While I strongly support maintaining a good serum level of Vitamin D through supplements, because it has several actions against viruses, I would still recommend playing it safe by getting a flu shot.
Importantly, this year, the flu season in the southern hemisphere was correctly predicted, and of the (typically) three strains in the vaccine, one was accurate for the dominant type of flu, but a lot of people still got sick after vaccination.
However, flu is very, very tricky, in that it has a unusually large number of “flexible” genes, allowing it to mutate at great speed, to either a weaker or stronger strain. This means that vaccines are dependent on what I would call the “cousins approach”.
Say a vaccine is prepared for a given strain. It will likely also work well on mutated strains descended from it, as well as “first cousin” strains, for which it will provide partial protection. But when you get to second and third cousin strains, its protection becomes marginal or ineffective at best.
And the immune system works much the same way. The closer a strain is to one the immune system has previously experienced, the faster and more measured the response. A novel strain, however, might be all over the body before the immune system even notices it.
And *then*, the immune system might overreact, with what is called the “cytokine storm”, which can be more deadly than the flu itself. This happened most recently in Ukraine, and doctors were horrified that it made the lungs of the deceased look “burned”.
Vitamin D helps a lot, because it opens “immune pathways” to fight infections, a breakdown product of it erodes the viral coat in the bloodstream, and it also acts as an ACE inhibitor, which moderates the immune system and prevents it from overreacting.
However, it can only do so much. So think of it together with vaccine as a “shield and sword”. Vitamin D offers some protection like a shield, but you need the sword of the vaccine stimulating your immune response to that particular virus, to have a much better defense.
You are much more likely to get sick from a flu shot for one reason or another than from Sunshine/D-3. I would venture to guess that if you do both shots and D you will be sick more likely than if you do the D alone. The government says each year that the shots are only 20% or 60% or some % effective and never 100%. I admit my sample is small, perhaps a couple of dozen people I have successfully proselytized and a few more I know who were already doing it. Over the time that all of them have been using the supplements there has been zero flu among them. I would say zero colds but some are subject to seasonal allergies and sometimes one can’t tell for sure. Three, including myself 9 years ago, used megadoses (900 iu per pound of body weight) to end almost immediately -12 hours or less- highly debilitating cases of flu.I got a flu shot in 1966 that almost knocked me out of the Air Force and have not had one since.
Vitamin D-3 prophylaxis does not depend even an iota on getting the strain of the flu right. Flu shots are charity for the drug companies. If you just send a check once a year to your favorite drug company, abjure the vaccinations, and take the supplements you will be healthier and will have the satisfaction of having helped a needy American Pharmacological company.