Vitamin D deficiency has been proposed as a risk factor for Covid-19 almost since the pandemic's start. Since vitamin D supplements are cheap and widely available, and sunlight is free, it would be fabulous news if this was true. However, despite many suggestive pieces of evidence, confirmation has been hard to find. A new study offers probably the best evidence yet that people with vitamin D deficiency are more likely to become infected, although it's still not as conclusive as scientists would like.
The pandemic has hit African Americans disproportionately hard, both in terms of Covid-19 infections and deaths. The same applies to Black and Asian citizens of the UK. Poverty and employment that can't be done from home is almost certainly a factor, but these patterns and the timing of infections at high latitudes might also be explained if vitamin D deficiency is a risk. Some theories about how SARS-CoV-2 operates within the body add credibility to the idea.
To test the hypothesis, Professor David Meltzer of the University of Chicago obtained the records of patients who had been tested for Covid-19 and had their vitamin D levels measured during the previous year at the University of Chicago Medicine. In JAMA Open Network Meltzer reports 21.6 percent of those who were deficient tested positive for Covid, but this was 12.2 percent among those whose vitamin D levels had been in the healthy range. Even with a sample size of just 489 patients, the difference was large enough to be statistically significant.
Since the tests were taken at different times, participants' vitamin levels may have changed by the point of exposure. It's also possible vitamin D deficiency is a proxy for underlying problems that cause increased vulnerability, so the study can't confirm the link on its own.
On the other hand, our chances of getting anything more definitive may be low. It's very hard to ethically run a randomized controlled trial, the gold standard for medical testing, in a case like this. To do so would mean not treating some people for a deficiency that, Covid-19 aside, is associated with a wide array of negative effects.
Trials are underway for vitamin D supplementation for people who have already caught Covid-19, but have yet to report findings.
Even before Meltzer's work, many health experts have recommended taking vitamin D for Covid protection, either through supplements or by spending more time in the Sun. A review in The Lancet quoted Rose Anne Terry of Trinity College Dublin saying: “We don't have randomized controlled trial evidence, but how long do you want to wait in the context of such a crisis? We know vitamin D is important for musculoskeletal function, so people should be taking it anyway”.
Accepting a connection is probable might force a rethink about lockdown rules, at least for apartment dwellers without access to direct Sun.
Nevertheless, experts warn even optimum vitamin levels are no substitute for avoiding exposure to the virus through masks or physical distance.