In non-tropical countries of the Northern hemisphere (e.g., the United States), flu rates are almost always highest during the winter;[1] there are various proposed theories for why this is.
One explanation involves humidity and temperature. During the winter, the air is colder and less humid. This is notable, because various experiments have found that aerosol and droplet transmission of the influenza virus (two major routes of infection)[2] occurs much more readily in colder and dryer (less humid) environments.[3][4]
Another factor could be a reduction in vitamin D synthesis during the winter as a result of reduced exposure to ultraviolet light.[5] Vitamin D has a well-established role in the immune system,[6] meaning a wintertime drop could increase susceptibility to influenza. In support of this, a 2022 meta-analysis of 10 clinical trials found vitamin D supplementation reduced the risk of getting the flu by 22%.[7]
Finally, other explanations exist, including changes in social behavior (e.g., large groups gathering inside more during the winter)[8] and an alteration to the circadian rhythm due to longer nights.[9] Ultimately, it’s likely a combination of factors working in tandem which make the flu a seasonal virus.