There is also this general wisdom that Fruit because of Vitamins (in particular Vitamin C) is supposed to be good against the common cold. Numerous studies show that there is no effect. I bet this wisdom doesn't come out of nowhere, but is rather an indicator how bad the average Vitamin (C) supply is.

FWIW older people are usually more often indoors and have both higher likelihoods of becoming hospitalized because of Covid and are more often indoors, thus prone to Vitamin D insufficiency.