I think everyone I know who has checked their Vitamin D levels has needed supplementation, and everyone I know who has checked their vitamin D receptors from their genetic testing (mostly white Americans) has been missing one or two of the 4 (VDR).
A doctor I respect once told me that except for the ones who die of skin cancer, people with the most sun exposure are generally the healthiest 🤷 Is it vitamin D? Is it something else? Is the drop in sun exposure a contributing factor in the "flu season"? If you do decide to supplement, my understanding is that it should be taken with k2. (I use a liquid that has both).
If you need further encouragement: it also looks like Covid outcomes are worse if you are vitamin D deficient, and it can take weeks to get your levels up.