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This season's flu vaccine was a poor match for the virus, CDC reports

But for the second year in a row, flu cases have remained abnormally low, staving off fears of a "twindemic."
A nurse administers a flu vaccination at a free clinic held at a local library on Oct. 14, 2020, in Lakewood, Calif.
A nurse administers a flu vaccination at a free clinic held at a local library Oct. 14, 2020, in Lakewood, Calif.Mario Tama / Getty Images file

This season's flu vaccine offers meager protection against mild cases of influenza, the Centers for Disease Control and Prevention reported Thursday.

Against the most common flu strain circulating this season, the flu shot reduced a person's chance of getting a mild case by 16 percent, which is "considered not statistically significant," the CDC authors wrote, though the shots should offer some protection against more severe illness.

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Put more bluntly, the flu vaccine was “essentially ineffective,” said Dr. William Schaffner, an infectious diseases expert at the Vanderbilt University Medical Center.

Indeed, research from earlier in the flu season found that the vaccine was a poor match for the H3N2 strain of the virus. That study, from researchers at the University of Pennsylvania, was posted as a pre-print, meaning it had not been peer-reviewed.

Thursday's CDC report confirmed that the dominant strain detected this season was H3N2 — a strain that flu experts say is particularly troublesome, as it tends to mutate faster than other variants of influenza and traditionally leads to more hospitalizations and deaths.

The findings come amid the nation's second flu season in a row with low flu activity overall. Flu cases did start to rise in the fall, sparking fears of a "twindemic" of Covid-19 and the flu, but cases never took off like they do in typical flu seasons.

Some flu experts had been particularly concerned that the nation could have a bad flu season this year after flu cases reached an all-time low last year, when most parts of the country were shut down because of the pandemic.

Schaffner said that it's possible the omicron surge in December and January had an impact on keeping flu cases at bay, by causing people to wear masks again, practice social distancing and other mitigation measures. Those measures likely kept flu levels low in the 2020-2021 season as well.

Still, Schaffner said, the CDC's new report underscores the need for a better flu vaccine, because "the flu is not going away. ... It will be back again next year and the year after that and the year after that."

Flu vaccines generally reduce the risk of illness from influenza viruses by between 40 percent and 60 percent, according to the CDC. Vaccine makers, including Pfizer and Moderna, are developing potential flu shots that use mRNA technology, the same platform used to create the Covid-19 vaccine, though those vaccines, if found to be effective, will likely not be available until late 2023.

The CDC recommends everyone over the age of 6 months get the flu vaccine because the shots can prevent more serious outcomes from influenza, such as severe disease, hospitalization and death.

Thursday's report noted that compared to the 2020-21 flu season, vaccination coverage this year was lower in certain groups, including pregnant women, infants and preschool-age children.

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