> The "correlation is not causation!!" bromide, while useful to understand in context, is overused.
Maybe then we should take a look at an actual double blind randomized controlled trial. Preferably one with thousands of participants. Luckily, exactly such a trial just ended. It lasted 5 years, it started in 2016 and ended in 2021. It considered acute respiratory infections; Covid was not one at the beginning of the trial, but was by the end of it.
Here's the conclusion:
Monthly bolus doses of 60 000 IU of vitamin D did not reduce the overall risk of acute respiratory tract infection, but could slightly reduce the duration of symptoms in the general population. These findings suggest that routine vitamin D supplementation of a population that is largely vitamin D replete is unlikely to have a clinically relevant effect on acute respiratory tract infection.