Seasonal Effect of Sunlight on COVID-19 among Countries with and without Lock-Downs

Objective: The main aim of the study was to determine whether COVID-19 epidemiological data reported by countries in different hemispheres correlated with the seasons of the year. Since stay-at-home orders could be a main factor affecting the time individuals spent outdoors, the progression of COVID-19 in countries that mandated the most stringent lock-downs and stay-at-home orders was compared to countries in the same hemisphere that did not order their citizens to remain at home. Methods: Infections attributed to COVID-19 per million inhabitants, deaths per infections × 100, and deaths per million inhabitants from different countries were analyzed utilizing national reports registered in the Johns’ Hopkins database together with the most recent world population data. The null hypothesis (no difference between countries with and without lock-downs) was tested (two tailed test, p < 0.01) for each paired set of data according to well established statistical analysis. Results: The shift of highest infection rates from countries in the northern-towards countries in the southern-hemisphere during early 2020 and the reverse in December of the same year correlates with the seasonal variation in the flux of germicidal sunlight. Mortality rate for the same virus among different countries did not show a seasonal component. COVID-19 infection mortality rate was considerably lower in developing countries of South America (11 of the largest countries) than in several (at least 8) developed European countries. Discussion: COVID-19 resulted in higher infections during winter than in summer. The finding of a seasonal component, correlating the progression of the pandemic with local solar flux, demonstrates that infectious virus in the environment plays a role in the pandemic since direct person-to-person transmission would afford little time for solar inactivation. Similar epidemiological data amongst “locked” and “unlocked” countries demonstrates that lock-downs and similar confining measures had no effect on the chances of healthy individuals becoming infected with SARS-CoV-2 or dying of COVID-19.