When Exactly is Flu Season?

At the start of the school year, I ran into a colleague in the elevator. Sam told me he enjoyed his summer break in spite of the hot and humid weather.  His summer would have been perfect except for the severe flu he contracted a couple of weeks prior. This story might sound strange to someone living in another part of the country where flu is a winter and springtime disease.  In Florida, we hear anecdotes about someone coming down with the flu anytime of the year. For example, there was a September flu outbreak at an evacuation shelter during Hurricane Irma.

We recently published the nation’s first systematic study of influenza seasonality. Using novel real time influenza tests, we could definitively identify the most common times when you could contract flu.  Interestingly, the flu transmission seasons were different in the southeastern U.S. and Hawaii compared to the rest of the nation. While you are still much more likely to contract flu cases in the winter/spring, the virus circulates throughout the year in the southeastern U.S.  

What could be responsible for this pattern?  Neither vaccination rates nor total population could account for this geographic contrast.  Interestingly, dryer air during the flu season was consistently related to a larger proportion of winter/spring versus summer time cases.  Dry air can be related to flu seasonality in two interrelated ways.

First, flu virus survival (e.g. think someone sneezing on a door handle) and the chance of infecting another person dramatically increases in dry air.  This may be one reason why flu peaks in the cold season in most of the U.S. Second, the moisture in the air may be related to the intensity of flu transmission and the proportion of people who catch the virus.  In cold and dry areas, the flu season may be intense, infect more people, and “burn itself out”. In warmer and moister areas (e.g. Florida), the wintertime flu season may be more subdued and infect fewer people.  Thus, there is a larger proportion of people who can contract flu in the summer.

Influenza still kills 250,000-500,000 people per year across the globe. The virus mutates rapidly which makes it difficult to develop a perfectly protective vaccination. Strengthening our understanding of flu dynamics could help forecast and target flu prevention activities.     

Learn more about this research at the PLOS ONE website. The paper is available for free.

Dr. Christopher Uejio is an Assistant Professor in the Department of Geography.

The feature image is from Science Magazine.

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