I have no reason to believe this is true. It can take years to fully remedy a serious backlog of nutritional deficits, but that's a completely different metric from "years for measurable benefit...to be realized."

For seeing an overall mortality benefit in normal times, maybe.

But it's the D-related-compound levels in your body right now (or seasonally) that have a noticeable effect on fighting current viral infections.

It takes about 7 days for ingested Vitamin D to be fully converted into calcifediol, an important active form measured in the blood, and with good results as a treatment preventing ICU admission in serious COVID cases.

So if you'd like a better chance of fighting off a COVID infection over the next few months with only mild effects, ensure you're on a daily regimen of Vitamin D now.