A new study reveals that the dominant strain of influenza circulating this season has evolved a dangerous mutation, rendering this year’s flu vaccine a “bad match.”
“From our lab-based studies it looks like a major mismatch,” says professor Scott Hensley, study coauthor.
Hensley and his colleagues have been keeping an eye out for genetic alterations in the H3N2 subtype of the influenza virus as it spreads. In thier observations, they recently detected a new H3N2 “clade,” or split in the virus’s family tree.
They dubbed the clade “3C.2a1b.2a2,” or 2a2, and published their findings on the preprint database medRxiv on Dec. 16.
The study has yet to be peer-reviewed or published in a scientific publication, and it only evaluated antibody responses in 40 people, the majority of whom were young and healthy.
However, Hensley told CNN that the new clade’s changes may render this year’s flu vaccine less effective against H3N2.
That doesn’t mean the flu shot you got was ineffective.
“Studies have clearly shown that seasonal influenza vaccines consistently prevent hospitalizations and deaths even in years where there are large antigenic mismatches,” the authors note in the paper.
Even if this year’s flu vaccine does not match the dominant strain, the vaccines will minimize the chances of severe illness and death.
So, what’s the big deal about the newly discovered 2a2 clade? Viruses in the clade had changes in genes that code for hemagglutinin (HA), a protein on the virus’s surface, the researchers discovered.
Flu vaccines teach the immune system to recognize the HA protein, which protrudes from influenza viruses like a lollipop on a stick. The difficulty is that the HA protein mutates so quickly that its structure can change between the time the flu vaccine is developed and when the flu season peaks, which is usually between December and February. And, of the four influenza subtypes covered by the annual flu shot — two influenza A viruses, H1N1 and H3N2, and two influenza B viruses, Victoria and Yamagata — H3N2 mutates the fastest.
As a result, the flu shot is least protective against H3N2, which has substantially harmed the vaccine’s efficiency in the past. The modifications in the H3N2 virus this year are reminiscent of the mutations that caused the immunization so ineffective during the 2014-2015 flu season, when it provided only 6% protection against H3N2, according to Hensley.
“Importantly, we found that antibodies elicited by the 2021-2022 Northern Hemisphere influenza vaccine poorly neutralize the new 2a2 H3N2 clade,” Hensley tweeted.
We found that one of these substitutions eliminates a key glycosylation site on HA. Although 2a2 viruses have a decrease in receptor specificity breadth, they replicate at least as efficiently as 2a1 H3N2 viruses in both primary and transformed cell culture systems. 5/10 pic.twitter.com/eis0GNSeS7— Hensley Lab (@SCOTTeHENSLEY) December 16, 2021
“55% of vaccinees had undetectable levels of neutralizing antibodies against 2a2 H3N2 after vaccination.”
Importantly, we found that antibodies elicited by the 2021-2022 Northern Hemisphere influenza vaccine poorly neutralize the new 2a2 H3N2 clade. 55% of vaccinees had undetectable levels of neutralizing antibodies against 2a2 H3N2 after vaccination. 6/10 pic.twitter.com/UfHBugfOKt— Hensley Lab (@SCOTTeHENSLEY) December 16, 2021
This may help to explain a recent influenza outbreak on the University of Michigan’s Ann Arbor campus, which was predominantly caused by H3N2. According to CNN, the surge affected over 700 people, with around a quarter of them having had flu vaccinations.
But not all is lost.
“While cases of 2a2 H3N2 infections are quickly rising in the United States and other parts of the world, it is possible that other clades of H3N2 will become predominant in the future,” the researchers write.
“It is also possible that H1N1 or influenza B viruses might dominate later in the 2021-2022 season.”
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