Are you more likely to get the flu if you had a vaccine?

The McCullough Foundation says flu shots make you more likely to get the flu, according to a Cleveland Clinic study.

• Different testing rates may have affected the Cleveland Clinic study’s results.
• Past flu infections and vaccines may have shaped people’s protection.
• Flu shots cannot cause flu, though they can cause mild side effects.

This Cleveland Clinic study is posted as a preprint. That means it was shared before it completed peer review, when other experts check a paper’s methods and conclusions. Its findings should be treated as early results. It’s been over a year, and this article still has not been published. Let’s look at why that is.

One concern is that vaccinated and unvaccinated people may have been tested for flu at different rates. This is called testing bias. Vaccinated people were 27% more likely to get tested, so more of their flu cases could be found. Meanwhile, cases among unvaccinated people who did not get tested could be missed. That could make the vaccinated group appear to have more flu, even if the groups’ actual infection rates were similar.

The study followed 53,402 employees. About 82% were vaccinated, and 1,130 people (about 2%) had a recorded case of flu. These numbers describe the cases found during the study. They do not tell us how many infections went untested.

Another question is how earlier flu infections and vaccines may have affected the results. The immune system can remember viruses it has met before. An earlier infection or vaccine may offer some protection against a similar flu virus in a later season. This is called cross-protection. Without a full history of past infections and vaccinations, it is harder to know how much protection people already had when the study began.

Flu shots cannot cause the flu. They contain killed viruses or parts of a virus that cannot make copies of themselves and make you sick. These ingredients teach the immune system what to recognize. A shot can cause a sore arm, mild fever, or body aches as the immune system responds, but those side effects are different from a flu infection.