This is unrelated to my parent comment: perhaps I wasn't clear that being outside (in the sun) has a larger impact on your Vitamin D levels than supplementation

This is unrelated to my parent comment: perhaps I wasn't clear that being outside (in the sun) has a larger impact on your Vitamin D levels than supplementation. Therefore the study's conclusion is invalid if the two groups studied were measured one before and one after lockdown kept them inside - since we would expect Vitamin D levels to drop, for this reason.