Flu vaccines' efficacy fluctuates each year depending on how accurately the vaccine is matched with circulating strains of the virus. On Wednesday, CDC presented data for the 2022-2023 flu season to the Advisory Committee on Immunization Policy (ACIP), outlining the level of protection provided by this year's flu shot.

How much protection did the flu vaccine provide this year?

After experiencing an early start to this year's flu season, activity peaked around Thanksgiving. During the 2022-2023 flu season, influenza A and H3N2 viruses were the dominant strains, with roughly three-quarters of the viruses studied being H3N2 viruses.

In previous seasons, vaccine effectiveness studies have found that the vaccine generated little or no statistically significant protection.

However, new data from CDC found that this year's flu vaccine provided significant protection from the virus, with effectiveness ranging from 45% to 55%. According to CDC, vaccine effectiveness fluctuated among different age groups and metrics, including whether there was protection against any infection or serious infection that required hospitalization.

According to Brendan Flannery, an epidemiologist in CDC's influenza division, this year's shot even provided a measurable benefit to individuals who typically do not gain much protection from the flu vaccine, including individuals over the age of 65 and immunocompromised individuals.

"When we've had H3 viruses for the last 10 years or so, those are groups where it's hard to show protection," Flannery said. "It's at least reassuring that the vaccination in those groups that are often vaccinated and still at high risk, that there was some protection."

Data from three vaccine effectiveness networks used by CDC and two studies conducted by researchers at Wisconsin's Marshfield Clinic Research Institute showed consistent evidence of protection by this year's vaccine.

The Marshfield study, which was supported by flu vaccine manufacturer CSL Seqirus and CDC, estimated vaccine effectiveness among individuals ages 6 months to 64 years to be 54%. In another study, vaccine effectiveness in children ages 1 to 17 was estimated to be 71% against symptomatic flu infection.

According to data from a network of seven children's hospitals, the vaccine was 49% protective against getting sick enough to either warrant hospitalization or ED care. In an analysis of data from a second surveillance network, vaccine effectiveness was estimated to be 46% for people aged 18 to 64 years, and 39% for people aged 65 and older who visited an ED or an urgent care center for care.

The U.S. findings aligned with vaccine effectiveness data from Canada that were published earlier this month in Eurosurveillance.

Flannery noted that the viruses targeted in this year's shot are well-matched to those that are sickening people. "It just works well against the viruses that are circulating," Flannery said.

Still, while this year's flu shot performance is better than in previous years, Sarah Long, a member of ACIP and professor of pediatrics at Drexel University College of Medicine, said she was underwhelmed by the vaccine effectiveness estimates.

"This was the year that we thought there was a very good match," said Long. "It's reassuring somewhat that it's a higher effectiveness than last year, for instance, or the year before. But it still is, for all of us, disappointing after all of this time and annual immunization. And although we're saving lives, and saving hospitalizations, we don't want to lose track of that." (Branswell, STAT, 2/22; CDC Influenza report, 2/22)