Austin, TX—Pharmacists might breathe a sigh of relief in spring and early summer when the typical influenza season ends, especially if the vaccination rate was high and the flu season remained relatively mild.

A new statistical simulation suggests, however, that late spring or early summer, at the end of the normal flu season, might be the likeliest time for influenza pandemics. The report in PLOS Computational Biology points out that all six flu pandemics since 1889 emerged in spring and summer months.

The University of Texas–led researchers posit that the late timing of flu pandemics might be caused by two factors that are seemingly in opposition. While flu spreads best under winter environmental and social conditions, infection by one influenza virus also can provide temporary immune protection against other flu viruses, limiting the spread of pandemic strains.

That means late spring and early summer might be most favorable for new pandemics to emerge. To test the hypothesis, the study team developed a computational model assuming that people infected with seasonal flu gain long-term immunity to seasonal flu and short-term immunity to emerging pandemic viruses. 

The same model incorporated real-world data on flu transmission from the 2008-2009 flu season and correctly predicted the timing of the 2009 H1N1 pandemic.

Running thousands of simulations in which new pandemic viruses emerged at different points throughout the flu season, researchers determined that the combination of winter conditions and cross-virus immunity tended to lead to warmer weather pandemics.

“We don’t know when or where the next deadly flu pandemic will arise,” explained principal investigator Lauren Ancel Meyers, PhD, of the University of Texas. “However, the typical flu season leaves a wake of immunity that prevents new viruses from spreading. Our study shows that this creates a narrow, predictable window for pandemic emergence in the spring and early summer, which can help public health agencies to detect and respond to new viral threats.”