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As Covid-19 swept the world, so did misinformation about how to treat it. But sometimes misinformation can develop even around ideas that have some truth to them - and that can be the most difficult kind to tackle.

Why vitamin D?

There are many treatments that have been suggested for Covid-19.

Hydroxychloroquine, Ivermectin and vitamin D - all are, or were, being studied. Suggesting that a treatment could be effective and then finding it isn't upon further research is all part of the normal scientific process.

But online, early or low-quality research can be shared out of context. And the confusion this creates can be exploited by people promoting conspiracy theories.

There is some logic behind why vitamin D might be useful in treating or preventing Covid.

It plays a role in immunity and it's already recommended that everyone in the UK take the supplement in the winter, with those at higher risk of vitamin D deficiency advised to take it all year round. So far, no research has shown a convincing enough effect to support higher doses to prevent or treat illness - although this doesn't mean that won't change in the future.
 
Should I start taking vitamin D?

So it's understandable it was taken up as advice by many online. Yet some have gone much further, suggesting on Reddit forums that governments are "barely mentioning" vitamin D's effectiveness, instead of focusing on "vaccines and police state tracking"; or alleging the vitamin is being ignored because the World Health Organization is in the "pay of Big Pharma".

But governments have taken up other cheap, effective treatments like dexamethasone, once proven. And vitamins themselves are a multi-million-pound industry.

What do studies say?

Many studies have shown an association between vitamin D and Covid outcomes, but the evidence is largely observational - meaning it looks at what happens to people with higher and lower levels of vitamin D without controlling for other factors.

This isn't gold standard evidence - that would a need randomised control trial, where people are allocated a treatment or a dummy version, so scientists can be clear an outcome is caused by the treatment.

Observational studies do show certain groups are both more likely to have vitamin D deficiencies and to catch Covid - older people, people with obesity, people with darker skin (including black and South Asian people).
 
It may be that a deficiency is the reason these groups are at higher risk, or there may be other health and environmental factors driving both a fall in levels of vitamin D and greater susceptibility to the virus.