Its common knowledge in regions with stronger winters that indoor humidity needs to be well managed (aka avoid too dry air) since upper breathing tract is more prone to infections. I've experienced it myself numerous times, so did my family and literally everybody in that region I know. Nobody ever bothered to look for peer-reviewed study of something one experiences every winter during ones whole life (just like ie eating raw strong garlic works very well as prevention of infections, not so much once sick).

Now that's not the sole reason for transmissions of course, but weakened outer defenses help infections a lot.