A post circulating on X claims that flu vaccines are a “total failure”, alleging that they do not protect against influenza and instead increase the risk of flu by 27%.

The post cites a Cleveland Clinic study that reportedly found negative 26.9% vaccine effectiveness during the 2024–2025 flu season, i.e., those vaccinated appeared more likely to catch the flu than those who had not been vaccinated at all.

While the post itself has modest reach, 5623 views at the time of writing, the claim is particularly consequential given that Singapore is heading into a peak flu period that typically runs from May to August, a time when vaccination decisions carry real public health weight. Misleading claims that discourage vaccination could undermine public confidence in an important preventive health measure, which is why we examined the evidence closely to assess whether the claim has merit.

Why this matters

The flu can cause high fever, cough, sore throat, headaches and muscle aches, and severe cases may lead to complications such as pneumonia, a lung infection that can be life-threatening for vulnerable individuals.

In Singapore, the flu vaccination is commonly recommended by doctors on an annual basis because the influenza virus mutates rapidly, meaning the strains circulating each season can differ significantly from those of the year before, and immunity from a previous vaccination may no longer be effective against them. Vaccination is highly recommended for vulnerable groups such as seniors, young children and people with underlying medical conditions, because they face a higher risk of serious complications, either due to weaker immune systems or health conditions that the flu can aggravate.

What is the study?

The study cited in the X post involved 53,402 Cleveland Clinic employees in Ohio, U.S. during the 2024–2025 winter months, when flu and other respiratory viruses are most actively circulating.

Among those studied, 82.1% had received the flu vaccine, and only 2.02% tested positive for influenza over the study period. After statistical adjustments, the researchers reported that vaccinated participants had a 27% higher rate of recorded influenza infection, producing a calculated vaccine effectiveness estimate of –26.9%. This means that more vaccinated employees in this specific study tested positive for flu than unvaccinated employees.

This figure is the basis of the viral claim. However, this does not automatically mean the vaccine caused more infections.

Why the claim is misleading

First, the study is a preprint, meaning it has not yet undergone peer review by independent experts, and the paper itself states clearly that its findings should not be used to guide clinical practice. Preprints are useful for early scientific discussion, but their findings should not be treated as settled conclusions or used alone to guide public health decisions.

A Cleveland Clinic spokesperson said it is misleading to use the study to draw broader conclusions about vaccines and susceptibility to infection. Public health agencies such as the US Centers for Disease Control and Prevention (CDC) note that flu vaccine effectiveness can vary from season to season depending on factors such as the match between the vaccine and circulating strains, the population studied, and the method used to measure outcomes. It is not useful to take the results of a single study in isolation as evidence that flu vaccines do not work.

Second, the study shows an association, not proof that the vaccine caused people to catch flu more easily. A higher recorded infection rate among vaccinated employees does not necessarily mean the vaccine made them more likely to fall ill. Several confounding factors could account for the finding.

For example, vaccinated healthcare workers may be more likely to seek medical care and get tested when they develop symptoms. This matters because the study measured effectiveness based on recorded positive tests, not on the total number of people who actually caught the flu. If unvaccinated employees were less likely to get tested or visit a doctor when sick, their infections may have gone uncounted. In other words, the study may partly reflect differences in testing behaviour rather than a direct biological effect of the vaccine itself.

 

How the flu vaccine works

The viral post also leaves out the main purpose of the flu vaccine. Robert H Hopkins Jr, medical director of the National Foundation for Infectious Diseases, a U.S.-based non-profit organisation that works to reduce the burden of infectious diseases through education and research, pointed out that the Cleveland Clinic study does not evaluate the primary reason most people are advised to get vaccinated: reducing the risk of severe illness, hospitalisation and death. Hopkins also noted that while vaccine effectiveness varies from year to year, vaccination can still make illness milder and prevent serious complications even when it does not prevent infection entirely.

The broader evidence supports this. For the same 2024–2025 flu season cited in the viral post, the U.S. CDC, drawing on multiple nationwide surveillance networks, estimated that flu vaccines reduced the risk of illness requiring medical care by 56%. That means for every 100 people who would otherwise have needed to see a doctor for flu, vaccination prevented around 56 of those cases.

Experts also stress that the flu shot cannot give a person influenza. Most flu vaccines use an inactivated form of the virus, meaning it cannot replicate in the body, making it biologically impossible for the vaccine itself to cause a flu infection. Some people may experience mild short-term side effects such as soreness, fatigue, or a low-grade fever, but these are signs that the immune system is responding to the vaccine, not evidence of infection.

Closer to home, a study by the National Centre for Infectious Diseases (NCID) and Tan Tock Seng Hospital found that elderly Singaporeans may be better protected with two flu vaccinations a year instead of one. With a second vaccination given six months after the first, the proportion of participants with antibody levels high enough to protect against all three influenza strains in the vaccine rose from 56.8% to 80.4%. Lead researcher Dr Barnaby Young, an infectious disease senior consultant at NCID, noted that the findings could help inform how frequently Singaporeans should get a flu jab, particularly given that unlike temperate countries, Singapore sees influenza circulating year-round, making sustained protection especially important for older adults.

Therefore, we rate the claim as mostly false. The Cleveland Clinic study did report a negative effectiveness estimate among one group of healthcare workers in a single season, but it is an unreviewed preprint that its own institution cautions against generalising. The post presents this finding as proof that flu vaccines increase infection risk, while ignoring confounding factors and the broader body of evidence showing that vaccination reduces severe illness, hospitalisation and death.

When we come across claims discouraging any medical treatment or health intervention, it is important to check what evidence they cite and whether it has been peer-reviewed, and when in doubt, refer to official health authorities or consult a medical professional directly.