What To Know About the Effectiveness of the Influenza Vaccine During the 2024-2025 Respiratory Viral Season Study

You may have seen this headline recently: “This year’s flu shot linked to higher flu risk in adults: Cleveland Clinic study.” Here’s what you need to know about the study the headline refers to, Effectiveness of the Influenza Vaccine During the 2024-2025 Respiratory Viral Season: 

• The study by Cleveland Clinic was released as a pre-print, meaning it has not been peer-reviewed. It says right on the pre-print that it “should not be used to guide clinical practice.” 
• The study looked at 53,402 Cleveland Clinic employees from Oct. 1, 2024, through March 26, 2025. 82.1% of those studied received a seasonal influenza vaccine at some point during the study period. ONLY 2.02% of those in the study were infected with influenza virus, with the vaccinated group having a significantly higher incidence of infection than the unvaccinated group, “yielding a calculated vaccine effectiveness of -26.9%” (yes—NEGATIVE 26.9%). The authors concluded they were “unable to find that the influenza vaccine has been effective in preventing infection during the 2024-2025 respiratory viral season.” The ratio of the proportion of positive tests was not significantly different between the two groups. 
• Here are the problems with this study: 
  • It has not been evaluated through peer review, a process which often calls methods and conclusions into question. 
  • There are many confounding factors that weren’t considered—selection bias (vaccinated healthcare workers more likely to be tested than unvaccinated, only counted tests done at Cleveland Clinic locations, the vaccinated group was 4.5 times larger than the unvaccinated group, etc.) One researcher found that vaccinated people are 27% more likely to be tested than unvaccinated people. 
  • The study did not look at morbidity or mortality from influenza, the avoidance of which is the primary reason for recommending seasonal influenza vaccination. 
  • The study only involved healthcare workers from northern Ohio, most of whom were young and female. Although the authors suggest the study’s findings are generalizable to relatively healthy adults in the U.S., it is difficult to make that case. 
  • The study does not mention the rate of influenza infection in the season prior to the study. Since immunity after infection is typically longer lasting than immunity after vaccination, individuals who developed illness from influenza the prior year may have had a decreased likelihood of developing the disease during the study year, even if unvaccinated. In other words, it’s possible that the unvaccinated group may have been less likely to develop clinical influenza due to infection last year. 
• Talking points and take aways:  
  • Studies need to be peer-reviewed before acting on data that is presented.  
  • There are many confounding variables in this study that raise questions about the validity of the conclusions that were made.  
  • Bright spot! Only 2.2% of healthcare workers were infected with flu! That’s amazing! 
  • At this time, there is no reason to change flu vaccination recommendations based on this study.  
  • We look forward to the peer-reviewed publication.