Vitamin D is produced by the action of UVB on the skin, and while controversy remains over its role in COVID-19, UVA radiation might play a more important role.

Solar radiation is significantly associated with the onset of epidemics of infectious diseases such as influenza. Moreover, simulated sunlight has been shown to rapidly inactivate COVID-19 and this effect has been attributed to UVB radiation. Although UVA radiation has little virucidal activity, it does induce release of nitric oxide (NO) in the skin, leading to a reduction in systemic blood pressure and higher UVA exposure has been linked to a lower incidence of myocardial infarction. Since cardiovascular disease and hypertension have become recognised as important risk factors among those with COVID-19, a team from the School of Geosciences, Centre for Cardiovascular Sciences, University of Edinburgh, wondered if ambient UVA radiation levels were related to COVID-19 deaths. The team modelled COVID-19 deaths in counties across the USA between January and April 2020 and compared the data with deaths in England and Italy. Interestingly, the researchers restricted their analysis to areas in which levels of UV were too low to limit potential confounding from UVB-induced vitamin D production during the period of study and obtained median UVA levels from satellite data. The team modelled the effect on mortality of UVA exposure and adjusted for several sociodemographic factors and infection susceptibility variables. The main outcome of interest was the estimated mortality rate ratio (MMR) per 100 (kJ/m2 ) increase in UVA.

Findings
Daily UVA levels varied between 450–1000 kJ/m2 across the three countries and were slightly lower in England compared with both Italy and the USA. The MMR estimate was 0.71 per 100 unit increase in UVA levels for the USA, 0.81 (Italy) and 0.49 (England) with a pooled estimate of 0.68, i.e., a 32% reduction in mortality from COVID-19 per 100 unit increase in UVA exposure. Overall, the analysis suggested that the association between higher ambient UVA exposure and lower COVID-19 mortality was independent of socio-demographic factors, temperature, humidity and UV radiation within the vitamin D action spectrum.

While this was an observational study, the authors point to data showing that NO has virucidal activity against COVID-19. In addition, their data offer a possible explanation for the higher mortality among black or Asian individuals. Since darker skin attenuates the effect of UVA, it therefore reducing the release of NO and any potential beneficial effects. The authors concluded that the data suggests a possible important link between UVA levels and COVID-19 mortality that requires further study.