Disclosures: The authors report no relevant financial disclosures.
October 30, 2020
2 min read

Egg-based mutations decreased efficacy of flu vaccine during severe 2017-2018 season

Disclosures: The authors report no relevant financial disclosures.
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More evidence indicated that mutations introduced by egg-based production led to a decrease in influenza vaccine efficacy during the severe 2017-2018 influenza season, researchers reported in The Journal of Infectious Diseases.

Almost 1 million people were hospitalized with influenza during the 2017-2018 season, and around 80,000 died.

flu shot
During the 2017-2018 influenza season, younger age, seasonal vaccination and female sex contributed to greater nAb titers against the seasonal vaccine virus but did not protect against the cell-grown H3N2 vaccine or circulating H3N2 viruses.
Credit: Adobe Stock

“The 2017-2018 season was a particularly severe influenza season in the United States and all over the world. One of the biggest questions we wanted to help answer was why? Why was this season infecting and killing so many people?” Rebecca L. Ursin, MS, PhD student in the department of biochemistry and molecular biology at Johns Hopkins Bloomberg School of Public Health, told Healio.

“The H3N2 strain of influenza was the dominant strain circulating during the 2017-2018 season, and it appeared that the H3N2 component of the vaccine was not effective at protecting people from this strain,” Ursin noted. “We know that more than 50% of Americans received the seasonal influenza vaccine, but the efficacy of the vaccine against H3N2 viruses that season was around 25%. We wanted to uncover the viral and host mechanisms that could be contributing to how the vaccine was not working well.”

Ursin and colleagues collected serum and nasal washes from influenza-positive and influenza-negative patients during the 2017-2018 influenza season to determine neutralizing antibody (nAb) titers and for influenza virus sequencing.

The study demonstrated that the circulating and vaccine H3N2 virus strains were different clades, with the vaccine strain being clade 3C.2a and the circulating viruses being 3C.2a2 or 3C.3a. According to the study, both H3N2-negative and H3N2-positive patients had greater nAb titers to the egg-adapted vaccine virus vs. the cell-grown vaccine but the H3N2-negative population had significantly greater titers to the circulating 3C.2a2 clade. Additionally, the study found that among H3N2-positive patients, vaccination, young age and female sex were associated with greater nAb responses to the egg-adapted vaccine H3N2 virus but not to the cell-grown vaccine or circulating viruses.

“My biggest take-home message is more of a call to action to appreciate that influenza vaccines should not be considered as a one-size-fits-all vaccine, and that we still have much to improve upon in terms of how we manufacture them and predict their efficacy,” Ursin said. “We should use our understanding that sex and age impact immunity to vaccines and therefore may require different vaccine formulations or doses.”

“Overall,” Ursin said, “we are doing a good job with the way we create immunogenic influenza vaccines, but they need to be more consistently protective from season to season and across strains.”