Could someone look at whether the underlying explanation for this could be SIRT-1 pathway, which involves zinc, nicotinic acid/niacin/B3 and D
Here is a list of studies* I collated which seem to support that possibility and point to effective low-cost actions.
BMI does have an impact on D3 requirement. More sunlight exposure is required to get adequate B3 serum levels in older people, and of course this exposure is down due to skin cancer advice, lockdowns and urbanisation.
Re Diabetes: it appears that covid infection may profoundly impact ketone levels in diabetes, so some endocrinologists are advising people to monitor ketone levels more than usual, and this might be part of what's going on.
*NB please ignore all the studies and reviews which focus on osteoporosis, which is a rather different issue - here we're looking at prophylaxis of respiratory viruses