My educated guess is that those who die from the flu do not have blood levels of vitamin D3 over 60. Sure, you can still get the flu. But to have a bad case means your vitamin D3 is deficient.
When my brother was a kid, in the late fifties, he was constantly getting sick. The doctor told my mom to take his shirt off, over the summer, and get him out in the sun. She did that every summer, and after that he didn’t get sick nearly as often.