Fact and Discovery

Your First Battle With Flu Matters Most – Here’s Why

3D computer-generated rendering of a whole influenza (flu) virus. Our immune response is triggered by the virus’ hemagglutinin (HA) and neuraminidase (NA) surface proteins, shown in semi-transparent blue. HA is a trimer comprised of three subunits, while NA is a tetramer, comprised of four subunits, with a head region resembling a 4-leaf clover. Credit: Illustration by Dan Higgins, courtesy of CDC/ Douglas Jordan

The first strain of influenza virus we encounter during childhood sets the course of how our immune system responds to exposures later in life.

How successfully a person can fend off the flu depends not only on the virus’ notorious ability to change with the season, but also on the strain first encountered during childhood, according to new research published in the open-access journal PLoS Pathogens on December 19, 2019.

The findings offer an explanation for why some patients fare much worse than others when infected with the same strain of the flu virus. The results also could help inform strategies aimed at curbing the impact from the seasonal flu.

“The last two flu seasons have been more severe than expected,” says study co-author Michael Worobey, head of the Department of Ecology and Evolutionary Biology and a member of the BIO5 Institute at the University of Arizona. “In the 2017-18 season, 80,000 people died in the U.S., more than in the swine flu pandemic of 2009. Influenza is a major, major killer – not just in this country, but worldwide.”

For decades, scientists and healthcare professionals were vexed by the fact that the same strain of the flu virus affects people to various degrees of severity. Then, in 2016, a team including Worobey and authors of the current study presented a paper in the journal Science showing that past exposure to the flu virus determines an individual’s response to subsequent infections, a phenomenon called immunological imprinting.

The discovery helped overturn the prior commonly held belief that previous exposure to a flu virus conferred little or no immunological protection against strains that can jump from animals into humans, such as those causing the so-called swine flu or bird flu. These strains, which have already caused hundreds of spillover cases of severe illness or death in humans, are of global concern because they could gain mutations that allow them not only to readily jump from animal populations to humans, but also spread rapidly from person to person.

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