It is important to take Vit. D in advance - studies have shown that is more effective than taking a huge amount when the illness strikes. In fact tune up immunity with supplements of all vitamins you might need is good too(my opinion).
In addition, people who are overweight, need to take ab extra dose of Vit. D and then taper to maintenance.
When you get sick, then boost it up again. Dr. Been explains:
https://www.youtube.com/watch?v=mg2eBhGB3T8
link to drs. that will prescribe!
https://covid19criticalcare.com/ivermectin-in-covid-19/how-to-get-ivermectin/