Flu vaccine linked to higher infections, says early research

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The flu vaccine is recommended annually for all Americans 6 months and older, per the Centers for Disease Control and Prevention — but a new study from Cleveland Clinic suggests that it might not have the protective effects people expect.

The preprint study, which was published on MedRxiv.org this week, looked at infection data for the 2024-2025 flu season.

Researchers found that among 53,402 Cleveland Clinic employees in northern Ohio, getting the influenza vaccine was associated with a 27% increase in flu infections.

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Vaccinated employees were compared to those who did not get the vaccine over a 25-week period.

The flu vaccine is recommended annually for all Americans 6 months and older, per the Centers for Disease Control and Prevention. (Joe Raedle/Getty Images)

"The influenza vaccine can be highly effective in reducing the severity of illness, preventing hospitalizations, and minimizing the spread of the virus, but its effectiveness can vary depending on the virus strain and individual factors, such as age and underlying health conditions," the Cleveland Clinic said in a statement provided to Fox News Digital.

While previous work by these researchers on vaccine effectiveness has been published in major journals, this most recent study is still in the pre-print stage, which means it has not yet been peer-reviewed.

Potential limitations

The researchers noted several limitations of the study, including that "trivalent inactivated influenza vaccine" was used in about 99% of the included employees.

"The possibility that other influenza vaccines might have been more effective cannot be excluded," they wrote.

It is also possible that home testing kits might have missed some infections.

"This study found that influenza vaccination of working-aged adults was associated with a higher risk of influenza during the 2024-2025 respiratory viral season, suggesting that the vaccine has not been effective in preventing influenza this season," the researchers wrote in the study. (iStock)

Additionally, the study did not compare the risk of flu-related hospitalizations or deaths, nor did it measure whether the vaccine decreased the severity of illness. 

The population studied also did not include children and contained very few elderly or immunocompromised individuals, the researchers stated.

"The results do not suggest that vaccination increases the risk of flu," Cleveland Clinic stated. "Instead, the study implies that the effectiveness of this season’s vaccine in preventing influenza may have been limited in relatively healthy healthcare workers."

Doctors weigh in

Dr. Marc Siegel, clinical professor of medicine at NYU Langone Health and Fox News senior medical analyst, reviewed the study findings and said the results are "misleading."

"It is observational, so does not prove that the vaccine doesn’t decrease spread," he told Fox News Digital.

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"The biggest bias I see is that it is comparing vaccinated healthcare workers (80% are vaccinated against flu) with general population trends, as the study doesn’t take into account that healthcare workers are more likely to be exposed to flu in their profession, so we would expect the infection rate to be higher, whether vaccinated or not."

Siegel also said he believes the study misuses the word "effectiveness." 

The study did not compare the risk of flu-related hospitalizations or deaths, nor did it measure whether the vaccine decreased the severity of illness.? (iStock)

"The flu vaccine is intended to decrease severity rather than decrease spread," he noted. 

"The real yardstick for flu vaccine effectiveness is the rate of hospitalization or visit to a doctor’s office or urgent care, which is usually down by well over 100,000 per year or more due to the vaccine." 

"The flu vaccine is intended to decrease severity rather than decrease spread."

This effectiveness varies year to year based on how well the vaccine fits the predominant strain, the doctor added.

"At the same time, the flu vaccine tends to decrease viral load, which should decrease overall spread within a community."

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Dr. Jacob Glanville, CEO of Centivax, a San Francisco biotechnology company, echoed that the study has not yet undergone peer review.

"We should await that process, as it will serve to check statistical methods and other study design considerations," he told Fox News Digital.

"I still tell my patients that a benefit from receiving the flu vaccine is that even though you may get the flu, your case may not be as severe as those who did not receive the flu vaccine," one doctor said. (iStock)

"For example, it would be important to rule out reporting avoidance by non-vaccinated Cleveland Clinic employees. Did they have any policies that would discourage non-vaccinated subjects to report illness?"

"The whole world is affected by influenza, so if this phenomenon is real, it should also be easily supported by other studies from this season."

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Dr. Mike Sevilla, a family physician based in Salem, Ohio, agreed that the higher rate of infection suggests that this year's influenza vaccine was not as effective as hoped.

"I wouldn't think that this is expected. However, in comparing previous years, the effectiveness of the annual flu vaccine can vary widely, from about 10% up to about 60%," he said in an interview with Fox News Digital.

"The effectiveness of the annual flu vaccine can vary widely, from about 10% up to about 60%."

Sevilla shared that in his office, he did see increased cases of influenza this season, but not necessarily as severe cases. 

"I still tell my patients that a benefit from receiving the flu vaccine is that even though you may get the flu, your case may not be as severe as those who did not receive the flu vaccine," he said.

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"If you have any questions about vaccines, you can always ask your family physician."

Fox News Digital reached out to the Cleveland Clinic researchers for comment.