After speculation as to whether this year’s flu vaccine could be poorly matched to the current strain of flu, new Centers for Disease Control and Prevention data released last week says otherwise.
The 2024-25 influenza vaccine reduced the risk for influenza-associated outpatient visits and hospitalization, the CDC’s findings said — despite weeks of a previously reported surge in flu cases.
Dr. Amesh Adalja, a Pittsburgh-based infectious disease physician and senior scholar at the Johns Hopkins Center for Health Security, said the new data is “consistent” with better years of match between the flu strain and flu vaccine, especially in hospitalization reduction.
“I was expecting to see a fairly good level of protection against hospitalization, which is where the current influenza vaccine’s value is concentrated,” he said.
About 54.9% of the influenza A H3N2 strains — which is causing about half of all cases this season — is well-matched to this year’s flu vaccine, Adalja said, according to this week’s most updated FluView data. And 100% of the H1N1 strains were matched to the vaccine, FluView said.
This refutes previous conjecture that stemmed from preliminary CDC data, saying the flu vaccine this season might be poorly matched, CBS News reported.
Vaccine effectiveness for the 2024-25 season was derived from four networks, the CDC reported, and the findings showed:
- For children and adolescents, vaccine effectiveness was 32%, 59% and 60% in outpatient settings, and 63% and 78% against influenza-associated hospitalization
- For adults, vaccine effectiveness was was 36% and 54% in outpatient settings, and 41% and 55% against influenza-associated hospitalization
Dr. Tom Walsh is an infectious disease physician who serves as the medical director of Allegheny Health Network’s Antimicrobial Stewardship Program. He explained that the flu vaccine contains three different flu strains expected to circulate the coming season.
“It changes every year,” he said. “The effectiveness depends on factors like age and health.”
The FDA considers factors like how the viruses are changing and disease trends, Walsh said. Companies that manufacture influenza vaccines rely on the FDA to pick out the strains to use in shots yearly.
“A lot is crystal-ball prediction,” he said. “The closer the match between the vaccine virus strains and the circulating virus strains causing disease during flu season, the better the protection that the vaccine provides.”
But the vaccine is never “a perfect match,” according to Walsh, due to ongoing mutations. A 50% or 60% match is not terrible to see typically, he said.
“Even though it’s never an exact match, doesn’t mean the vaccine doesn’t benefit people,” he said.
Studies prove that the vaccine reduces the severity of illness in people who get vaccinated but still contract the virus, according to Walsh.
Based on insights from the FDA, the Centers for Disease Control and Prevention then recommends certain vaccines to the public yearly.
However, the CDC’s updated data was released last week in the wake of the U.S. Food and Drug Administration’s abrupt cancellation of its committee meeting to recommend the makeup of seasonal flu shots for the 2025-26 influenza season.
The meeting was supposed to take place March 13. Adalja previously told TribLive it’s unclear what the prospect is for rescheduling. He connected the action to new Department of Health and Human Services Secretary Robert F. Kennedy Jr., a vaccine skeptic.
“It is critical that manufacturers have enough lead time to scale up manufacturing in time for the start of influenza season,” Adalja said, which is typically six months.
Continuous impact
Walsh treats flu patients in Western Pennsylvania daily, and he said he’s seen more patients hospitalized this February compared to the past decade.
“We have an enormous amount of people sick in our hospitals now,” he said, which can put hospitals at capacity.
As of Friday, AHN has 104 patients hospitalized with the flu. Last Friday, AHN had 134 hospitalized, so the numbers are moving in the right direction.
“I don’t recall seeing this many patients at this time of the year with severe disease requiring hospitalizations,” Walsh said.
The last time he saw similar numbers was in 2009 during the swine flu pandemic, which was the first major outbreak of the 21st century.
“This is still seasonal influenza,” Walsh said, though. “This is the largest seasonal influenza that we’ve seen really since that time.”
Featured Local Businesses
But Adalja said he would still differentiate the severity of this season from the 2009 pandemic.
“While there may be comparable levels of influenza related health care visits and hospitalizations, there is not the same ICU burden as there was during the pandemic.”
This flu season marks the first time since the covid-19 pandemic that flu deaths and hospitalizations have been more numerous than covid, according to Walsh.
In Pennsylvania, influenza activity remains “very high,” but it’s decreasing from previous weeks, according to the state’s Respiratory Virus Dashboard.
As of Feb. 22, there were 172,271 confirmed flu cases across the state since Sept. 29 and 453 flu-related deaths, the dashboard showed. That’s up from 125,640 cases during the same period a year earlier.
During that period, there have been 14,553 flu cases in Allegheny County and 3,801 cases in Westmoreland, according to the dashboard.
The state also reported that, as of Feb. 15, new hospital admissions for the flu were high for both pediatric and adult patients — and increasing for adult patients.
Vaccine hesitancy
Efficacy aside, Walsh said the biggest issue he’s witnessed this flu season is vaccine hesitancy — as just 45% of adults and 40% of kids have received a flu shot.
“We used to have somewhat higher,” he said. Even though the flu vaccine has always not been the most popular vaccine, “this has certainly trended down since the pandemic.”
The bird flu, or H5 virus, also concerns Walsh, even though it hasn’t reached person-to-person spread.
“That’s a concern if it would develop person-to-person spread,” he said. “We would have a whole population with no immunity to it.”
Walsh attributes this decline to vaccine misinformation circulating, especially with the covid-19 vaccine. It’s discouraging to Walsh during his job as an AHN infectious disease physician, he said.
“The covid vaccine led to a lot of vaccine hesitancy,” he said. “(It) has made folks even less likely to take any of the respiratory vaccine viruses at this time.”