Weather’s impact on the spread of influenza

According to the Center for Disease Control (CDC), flu season here in the Borderland and the rest of the United States is roughly from October to April, the coldest 7 months of the year. So why does the flu peak in winter?

A 2014 study in the Journal of Virology found that the spread of influenza is dependent on both temperature and humidity. It concluded that flu transmission was highly efficient at 41 degrees Fahrenheit, but blocked or inefficient at 86 degrees.

Drier conditions (below 35% relative humidity) were found to be more favorable for the spread of influenza than intermediate (~50%) or humid (~80%) conditions. Even though outdoor relative humidity is similar or, in some cases, higher in winter, indoor relative humidity is usually much lower.

This is because it is warmer indoors in winter, and warmer air can hold more moisture, thus decreasing relative humidity inside.

Here in El Paso, January outdoor relative humidity averages 50%. However, if you take that same air and heat it to indoor temperature, that would bring relative humidity down to 20%.

It is important to note that cold or dry air does not cause the flu, it is just easier to spread in those conditions. People also spend more time indoors with other people in winter, which likely contributes to the spread of the flu as well, according to Dr. William Schaffner in an article by ABC News.