Does the Flu Shot Raise Infection Risk? Here’s What a New Study Actually Found

New research from the Cleveland Clinic has fueled vaccine skepticism and called into question the effectiveness of the 2024-25 flu shot, but experts said the results don’t imply what they seem to at first glance. 

The study, which was released April 4 on the preprint server medRxiv and hasn’t been published in a peer-reviewed journal, aimed to test the flu vaccine’s effectiveness in about 53,000 Cleveland Clinic employees. Researchers found that those who received the flu shot—82% of the employees—had a 27% higher risk of testing positive for the flu than the unvaccinated group.

Some people are using the study as evidence that the flu vaccine can actually cause influenza. In a post on X with over 700,000 views that referenced the research, one user wrote, “GET THE SHOT GET THE DISEASE!”

However, experts disputed this notion—as well as the assumption that the study indicates the flu shot hasn’t been effective.

Here’s what’s really going on.

In a statement to Health, the Cleveland Clinic said that the study’s “results do not suggest that vaccination increases the risk of flu.”

In fact, the flu shot cannot cause the disease, experts stressed. The flu vaccine contains a harmless form of the virus.

“In the inactivated and recombinant [vaccine]—which is what most people get—the essential parts of the virus are not there,” explained Geeta Sood, MD, ScM, an assistant professor of medicine and hospital epidemiologist at Johns Hopkins Bayview Medical Center. “The virus cannot replicate. It’s just not possible for it to do that. So, you cannot get influenza from the vaccine.”

Instead, the Cleveland Clinic said the study’s discovery “implies that the effectiveness of this season’s vaccine in preventing influenza may have been limited in relatively healthy healthcare workers.” 

Before you make too much of the findings, though, it’s important to consider two key limitations that experts say taint the results. 

First, this study is a preprint, meaning it has not been peer-reviewed.

“Often preprints don’t turn into manuscripts,” Shira Doron, MD, chief infection control officer for Tufts Medicine and hospital epidemiologist at Tufts Medical Center, told Health. “Based on the track record of this group, it will, but sometimes the conclusions change dramatically from preprint to peer-reviewed.”

Second, testing bias, or the likelihood of someone seeking a flu test, also impacted the study results. The study measured the flu shots’ effectiveness based on how many people tested positive for the disease, but not everyone who catches the flu will get a test.

“It’s highly likely that somebody who chooses not to get vaccinated may also choose not to get tested or go to a doctor,” Sood told Health.

In fact, Doron pointed out that Jeffrey S. Morris, PhD, director of the Division of Biostatistics at Perelman School of Medicine, analyzed the research and found that vaccinated people are 27% more likely to test for flu than unvaccinated people, a statistic he later posted on X.

“That confounding [variable] makes it very, very challenging to interpret the results of that study,” Sood added.

Yes, you should still get the flu vaccine, experts recommended.

As explained above, even if a flu shot is ineffective, it won’t cause the disease. The main reason why a flu shot may not be effective is if the vaccine does not match the circulating strains of the virus that year.

“Because influenza mutates so much from year to year, and we have to start the manufacturing process of the vaccine well in advance of flu season, we have different success rates from year to year in the efficacy of that vaccine—or the closeness of the match between the vaccine and the circulating strain,” Doron said.

Even so, the main purpose of the flu shot is not to prevent you from contracting the disease—it’s to stave off severe cases of the flu that result in hospitalizations and death. 

In the statement to Health, the Cleveland Clinic noted that “the study only focused on infection rates and did not assess severe illness or hospitalization, which vaccines have historically been shown to reduce.”

“I don’t think it should change how we recommend the flu vaccine—to everyone over the age of 6 months—because people die every year from the flu who have zero risk factors,” Doron said. “We need people to understand that it’s to prevent severe disease, hospitalization, and death.”