Why Do Flu Outbreaks Happen in the Winter?

Why is it when the temperatures outside become colder we see a peak in illnesses, like the flu?

Many of us may ask ourselves this question and wonder why people tend to be sick especially during the winter months. And, if you have asked yourself that question, you are not alone. In fact, researchers have always sought to find the answer.

There are numerous environmental drivers that contribute to the seasonality of influenza, or the flu. These include: absolute humidity, relative humidity, temperature, solar insolation, and even the school calendar. With so many different factors, it can be difficult to distinguish if one has a more significant impact over another. Yet, there is one that stands out.

A Change in Humidity

For some time, the predominance of influenza in the winter months was attributed to a decrease in indoor relative humidity, or how much humidity there is in the air compared to how much there could be. Such assertions, however, do not square with the fact that outdoor relative humidity peaks in winter. In contrast, absolute humidity, which is an actual measure of the water vapor content of air, is better suited for determining the influence of environmental factors on the influenza viral persistence and transmission.

Research findings indicate the seasonal cycle of absolute humidity may be the reason we see flu cases spiking during the colder months. Specifically, low levels of absolute humidity, both indoors and outdoors, increase influenza virus survival and thus increase influenza virus transmission efficiency. The findings indicate that Influenza virus survival responds to the amount of water vapor in the surrounding air. So, the higher the humidity, the better the chances of the flu virus not surviving.

While we may not be able to rush to or live in climates with high humidity during the winter months, there are measures we can all take to reduce our risk of becoming ill; these include the practice of good hand hygiene (handwashing or hand sanitizing with an alcohol-based hand sanitizer when soap and water are not available) at key moments and surface disinfection. Learn more about tips to help you stay well this cold and flu season.

Shaman J, Pitzer VE, Viboud C,  Grenfell BT,  Lipsitch M (2010) Absolute Humidity and the Seasonal Onset of Influenza in the Continental United States. Journal PLoS Biol 8(2):

Jeffrey Shamana and Melvin Kohnb (2009). Absolute humidity modulates influenza survival, transmission, and seasonality PNAS I March 3, vol. 106 I no. 9 I  3243–3248