There is not a baseline for vitamin D only a range of 20 ng/ml to 50 ng/ml for "normal". There are no studies to determine "optimal" levels of vitamin D and the amount needed as a supplement varies from one person to the next. How much time one spends outdoors and ones age makes a great deal of difference. Best to get the blood serum level tested so you know what it is and whether to take a supplement.
The RDA numbers are worthless and only designed to prevent ricketts in children and not to insure a healthy immune system for adults. If I took only the RDA my blood serum level would be at the bottom of the "normal" range and I need to take 7 times the RDA to get my blood serum level of vitamin D inside the "normal" range.
The medical community hopes that annual booster shots will be effective for Covid but there is also a growing awareness that as the mutations evolve that vaccines may become less and less effective. Prudent course is to minimize exposure in the first place.
The risks of getting Covid include needing to be hospitalized and then acquiring a nosocomial infection. Nosocomial infections kill thousands each year and the patient has no control over the procedures or lack thereof at a hospital.
One medication that has been very effective, particularly with patients 75 years of age or older is pavlovid. Few doctors are bothering to proscribe it for their patients which is most unfortunate as it has been shown to reduce hospitalizations by 90% and reduce rebound Covide to the same degree.