Vit D has half a dozen anti-viral properties. It opens up immune pathways, and even a breakdown product in the blood erodes viral coats. And it also inhibits the immune system from running wild, an ACE-inhibitor effect.
Basically at every step of the process, from recognition of novel pathogens to conducting an orderly immune response, Vit D is a major player.
I’ve been doing Vit D3 and Zinc as a cold/flu protocol for years. When I got Covid, it was a cold with some heart issues. The Chinese knew what they were doing.