Dec. 17, 2021 -- Flu vaccines aren’t providing as much as much protection as hoped for against the most widely circulating strain of influenza, a new study says, but it appears the vaccines will still prevent severe illness in people who are infected.

The vaccines were designed far in advance to fight the flu strains expected to dominate the United States during the 2021-22 flu season: H3N2, H1N1, and two strains of influenza B.

H3N2, the major strain, mutated in a way that means the flu vaccines don’t match up against it very well, Scott Hensley, PhD, a professor of microbiology at the University of Pennsylvania who led the study, told CNN.

"From our lab-based studies, it looks like a major mismatch," he said.

That mismatch may have contributed to influenza outbreaks on college campuses.

An outbreak at the University of Michigan affected more than 700 people, with about 26% of those who tested positive having been vaccinated against the flu, according to CNN.

Though vaccines may not stop H3N2, it looks like they’ll prevent serious infection from that strain, the researchers said.

“Studies have clearly shown that seasonal influenza vaccines consistently prevent hospitalizations and deaths even in years where there are large antigenic mismatch,” they wrote on medRxiv, an internet site that publishes reports that are yet to be peer-reviewed.

The news about H3N2 is worrisome because the U.S. went through an easy flu season in 2020-21, apparently because COVID safety measures cut down on the spread of flu viruses.

“Population immunity against influenza viruses is likely low since these viruses have not circulated widely during the COVID-19 pandemic,” the researchers wrote. “Social distancing, mask wearing, and decreases in international travel have likely contributed to reduced global circulation of influenza viruses.

“Once COVID-19-related restrictions are eased or lifted, it is possible that influenza viruses will circulate widely due to lack of infection-induced population immunity over the past two years.”

In the 2019-20 season, more than 22,000 people in the U.S. died from the flu; last year, deaths decreased to about 700 for the 2020-21 season.

Researchers are now wondering if the U.S. will go through another easy flu season or go through a “twindemic,” with people getting sick from the flu and COVID.

Show Sources

medRxiv. “Antigenic and virological properties of an H3N2 variant that will likely dominate the 2021-2022 Northern Hemisphere influenza season.”

CNN. “Flu vaccines don't match the main circulating flu virus strain, researchers find.”

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