D3 might be a proxy for age, as I thought older folks were to take D and Calcium to reduce the chance of osteoporosis. It might also be a proxy for overall health, as those short of D3 are probably not eating "well-balanced diets, ..." that marketeers are fond of talking about. It's also possible that it has a direct effect on the result, which would (as mentioned already) make it an excellent complement of vaccination. One reduces the chance of the ultimate problem, one reduces the chance of duration/problems. And it's cheap.