Sun, skin and COVID-19 mortality
Sunlight may reduce deaths from COVID-19 infection. This may sound like misinformation, but a recent paper in the British Journal of Dermatology made for interesting reading based on known principles of skin physiology which could potentially impact COVID replication and subsequent infection rates.
Writing in the British Journal of Dermatology, Cherrie et al.,  suggest that UVA radiation levels may affect COVID death rates. The concept that the two are linked is based on the premise that many infectious diseases are seasonal, often related to humidity, ambient temperature or ultraviolet radiation levels.
In earlier work, authors on this paper showed how sunlight (UV) exposure to the skin releases nitric oxide (NO)  which has a blood pressure-lowering effect reducing hypertension and the risk of myocardial infarction across a population . It is known that COVID-19 affects patients with cardiovascular disease and metabolic syndrome more severely so any UV-related effect in reducing the viral transmission would be beneficial. The authors cite work suggesting that UV light levels can potentially affect the growth rates of COVID-19 and its viability in droplets and fomites potentially reducing the spread of the disease, viral load and consequently, disease severity. Interestingly, the antiviral effect of UV light on COVID transmission is from UVB radiation, with little response shown to UVA exposure.
However as explained earlier, UVA has an alternative action. When the skin is exposed to UVA light, nitric oxide stored in the skin is liberated. Its release has been shown to inhibit COVID-19. The coronavirus is known to enter human cells, causing infection, by the use of its “spike” protein on its surface which is able to fuse with the host membrane and enter the cell. Released nitric oxide binds to this spike protein (a process called nitrosation) and has a double effect – preventing the virus from binding to the host cell membrane and by inhibiting its replication along with other cytopathic effects.
Consequently, the researchers undertook an ecological study looking at death rates and UVA levels. To do this they selected local areas across the USA, along with selected regions of the UK and Italian and derived the mean UVA levels for each of the areas for the period January to April 2020. Adjusting for a range of variables they were able to compare UVA levels with COVID-19 deaths.
Ultimately, the results demonstrated that higher ambient UVA exposure was associated with lower COVID-19 specific mortality. As the authors state this is only an observational study, so is only the start of a potential research trail and it is possible that the observed effect is due to something entirely different  but nevertheless this timely work is an important step forward in our understanding of the potential interaction of virus and the environment.
1. Cherrie M, Clemens T, Colandrea C, Feng Z, Webb DJ, Weller RB, Dibben C: Ultraviolet A radiation and COVID‐19 deaths in the USA with replication studies in England and Italy*. Br J Dermatol 2021, 185(2):363-370.
2. Liu D, Fernandez BO, Hamilton A, Lang NN, Gallagher JMC, Newby DE, Feelisch M, Weller RB: UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. J Invest Dermatol 2014, 134(7):1839-1846.
3. Weller RB, Wang Y, He J, Maddux FW, Usvyat L, Zhang H, Feelisch M, Kotanko P: Does Incident Solar Ultraviolet Radiation Lower Blood Pressure? Journal of the American Heart Association 2020, 9(5):e013837.
4. McKenzie RL, Liley JB: Yet another benefit from sunlight in the fight against COVID-19? Br J Dermatol 2021, 185(2):246-247