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Sunshine may reduce a person's risk of dying with the coronavirus, research suggests.
Scientists from the University of Edinburgh analysed all coronavirus deaths that occurred in mainland US between January and April 2020.
These were compared against the UV levels of more than 2,000 US counties over the same four months.
Results suggest people who live in areas with the highest UVA exposure – a ray behind 95% of the sun's ultraviolet light – are less likely to die with the coronavirus than residents of less sunny counties.
Repeated experiments in England and Italy, at different latitudes to the US, produced similar results.
The findings cannot be explained by higher vitamin D exposure, according to the scientists. Many experts have called for the inexpensive supplement to be made a strategy against the pandemic.
UVA may cause the release of nitric oxide, which has been shown to reduce the coronavirus' replication in laboratory studies.
"There is still so much we don't understand about COVID-19 [the disease caused by the coronavirus], which has resulted in so many deaths worldwide," said study author Dr Richard Weller.
"These early results open up sunlight exposure as one way of potentially reducing the risk of death."
When it comes to vitamin D, one study suggested the so-called sunshine supplement may reduce Black people's risk of catching the coronavirus.
The vitamin has also been linked to lower odds of dying with the infection.
To adjust for the potential effects of vitamin D, the Edinburgh scientists only looked at counties with insufficient UVB exposure. UVB is "the primary source of vitamin D for the body".
In the UK, the NHS recommends "everyone consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter".
The Edinburgh scientists estimated the US counties' UV rays via satellite data.
The results, published in the British Journal of Dermatology, suggest coronavirus deaths in the US fell by 29% for every 100 kilojoules (KJ) per metre squared increase in daily UVA. KJ is a measure of energy.
When pooling the English and Italian findings, the fatality decline was estimated at 32%.
Watch: The link between coronavirus and vitamin D
The Edinburgh scientists took into account factors that are known to be linked with increased coronavirus exposure and death, like age, ethnicity, socioeconomic status and infection levels in the local area.
UVA is said to release nitric oxide from "stores" in the skin, "whence it is mobilised to the systemic circulation", causing blood vessels to dilate and blood pressure to drop, wrote the team.
Previous research has linked increased sunlight exposure to improved heart health and lower blood pressure.
With heart disease a known risk factor for coronavirus complications, this may also explain the results, according to the scientists.
They stressed the US study was observational, and therefore does not show cause and effect.
Nevertheless, co-author Professor Chris Dibben added: "The relationship between COVID-19 mortality, season and latitude has been quite striking, here we offer an alternative explanation for this phenomenon."
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Coronavirus levels were considerably lower during the UK's 2020 summer, spiking again as the weather turned.
Although unclear, research suggests UV rays may break down the norovirus, or winter vomiting bug, which could explain why it is less common during the summer months.
Warm weather may also encourage people to socialise outside, rather than huddling close together indoors with little ventilation. The Edinburgh scientists accounted for temperature in their study.
"It seems unlikely time spent outdoors would be sufficiently linearly associated with UVA for the same calendar period, after controlling for temperature, over such a wide range of latitude and UVA to produce the same effect on time spent outdoors (e.g. between Newcastle and Naples in March)," they wrote.
Some have pointed out, however, coronavirus cases have also spiked in tropical regions. Brazil is the world's second worst affected country, with more than 13 million confirmed cases since the outbreak was identified and over 340,000 deaths.
Skin pigmentation via melatonin can "block UV penetration", wrote the scientists.
This may partially explain why more Black people become seriously ill with the coronavirus, however, "social factors probably account for much of this".
The scientists concluded: "If the relationship identified proves to be causal, it suggests optimising sun exposure may be a possible public health intervention.
"Given the effect appears independent of a vitamin D pathway, it suggests possible new COVID-19 therapies and the importance of exploring the role of circulating NO [nitric oxide]."
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