While scientists race to develop effective antiviral drugs and vaccines against Covid-19, the potential role of sunlight in fighting the deadly virus has begun making headlines.
They follow research that suggested the ultraviolet radiation in sunlight can boost the disease-fighting immune system, and may even sterilise the air we breathe.
What connects Covid-19 infection and sunlight exposure?
Most attention is focused on the role of vitamin D, which is produced by the skin when exposed to the ultraviolet radiation in sunlight.
While often linked to bone health, vitamin D is also known to work with cells in the immune system that combat respiratory viruses.
This has led to suggestions that people low in vitamin D may be at higher risk of contracting Covid-19.
Does this explain why countries like the UAE have less Covid-19?
It seems obvious that people in sunnier countries will be at less risk of vitamin D deficiency, and thus better protected against the virus.
But as so often with human health, it’s not that simple.
Low levels of vitamin D are a global health problem, with sunny countries such as Italy and Spain having lower vitamin D levels than average.
The UAE is a case in point. A study published in 2017 estimated that 90 per cent of the population may be deficient.
One reason may be the sheer intensity of the sunlight in hot countries, driving people to seek shelter.
Who is at most risk from vitamin D deficiency?
Surveys suggest low vitamin D levels are most common among the elderly and also people from Black, Asian and minority ethnic communities, whose higher levels of the skin pigment melanin cuts vitamin D production.
The fact that both these groups are also at substantially higher risk of serious illness from Covid-19 may not be a coincidence.
But once again, the link is likely to be complex and involve more than just lower vitamin D levels, such as higher prevalence of hypertension and other Covid-19 risk factors.
How can you boost your vitamin D levels?
While the best source is exposing the skin to sunlight, supplements can help.
Guidelines vary. In 2018 an international team of researchers gave detailed recommendations for the UAE, ranging from 1,000 to 2,000 international units (IU) daily for adults, to 2,000 IU/day for over-65s and the obese (BMI over 30).
The recommended levels in the UK are, paradoxically, just 400 IU/day. Eating more of certain food groups can also help, including oily fish, margarine and mushrooms (especially if exposed to sunlight).
How effective are adequate vitamin D levels against Covid-19?
Recent studies claim to show that low vitamin D levels can have dramatic effects on the risk of contracting Covid-19 and succumbing to it.
The flipside is that adequate vitamin D can bring dramatic benefits.
In one study of patients in three southern Asian hospitals, 29 per cent of patients with inadequate vitamin D became critically ill, compared to just 4 per cent of those with normal levels – a seven-fold improvement.
But these findings have yet to be peer-reviewed, and involve relatively small numbers of patients.
A 2017 review of the outcome of rigorous clinical trials found vitamin D supplements produce only 20 per cent greater protection from respiratory infections.
Studies specifically focused on Covid-19 are under way in the UK, Spain and France and have yet to report.
Can sunlight sterilise contaminated surfaces?
Ultraviolet light has long been used for decontamination of surfaces, as the radiation tears apart the genetic material needed for pathogens to replicate.
As sunlight contains UV light, it therefore seems likely to kill the Covid-19 coronavirus.
The idea gained traction last month when the US Department of Homeland Security announced that sunlight has a “powerful effect” on the virus on both surfaces and the air.
Yet independent experts pointed out that sunlight mainly contains UVA – a relatively feeble form of the radiation already shown to be useless against the closely-related Sars virus.
In contrast, sunlight has no UVC, the high-energy form used in decontamination.
However, a DHS official hinted the results – yet to be published in a scientific journal – show that the heat of the sunlight and humidity also play a role, with the virus succumbing faster at higher temperatures and humidity.
Will the spread of Covid-19 be affected by summer weather?
Despite the scepticism about the effect of sunlight, researchers have found geographical support for the idea.
Researchers at the University of Maryland have pointed out that most outbreaks of Covid-19 have occurred within a band of latitude from 30 to 50 degrees north of the equator with similar temperature and humidity levels.
At the same time, outbreaks in southern latitudes, notably in New Zealand and Australia, have been relatively mild.
With summer in the UAE being hot and humid, the claim that such conditions help to kill the virus sounds like good news.
But the need to maintain hygiene measures and social distancing remain crucial to preventing a resurgence of the virus.
Robert Matthews is visiting professor of science at Aston University, Birmingham, UK