I'd like to bring home a case for vitamin D as a probable prophylactic for seasonal respiratory viruses like influenza or coronaviruses. Medical authorities already advise supplementing vitamin D in the winter but they haven't emphasized why it especially matters now. This is a no risk, low cost, low effort intervention so our standard of evidence doesn't need to be all that high here. We only need to have good cause to suspect that it helps, even if only just a little.

There quite a few studies showing that vitamin D deficiency is strongly associated with severity of infection. Here is a nice one. It found "VitD deficiency was associated with a 6-fold higher hazard of severe course of disease and a ~15-fold higher risk of death." in its decently sized cohort of 185 patients.

These are very large numbers. They really are not the sort you expect to see from a parameter that is merely correlated with the true causes and not itself causative. So perhaps there is something to this? A meta-analysis of studies relevant to severity paints a similar, but less stark, picture.

Influenza and coronaviruses are both enveloped respiratory viruses and both are prevalent in fall and winter, precisely when vitamin D levels are low and lowering. In tropical countries, the rainy season brings clouds and drives people indoors. It also brings their flu season, but its not as stark a difference as it is for temperate climes. As a spot check, let's look at the most populous nation with a rainy season. India's season generally ends in September which is when they peaked at ~100k cases per day. This is the month vitamin D levels would be expected to start to increase. It is down to ~25k now.

India isn't alone. Broadly speaking, COVID-19 appears so far to have seasonality that syncs up closely with that of influenza. We have been seeing a large increase in cases as we enter the dead of winter across Europe and USA. But perhaps this is due to the cold outside and dry conditions inside. Our protective mucus dries up when humidity is below ~40%, making us more vulnerable to infection. As temperatures lower, the virus's outer envelope solidifies like butter. This obstructs ozone and other reactive chemicals from penetrating inside and damaging its vulnerable interior. However, these two commonly cited factors obviously can't explain why it has long been observed that influenza pr...

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