I think there are multiple factors, each of which only increases R by a little bit, but when combined produce a noticeable increase. Lack of ventilation is cer

I think there are multiple factors, each of which only increases R by a little bit, but when combined produce a noticeable increase. Lack of ventilation is certainly a major factor. The temperature difference between indoors and outdoors is much larger in the winter, so people are more reluctant to let in fresh air. We also don't wash winter clothes as often as we do summer clothes, so viruses that stick to fabric might have more chances to get into our bodies. Homes and public spaces get cleaned less often, too, as they don't feel as sticky and smelly as they do in the summer. Ditto for our own hands, which is probably more important that any of the other factors mentioned here. In the United States, two of the largest mass migration and shopping events of the year take place in the early to mid-winter: Thanksgiving and Christmas. In China, COVID-19 began to spread in earnest outside of Wuhan around Chinese New Year. Even societies that don't have a special holiday in the wintertime might be impacted by cascading infections from those that do.