This season’s influenza vaccines have been 42% effective so far, according to a new interim estimate from the Centers for Disease Control and Prevention, amounting to protection against the virus that appears as good or better than seasons going back to 2016.

First previewed Wednesday at a meeting of the CDC’s Advisory Committee on Immunization Practices, details of the latest vaccine effectiveness, referred to as VE, estimates were published Thursday in the agency’s Morbidity and Mortality Weekly Report. 

“We’re right in the range that we typically see when the vaccine is a good match with the viruses that are circulating. Good VE, and it’s working consistent with past years,” said Sascha Ellington, head of the CDC’s influenza prevention and control team. 

The exact strains selected to be targeted by flu vaccines are tweaked each year based on what health authorities project will be the best match to the circulating viruses each season. In recent years, vaccines have been designed to target four different subtypes of flu: two from the influenza A group of viruses and two from influenza B.

The estimates are from four ongoing studies backed by the agency which put together actively test patients and draw on records from immunization registries, clinics, urgent care services, emergency rooms, hospitals and health insurance claims around the U.S. 

Estimates show vaccines this season were between 52% and 61% effective in protecting children against influenza hospitalization. In adults, the shots were estimated to be 41% to 44% effective.

While effectiveness looks good for this season, Ellington warned that declining vaccination rates means the U.S. could still see fewer hospitalizations and deaths prevented by vaccines this season.

“To prevent flu hospitalizations and deaths on the population level, we need both good vaccine effectiveness and we need people to get vaccinated,” she said. 

Ellington said the agency continues to recommend that people get a flu vaccine if they have not yet this season. The CDC says significant flu activity can last until May. 

Some regions of the country have reported renewed increases in flu activity for recent weeks, after a slowdown from a peak during the winter holidays.

High effectiveness for influenza B

Effectiveness looked especially high so far this season for influenza B infections, Ellington said.

This season marked the first since before 2020 with significant amounts of influenza B cases, after the COVID-19 pandemic upended the usual spread of the virus. 

Based on data from outpatient settings, like urgent care clinics and emergency rooms, the vaccines were 78% effective in adults and at least 64% in kids for cutting the risk of a visit from influenza B.

“We really have to go back a number of years to look at influenza B effectiveness. And when you go back for those older years, you do see it ranging usually in the 40 to 60 percent range,” said Ellington.

Usual effectiveness for influenza A

Effectiveness estimates for influenza A, which typically makes up the lion’s share of cases, looked similar to previous years overall: from 46% to 59% in kids and 27% to 46% in adults for outpatient settings.

Overall, a majority of tests reported so far this season from public health labs have been from a subtype of influenza A known as A(H1N1)pdm09, the descendant of the swine flu virus that drove a flu pandemic in 2009. That is different from last year, when the influenza A(H3N2) virus dominated cases. 

Ellington said that experts sometimes see vaccine effectiveness trend higher during seasons dominated by H1N1. But she cautioned that other factors, like changes to the virus and what strains were selected to be in the season’s shots, muddy the picture.

“I think the general consensus is that they would expect perhaps a little higher VE when it’s an H1N1 season, but that doesn’t always come to fruition,” she said.

Major change to the influenza vaccines coming

The new estimates come as the Food and Drug Administration is set to vote on the recipe used for next season’s influenza vaccines, at a meeting of its Vaccines and Related Biological Products Advisory Committee.

A major change could be in store. FDA and World Health Organization panels have called on flu manufacturers to strip out an obsolete component of the vaccines targeted at the influenza B Yamagata subtype, which disappeared during the COVID-19 pandemic.

That could effectively open up one of the seats in the vaccines, dropping them next season from quadrivalent formulations – targeting four different antigens in a single shot – to trivalent.

“As a result, it is likely that in the United States, all influenza vaccines in the 2024–2025 season will be trivalent,” wrote committee member Dr. Arnold Monto Wednesday, in an article published by the New England Journal of Medicine co-authored by officials from the FDA and the U.K. Health Security Agency.

Removing influenza B Yamagata could make room for new components in the flu vaccine recipe that might boost effectiveness, though these additions could be years away.

“Replacing the B/Yamagata component with another component or formulation will require further stepwise planning and is more of a long-term goal for improving vaccine effectiveness,” they wrote.