Why flu pandemics are more likely to occur at the end of flu season

Influenza pandemics in the Northern Hemisphere are more likely to occur at the tail end of flu season, according to a study published in PLOS Computational Biology.

While the flu virus is most abundant and more likely to spread in winter, all six influenza pandemics that have occurred in the Northern Hemisphere since 1889 spread in late spring and early summer.

Researchers believe the seasonal flu virus can provide temporary immunity to other strains of flu, so a pandemic spread is less likely to occur when this type of immune protection has peaked. However, researchers suspect this brief stint of immunity could provide a window for pandemics toward the end of flu season when immunity has waned.

To test this theory, researchers developed a computational model for viral transmission and plugged in data for the 2008-09 flu season. The model, which operated under the assumption that infected individuals develop long-term immunity to seasonal flu and short-term immunity to pandemic flu strains, accurately predicted the timing of the 2009 H1N1 flu pandemic.

"We don't know when or where the next deadly flu pandemic will arise," said Lauren Ancel Meyers, PhD, a professor of integrative biology with the University of Texas at Austin and the study's principal investigator. "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."

More articles on infection control: 
CDC updates care recommendations for infants with possible congenital Zika syndrome: 5 things to know 
Health officials confirm 2 Legionnaires' cases at Illinois VA facility 
San Diego VA administers 1.5k ineffective flu shots

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