This year's flu shot was up to 78% effective at preventing hospitalization in kids, early data finds

This year's flu shots have dramatically reduced hospitalizations for seasonal influenza, early data suggest.

For kids and teens, the shots were at least 63% effective against flu hospitalizations, and up to 78% in some regions. That's according to data pulled from two networks of health care centers that the Centers for Disease Control and Prevention (CDC) monitors to assess flu shot effectiveness.

"Vaccine effectiveness" reflects how much a shot lowers the chances of a particular health outcome in the real world, and it's calculated by comparing how often that outcome occurs in vaccinated versus unvaccinated people. So in this case, vaccinated kids had a 63% to 78% lower chance of being hospitalized for flu.

For people ages 18 and up, the shots were 41% and 55% effective against hospitalization, respectively, in two of the networks.

Related: Flu shots have changed this year — here's why

The shots have also reduced the likelihood of outpatient visits for flu, which include normal doctor's visits, as well as urgent care and emergency department visits. For kids and teens, the level of effectiveness was 32%, 59% and 60% in three networks, and for adults, it was 36% and 54% in two networks.

These estimates, shared Thursday (Feb. 28) in a Morbidity and Mortality Weekly Report (MMWR), are subject to change as flu season progresses.

At a glance — compared with estimates provided for last season's flu shots at this time last year — the 2024-2025 vaccines appear to be providing greater protection against pediatric hospitalizations. However, they may be less protective against outpatient visits for some kids. Adults' protection, on the other hand, appears to be slightly higher for both types of health care this season.

"Findings in this report show that vaccination with the 2024-2025 influenza vaccine reduced the likelihood of medically attended influenza and support CDC's recommendation that all persons aged ≥6 months be vaccinated against influenza," the MMWR noted.

"These findings also support the strong protective effect influenza vaccination has against influenza-associated hospitalization, demonstrating the importance of vaccination to reduce more severe influenza-associated complications," it concluded.

The authors of the MMWR proposed that a subtype of flu known as H3N2 may explain why the shots have not been as protective against pediatric outpatient visits this season.

Human influenza viruses come in two broad types — classified as either "A" or "B" — which are then divided into subtypes and variants. CDC data show that the vast majority of flu cases this year have been caused by influenza A viruses. Officials have genetically analyzed a subset of these A viruses and found that they fall into two subtypes: H1N1 and H3N2.

At a national level, H1N1 and H3N2 are each responsible for roughly half of the influenza A cases. However, the ratio of H1N1 to H3N2 varies between regions, and thus different networks. The MMWR suggests that the shots were less protective against pediatric outpatient cases in areas where H3N2 was more prevalent. Lab experiments conducted with ferrets had already suggested that the shots were not nearly as good a "match" for the circulating H3N2 viruses as they were for the H1N1 viruses.

Nonetheless, this season's flu shots are showing levels of protection "consistent with those from the 2023-24 season and seasons associated with higher VE [vaccine efficacy] over the last 15 years," the MMWR says. Flu season in the United States can run through May, so it's not too late to get a flu shot.

Next season's flu shots

Meanwhile, the fate of the next round of flu shots — for the 2025-2026 season — is uncertain.

Typically, an advisory panel to the U.S. Food and Drug Administration (FDA) meets around this time of year to determine which strains should be included in the upcoming season's flu shots. That meeting, scheduled for March 13, has now been canceled without a given reason, Reuters reported, though notably, the newly confirmed head of the U.S. Department of Health and Human Services, Robert F. Kennedy Jr., is a longtime vaccine skeptic.

Members of the FDA advisory panel and other experts have expressed concern that this cancellation could throw off the carefully timed planning and production cycle for flu shots, The New York Times reported.

Flu shot production involves growing viruses in chicken eggs, and that takes time — there's typically a six-month buffer between the FDA selecting strains and the start of the vaccine rollout in late July or early August, according to NBC. Delaying that strain selection for the U.S. market could mean that manufacturers don't have time to make enough vaccine for the season's start, which could translate to higher rates of serious illness and death.

The cancellation of the FDA advisory panel meeting follows the postponement of a different meeting of vaccine advisers to the CDC. That said, the CDC and FDA did recently participate in a World Health Organization (WHO)-led meeting about flu shots, despite the Trump administration's stated intention to withdraw from the WHO, STAT reported.

The WHO meeting typically guides the FDA's strain selection for the U.S. market, although the agency can make its own selection for America's vaccines if it chooses. Time will tell if the FDA's vaccine advisers will meet as usual, allowing a window of time for public comment, or if strain selection might otherwise proceed without the meeting.