daily weather conditions. Our analysis indicates a strong effect of ultraviolet radiation (UV) on daily COVID-19 growth rates: a 1 kJ m−2 increase in hourly UV decreases the growth rate of confirmed COVID-19 cases by .09 percentage points (±.04, p =.01), with a delayed effect that manifests over two weeks. We find weak or inconsistent lagged effects of local temperature, specific humidity, and precipitation. To illustrate the seasonal implications of our findings with respect to UV, we show that estimated UV effects imply decreases in COVID-19 growth rates of 1.18 percentage points (±0.47) in the extra-tropical Northern Hemisphere and increases of 2.1 percentage points (±0.83) in the extra-tropical Southern Hemisphere between April and July 2020. Seasonality in UV dictates the inverse pattern for January 2021, with COVID-19 growth rates rising by 5.5 percentage points (±2.18) in the extratropical Northern Hemisphere and falling by 4.82 percentage points (±1.91) in the extra-tropical Southern Hemisphere, relative to April 2020. These effects are substantial when compared to the average in-sample COVID-19 growth rate of 13.21 percent. The total seasonal effect of all climate variables investigated is indeterminate in sign due to uncertainty in the effects of temperature and specific humidity. Although many factors will influence future COVID transmission, our findings suggest a need for adjustment of COVID-19 containment policies for the seasonality of UV.