Cold and humidity are definitely important – scientists can make flu spread faster or slower in guinea pigs just by altering the temperature and humidity of their cages. But it can’t just be cold and humidity. But if it was just cold, you would expect flu to track temperature instead of seasonality. Alaska is colder in the summer than Florida in the winter, so you might expect more summer flu in Alaska than winter flu in Florida. But Alaska and Florida both have lots of flu in the winter and little flu in the summer.
(if it was just humidity, same argument, but change the place names to Arizona and Florida.)
It’s the same story with people being cramped indoors. Common-sensically, this has to be some of the story. But if it were the most important contributor, you would expect to see the opposite pattern in very hot areas, where nobody will go out during the summer but it’s pleasant and balmy in the winter. Yet I have never heard anyone claim that any winter diseases happen in summer in Arizona or Saudi Arabia or terrible places like that.
If it was just vitamin D…look, it’s not vitamin D. Nothing is ever vitamin D. People try so hard to attribute everything to vitamin D, and it never works. The most recent studies show it doesn’t prevent colds or flu, and I think the best available evidence shows it doesn’t prevent coronavirus either. African-Americans, who are all horrendously Vitamin D deficient, don’t get colds at a higher rate than other groups (they do get flu more, but they’re vaccinated less, so whatever).
Author(s): Scott Alexander
Publication Date: 7 Dec 2021
Publication Site: Astral Codex Ten