There was an uptick in COVID-19 cases in the winters after the initial pandemic surge in 2020. Like last winter, this winter is seeing a surge in post-holiday cases.
The holidays are a time when people get together. In colder parts of the country, the gatherings are often indoors. The closeness of people and lack of ventilation can put people at risk of contracting COVID-19. But that’s only a part of why there is a COVID-19 surge after the holidays.
The cold weather does play a part in the increase in COVID-19 cases. Ran Xu, an assistant professor at the University of Connecticut, co-authored a study showing the correlation between cold weather and the rate of COVID-19 cases. Dr. Xu says he discovered biological evidence that COVID-19 is more active during the colder months. During these months, the combination of cold weather and high ultraviolet (UV) radiation can spike the rate of COVID-19 cases.
Another factor in the increased level of COVID-19 during the winter months is the decrease in humidity. Colder weather affects the nose. People’s noses dry up, impairing the function of the cilia, the tiny hairs in the nose. The decreased function means the cilia may fail to remove a virus from the nose.
Approximately 87% of Americans spend their time indoors during the winter when indoor humidity levels are about 10-40%. Researchers believe the optimal humidity level to decrease the risk of COVID-19 transmission is between 40-60%. The increased time spent indoors and the decreased humidity are ideal for transmitting COVID-19.
Because coronavirus surveillance data was scaled back when the public health emergency ended in May 2023, it’s hard to tell the number of cases that have occurred or are occurring. As a result, the Centers for Disease Control and Prevention (CDC) is cautiously discussing the winter surge in cases.