Flu vaccine effectiveness varies by how well it matches the circulating virus strains
Last year's flu vaccine effectiveness was just 42%
This year's flu vaccine may also provide inadequate protection, some experts say
Last year’s seasonal flu vaccine effectiveness was just 42%, the US Centers for Disease Control and Prevention estimated. Even if vaccinated, people had inadequate protection against the flu.
This limited effectiveness was due to a mutation that occurred in the influenza A (H3N2) vaccine strain, according to a study published Monday in the journal Proceedings of the National Academy of Sciences. This vaccine mutation resulted from an egg-based manufacturing process commonly used today.
This year’s flu vaccine may also be imperfect, said Scott Hensley, author of the new study and an associate professor at the University of Pennsylvania. Add to that, he said, “this could end up being a pretty bad flu season.”
Still, he said, “our best protection” against severe illness is getting vaccinated with the flu shot we have today.
Each year before flu season begins, a vaccine is made based on whichever virus strains are expected to circulate. The selected seed strains are distributed to vaccine manufacturers, which produce their formulations and make them available to health care professionals before the season begins.
During the 2015-16 season, vaccine effectiveness was 47%, but for the 2014-15 season, effectiveness was just 19%, according to the CDC. While the overall effectiveness of last season’s vaccine was 42%, it was only 34% effective against the H3N2 viruses that dominated the season.
Vaccine effectiveness varies based on how well it matches the circulating virus strains. Sometimes, a vaccine corresponds to the predominant virus, yet its effectiveness is still not what scientists would expect. Trying to understand which element of the vaccine failed is difficult.
Hensley and his team began their investigation of last year’s vaccine by looking at the seed strains that had been distributed to vaccine manufacturers. These seed strains had been propagated in chicken eggs, the common method used today.
“The sequences of these viruses are available, and when we did an alignment to see what the sequence of these vaccines were compared to the viruses that were circulating, it became very obvious that there was this mutation,” Hensley said.
To see the effects of the mutation, the team next looked at how the immune systems of both animals and humans who’d been inoculated with an egg-based flu vaccine responded to the actual circulating viruses.
The antibodies – immune system proteins that fight invading pathogens – elicited in both animals and humans failed to bind to and neutralize the flu viruses, Hensley and his colleagues found.
Though most vaccines in the United States are made in chicken eggs, a small fraction are produced in insects or mammalian cells, Hensley explained. (These are given to people with egg allergies.) He and his team compared immune responses in animals and humans who had received a cell-based vaccine: in this case, Flublok ,made by Protein Sciences Corp.
“And we found both animals and humans receiving that (cell-based) vaccine had superior antibody responses that could bind and neutralize these circulating H3N2 strains,” Hensley said.
Making a better vaccine
“Most of the infrastructure to produce vaccines in the US is based on chicken eggs,” Hensley said. There are good reasons for this, including the fact that egg-based propagation allows manufacturers to quickly produce large quantities of vaccine.
Though egg adaptations have always been a problem, beginning last year, it had become a “huge problem,” he said. “As soon as you try to grow this virus in eggs, within a few hours, the virus will acquire this kind of mutation.”
This is not an easy problem to fix, he said. To produce vaccines in cells means “a very expensive process for companies to just change their overall manufacturing process,” Hensley explained. “You can’t really do that on the drop of a dime.”
Meanwhile, the same seed strains used last year are being used this year to make the current vaccine, Hensley said.
“This year may be especially difficult because, in addition to this egg adaptive mutation which was present last year, there’s indication that the H3N2 viruses are actually evolving,” he said.
Not only will the vaccine be a mismatch with the actual circulating viruses due to egg adaptation, it could also be a mismatch due to unexpected viral evolution.
What kind of flu season is ahead?
It’s too early to speculate which viruses will become dominant in the United States over the course of the coming flu season, Hensley said, “but it’s starting to look like it will be H3 viruses.” H3 viru