How Sunlight, the Immune System, and Covid-19 Interact

For thousands of years, humans have recognized that the sun plays a role in the emergence and transmission of viruses

Illustration: Shuhua Xiong

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

The sun’s ultraviolet rays damage the cells of the skin in ways that promote wrinkles, blemishes, and other signs of aging. UV damage also raises a person’s risk for skin cancer, which is the most common form of cancer in the United States. These dangers are well established, and so virtually all public health messaging advises people to apply sunscreen, don protective clothing, and take other measures to shield skin from the sun’s rays.

But some doctors who have studied the interaction between sunlight and human health say that “avoid the sun” recommendations are too strident, and that the benefits of moderate sun exposure without sunscreen may counterbalance — or even outweigh — the risks.

“Making people phobic about being outdoors in the sun is just so counter to our evolutionary basis — it just doesn’t make sense,” says James O’Keefe, MD, a cardiologist at Saint Luke’s Mid America Heart Institute in Kansas City who has studied the interactions between sunlight and human health. He says human beings evolved to live outdoors — “We’re not moles,” he says — and that our absence of hair or fur suggests that our skin is meant to encounter some direct sun exposure. “I think there are a lot of potential mechanisms by which sunlight could benefit health.”

Asked for specifics, O’Keefe says that sun-exposed skin releases large amounts of nitric oxide into the bloodstream. “Nitric oxide keeps vessels soft and supple, and it gives them a Teflon-like surface so that platelets don’t stick,” he says. “The vessels naturally produce a lot of nitric oxide when you’re healthy, and especially when you’re young.” He points out that deaths due to cardiovascular disease — the most common cause of death in the United States — tend to peak in winter both in the United States and in Europe, and that the absence of sun and its attendant nitric oxide boost may be a contributing factor.

Weller, the University of Edinburgh dermatologist, has studied the relationship between sunlight and nitric oxide, as well as the effects of both on human health. He says that sun-triggered elevations in nitric oxide could help protect people from Covid-19, and his belief is based in part on a 15-year-old Swedish study that examined another deadly coronavirus: SARS.

The first SARS outbreak occurred in 2002 in the Guangdong province of China. Like its close cousin Covid-19, SARS is a respiratory illness. The Swedish group showed that, in lab models, nitric oxide prevents the SARS virus from reproducing. “Covid-19 gets into the body by binding to the same receptor as the SARS virus,” Weller says. “And this [Swedish] group found that nitric oxide stops SARS from doing damage because it stops it from binding to this receptor.”

If this lab work turns out to be accurate and applicable to Covid-19 — both big ifs — this could be one way in which sunlight defends the body against Covid-19.

Another possibility, Weller and others say, has to do with “the sunshine vitamin.”

Vitamin D and Covid-19

Vitamin D is an essential nutrient, which means the human body needs it but can’t make it. While some foods contain vitamin D, people have traditionally gotten most of their vitamin D from the sun: When exposed to ultraviolet light, a chemical reaction takes place in the skin that results in the production of vitamin D.

For a just-published study in the journal Aging Clinical and Experimental Research, researchers examined the average vitamin D levels among residents of different European countries. They found a correlation between low vitamin D levels and higher rates of Covid-19 infections and — even more so — Covid-19 deaths.

“Previous studies have shown that vitamin D protected against acute respiratory tract infection overall, and older adults — the group most deficient in vitamin D — are also the ones most seriously affected by Covid-19,” says Petre Cristian Ilie, PhD, co-author of the study and a research director at the Queen Elizabeth Hospital in the U.K. “Our finding was that getting vitamin D levels into the normal range might help.”

Ilie says there are several mechanisms by which vitamin D could counteract Covid-19. First, vitamin D enhances the expression of an enzyme called angiotensin-converting enzyme 2, or ACE2. “Previous studies identified associations between higher levels of ACE2 and better coronavirus disease health outcomes,” Ilie says, adding that, in the lungs, ACE2 has demonstrated the ability to protect against acute lung injury.

Ilie also says vitamin D has “multiple roles” in the immune system that may strengthen its ability to repel Covid-19. One example: Low levels of vitamin D seem to impair the development of macrophages — white blood cells that eat invading pathogens, including viruses. He says vitamin D also helps prevent inflammation from running amok. Furthermore, there’s evidence that the low vitamin D levels are associated with immune system–related dysfunction and disease.

