Egg-based production faulted as experts predict flu vaccine might be just 10% effective this season

Based on data from Australia, which already had its flu season, scientists warn that this season’s flu shot might be only 10% effective. And the reason for such a low level of protection might lie in the method by which the majority of flu vaccines are made: in eggs.

But with 150 million doses needed every year in the U.S. alone, egg-based vaccine production remains the best option, according to a Sanofi Pasteur executive.

Health officials picked the same viral composition this year for flu vaccines in both the Northern and Southern Hemispheres, so scientists used Australia’s health records to get an idea of what the north might expect heading into its season. As Australia had already reported 215,280 influenza cases by mid-October, far more than the 59,022 recorded during its 2009 pandemic, a team disclosed the 10% estimate in a New England Journal of Medicine commentary. Anthony Fauci, M.D., head of the National Institute of Allergy and Infectious Diseases, co-authored the piece.

The U.S. is currently experiencing worse influenza prevalence than in previous years, with seven states already seeing widespread flu activity as of Dec. 2, according to the CDC’s surveillance.

Influenza viruses are notorious for their mutations, a phenomenon known as antigenic drift, and each year experts try to predict the strains that will circulate months ahead of the coming season so vaccines can be manufactured in time. If the virus evolves, creating a mismatch between circulating strains and the vaccine composition, it can lead to lower effectiveness.

But for this season, scientists perceive a different problem. Flu viruses didn’t significantly change after the vaccine composition was determined, the CDC’s most recent Morbidity and Mortality Weekly Report reported, and a preliminary analysis of the Australian data suggests that the low effectiveness was not primarily due to any difference between the vaccine strain and circulating viruses. Instead, it seems to be the egg-based vaccine production technology that caused the mismatch, according to the team.

“[C]irculating A(H3N2) viruses are antigenically less similar to egg-grown A(H3N2) viruses used for producing the majority of influenza vaccines in the United States,” the CDC report said.

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In fact, a recent study has already attributed last year's low flu shot efficacy to the egg-based production process, a theory that the NEJM study supports. As the earlier group of researchers concluded, flu viruses propagated in eggs undergo certain changes in the hemagglutinin protein—the primary target of neutralizing antibodies—and that could negatively impact our body’s virus-killing responses.

With that, scientists now stress the importance of a universal flu vaccine and are championing new paths away from the egg-based manufacturing process.

Recombinant and cell-based platforms are two other vaccine technologies currently available. Sanofi, through its recent acquisition of Protein Sciences, markets the Flublok family, the only recombinant-based flu shots approved in the U.S., and the platform grows vaccine virus in cells. Seqirus’ Flucelvax is also the only FDA-approved cell-based flu vaccine. Seqirus just started manufacturing its entirely cell-based flu vaccines on a commercial scale this season.

In a statement sent to FierceVaccines, Sanofi Pasteur's associate VP and North America regional medical head, David Greenberg, M.D., noted that no clinical studies have conclusively proved that cell-culture vaccines are more effective. He also pointed out that, with 150 million doses of flu shots needed each year in the U.S. alone, “egg-based vaccine production remains the most reliable, time-proven method and represents the vast majority of vaccines used globally to help prevent influenza.”

But the company also said it “continuously assesses new technologies to improve production capabilities,” including the benefits of Flublok and development of a universal flu vaccine that would be effective despite antigenic drift and mismatch.

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Published last month, the University of Pennsylvania study found that most people among the studied group who had strong antibody responses in the 2016-17 season had received the recombinant flu vaccine.

Regardless, before alternative technologies become widely adopted, experts still encourage people to get the flu shots, even if it only offers 10% protection. “However imperfect, though, current influenza vaccines remain a valuable public health tool, and it is always better to get vaccinated than not to get vaccinated,” the NEJM authors wrote.

Editor's Note: The story has been updated with two statements from Sanofi Pasteur. It also clarifies that recombinant flu vaccines are also grown in cell culture.