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Last month, during a now-infamous press conference, Donald Trump speculated about the ways in which sunlight and chemical disinfectants could help protect people from the threat of Covid-19. Trump seemed to suggest that injecting disinfectants could have some utility — a comment that drew immediate scrutiny and scorn. Much less attention was paid to the president’s statement that sunlight might safeguard people from the virus. “Supposing we hit the body with a tremendous — whether it’s ultraviolet or just very powerful light,” Trump said. “Supposing you brought the light inside the body, which you can do either through the skin or in some other way.” When it comes to potential Covid-19 treatments, the president’s speculations have been numerous and frequently misguided. But the idea that sunlight could counteract Covid-19, both inside and outside the body, is not all that far-fetched. Richard Weller, MD, is a dermatologist and sunlight researcher at the University of Edinburgh in the U.K. Weller says he’s looked at Covid-19 data in the United States, and that there seems to be a correlation between states that get a lot of sun and lower rates of death. “I think there are probably several pathways by which sunlight and sun exposure may exert beneficial effects,” he says. For thousands of years, humans have recognized that the seasons play a role in the emergence and transmission of certain illnesses, including viruses. “Annual epidemics of the common cold and influenza disease hit the human population like clockwork in the winter,” write the authors of a 2020 review paper from a team at the Yale University School of Medicine. They also point out that two deadly coronaviruses — first SARS, and now Covid-19 — both emerged during the winter months. “[This indicates] that the winter environment promotes the spread of a variety of respiratory virus infections,” they write. While the seasonality of many common respiratory illnesses is well-established, it may surprise some to learn that experts haven’t nailed down the exact causes of this phenomenon. For the most part, they tend to agree that a mix of environmental factors — such as temperature and humidity — play a role in pathogen transmission. So does human behavior; people are more likely to crowd together indoors during the winter months, and stagnant indoor air can increase the likelihood of spreading germs. But that Yale review also lists sunlight as a possible explanation for the seasonality of certain pathogens. “Our best model predicts that Covid-19 risk will decrease this summer in the U.S., largely due to the increase in UV light as days become longer.” In a recent preprint,’ which is a scholarly work that has not yet undergone peer review or formal journal publication, researchers at the University of Connecticut used country-level climate and infection-rate data to estimate the likelihood that certain environmental factors — namely temperature, humidity, and ultraviolet light — will lead to lower rates of Covid-19 infection during the upcoming summer. That doesn’t mean the virus will be wiped out; it just means its harmful effects may be somewhat muted. “As we were reading the literature on other viruses — particularly SARS, the earlier coronavirus — there were indications that UV light could at least inactivate the virus on surfaces and could also either decrease the risk of getting the virus or reduce symptoms,” says Mark Urban, PhD, co-author of that preprint and director of the Center of Biological Risk at the University of Connecticut. “Our best model predicts that Covid-19 risk will decrease this summer in the U.S., largely due to the increase in UV light as days become longer.” Urban says his models include a high level of uncertainty; no one is suggesting it’s a foregone conclusion that the United States will experience relief from Covid-19 this summer. But Weller says there are a number of plausible mechanisms by which increased sunlight could counteract Covid-19. Along with inactivating viruses on surfaces, he says UV light can also kill airborne virus particles — a view supported by research from Columbia University and elsewhere. But Weller says the benefits of sunlight may extend beyond its ability to slay Covid-19 outside the body. When people are exposed to UV light, he says, this may cause changes inside the human body that both strengthen the immune system and block Covid-19 from replicating and causing severe illness. “The data are strongly suggestive that sunlight does indeed have benefits,” he says. How the human body reacts to UV light