Although his study did not look specifically at sun exposure, Ilie says sunlight is a natural source of vitamin D. His analysis partly relied on a 2019 study from the European Journal of Endocrinology finding, somewhat counterintuitively, that older adults living in Portugal tend to have much higher vitamin D levels than the same demographic in neighboring Spain, and that older adults in the Nordic countries tend to have higher levels than those living in Italy and other sunnier Southern European countries. While both Spain and Italy have been hit hard by Covid-19, Portugal and the Nordic countries have thus far experienced relatively light rates of death and infection.

There are far more questions than answers when it comes to the relationships between sunlight and Covid-19. But the evidence to date hints that getting some sun may help protect people from the virus.

What explains the vitamin D discrepancies among these countries? The authors of that 2019 study point out that diet, behavior, clothing choices, and skin color all affect vitamin D status. The darker a person’s skin, the more sun they require to make vitamin D. If relatively dark-skinned Europeans in countries such as Spain and Italy avoid the sun, slather on sunscreen, and wear clothing that covers much of their body, this could partly explain why their vitamin D levels may be lower than those of the Portuguese. That study also points out that the predominantly light-skinned residents of Nordic countries need relatively little sun to produce vitamin D. They also tend to eat diets rich in cod liver oil and other seafood sources of vitamin D, and many foods in those countries are fortified with vitamin D — all of which could explain that population’s enviable blood levels of the vitamin.

Also noteworthy, though highly speculative: African Americans tend to have much lower levels of vitamin D than white Americans. Some researchers have posited that, in addition to long-standing racial and socioeconomic factors, these vitamin D discrepancies could partly explain why black Americans are at greater risk than whites for heart disease, certain forms of cancer, and other diseases linked to vitamin D deficiencies. Black Americans have also suffered disproportionately from Covid-19.

“Right now, we know that 17% of African Americans have levels of vitamin D below 10 ng/ml, which virtually everyone agrees is seriously deficient,” says Walter Willett, MD, PhD, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Medicine.

Willett has studied vitamin D and human health extensively. He says it’s possible, though far from proven, that vitamin D shortfalls may help partially explain the Covid-19 imbalances that are showing up between white and black Americans. He says it’s also possible that a vitamin D supplement may provide some protection against Covid-19 for those Americans who are deficient — which may be most of the population. While there isn’t broad expert consensus on what constitutes “normal” or “low” when it comes to vitamin D in the human body, a 2018 study concluded that up to 40% of U.S. adults may be deficient in the vitamin.

What should people do?

There are few risks associated with moderate doses of a vitamin D supplement. Willett says that taking one could, potentially, offer some protection from Covid-19. He recommends a daily supplement containing between 1,000 and 2,000 IU and says that Americans with dark skin may require more. He says that taking up to 4,000 IU is still considered safe.

But it’s not clear that getting vitamin D from a pill is as effective as sunlight at raising levels of vitamin D in the body. Ilie says there may be “limited absorption” from supplements, and some past work on the health benefits of vitamin D pills has been mixed. (A group in Australia is in the midst of a large trial that is comparing the effects of sunlight to the effects of a vitamin D supplement. But those results aren’t yet available.)

Even if a supplement could effectively raise vitamin D levels, it would not provide the nitric oxide boost associated with sun exposure — the one that may stop the spread of Covid-19 in the body. While sun exposure comes with risks, the benefits of spending time in the sun could prove to be a multipronged weapon against Covid-19.

Some dermatologists who have examined the benefits and risks of sunlight say, controversially, that most serious skin cancer concerns arise when people experience blistering or peeling sunburns. “I think the benefits of [non-burning] sun exposure may outweigh the skin cancer risks,” says Matthew Zirwas, MD, an Ohio-based dermatologist who has published research on UV light and skin disease. Especially in the midst of Covid-19, he says getting some sun — not enough to burn, but enough to tan — makes a lot of sense.

“I wish I could say how much sun is enough, but I don’t know,” says the University of Edinburgh’s Weller. “I know from a study we’ve just done that the equivalent of 20 minutes of summer sunshine in London or Edinburgh is enough to have an effect on blood pressure,” which suggests a boost in nitric oxide levels. But, he adds, the amount of sun a person needs to increase vitamin D and nitric oxide will depend on that person’s skin color, the amount of exposed skin, the time of day, the time of year, and many other factors.

There are far more questions than answers when it comes to the relationships between sunlight and Covid-19. But the evidence to date hints that getting some sun may help protect people from the virus.

I write about health and science. I live in Detroit with my wife and kids. I’m trying to learn German, but my progress so far is nicht gut.

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