The Covid-19 pandemic killed off one strain of the flu, and that will change the next vaccines

CNN — For 10 years, Americans have had access to flu shots that protect against four strains of the virus: two A strains and two B strains.

Starting this fall, however, all the flu shots distributed in the United States will probably contain only three strains, and the change is because of Covid-19.

In 2020, all the precautions that helped people avoid Covid had an unexpected benefit: An entire branch of the flu’s family tree, a B strain that geneticists call the Yamagata clade, disappeared, and it hasn’t been detected since.

A Yamagata strain was typically included in each year’s flu shot recipe, so vaccine designers faced a quandary: Should they drop the strain from the formula or keep it in, since B-viruses are known to be cagey?

In the 1990s, when Yamagata was in its heyday, another branch of B-strain flu viruses called Victoria was seen only sporadically in testing, but it had a resurgence in the 2000s. What if Yamagata came back after a lengthy absence? It’s not quick or easy to change how flu vaccines are manufactured, and those changes require regulatory review and approval.

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In September, the World Health Organization said that “inclusion of the Yamagata-lineage antigens in influenza vaccines is no longer warranted,” and in October, vaccine experts who advise the US Food and Drug Administration also said the Yamagata strains should be dropped as quickly as possible.

“We’ve been talking about this for four years,” said Dr.
Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a member of the FDA’s Vaccines and Biological Products Advisory Committee, or VRBPAC.

The committee is meeting Tuesday to discuss next steps and vote on flu vaccine recommendations for the fall.

Offit said he expects all flu vaccines available in the US this fall to be three-strain, or trivalent, vaccines with two A strains and a B/Victoria strain but no B/Yamagata strain, in line with the WHO and VRBPAC recommendations …

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