Most vets have no expertise on COVID-19, but I understand how disease and viruses work in general. Pathogens are in competition with the animal much like a tug-o-war match.
Most of the time the animal’s immunological system gives them the edge. Anything that weakens that system gives the advantage to the bacteria or viruses in the environment. The weather has a big part to play in that. When we have wet conditions in springtime animals that would ordinarily be turned outside to pasture end up being housed for longer.