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.

By Brenda Goodman, CNN

Published Mar 5, 2024 11:46 AM EDT | Updated Mar 5, 2024 11:46 AM EDT

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Vaccine experts who advise the FDA will vote on recommendations for this year's flu vaccine. They are expected to change the formula for the shots from four strains to three. (Joe Raedle/Getty Images/FILE via CNN Newsource)

(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.

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.

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.

There are good reasons for dropping the Yamagata strain, Offit said.

“You don’t want to be vaccinating people for something they don’t need,” he said.

There may also be some harms in continuing to include it, said Dr. Jodie Guest, senior vice chair of the Department of Epidemiology at Emory University’s Rollins School of Public Health.

“Anytime these flu vaccines are being produced, they are – depending on which vaccines you are talking about – using live or attenuated virus, and you do have to grow it,” she said. Growing something in a lab also means it could escape from that lab.

“So while it would be an anticipated, incredibly small risk, there is the possibility you could reintroduce it into the population by having it contained in a vaccine,” Guest said.

Other researchers have pointed out that dropping the Yamagata strain would free up production capacity to increase the number of doses made globally, something that would benefit countries affected by shortages.

In an article on the expected changes published February 28 in the New England Journal of Medicine, Dr. Arnold Monto, one of the FDA’s vaccine advisers, Dr. Maria Zambon from the UK Health Security Agency and the FDA’s Dr. Jerry Weir said the move opens the door to considering new vaccine formulas.

Since the shot’s B/Victoria and A/H1N1 strains are often more effective than the A/H3N2 component, some experts have suggested doubling the dose of H3N2 or perhaps slipping in a second member of that family.

But as the authors note, any such change would require testing and regulatory approval, and for that reason, it’s not likely we’ll see the return of four-strain flu shots any time soon. Instead, they say, it will be “more of a long-term goal for improving vaccine effectiveness.”

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