Read the article. It has nothing to do with being in closed quarters with people.
Freaking scientists, they could have just asked me. In the winter:
1. People spend more time indoors in close contact.
2. Less vitamin D and generally less other vitamins too in the body.
3. Thanksgiving, Christmas, New year's Eve, Superbowl all super spreader events.
In Florida it is opposite, because of the summer heat people spend more time indoors in AC.