This claim has been circulating again on social media. Current science suggests it is false. Blood clots were a rare side effect in patients who received the vector adenovirus COVID-19 vaccines by AstraZeneca and Johnson & Johnson. The AstraZeneca vaccine was never authorized for use in the U.S. and use of the Johnson & Johnson vaccine in the U.S. was halted in 2023. Multiple studies show no relationship between the mRNA vaccines from Moderna and Pfizer-BioNTech and blood clots.

Three types of COVID-19 vaccines have been distributed in the U.S. These three types are mRNA (Pfizer-BioNTech, Moderna), vector or adenovirus (Johnson & Johnson), and protein subunit (NovoVax). Johnson & Johnson shots have not been distributed since April 2021.

In the rare cases where patients developed blood clots as a result of the Johnson & Johnson vaccine, people had an antibody present in their blood that interacted with the adenovirus vaccine to cause clotting. There were no predictors of who would have this antibody, and the vaccines were taken off the market due to the potential severity of the side effects and its unpredictability.

In some studies of mRNA vaccines, blood clots did occur in people after vaccination. However, these results showed that the number of people who got blood clots after receiving the vaccine was not greater than the number of people who could have gotten blood clots without the vaccine.

An additional type of vaccine, NovoVax, became available in the U.S. in 2022. This is a protein subunit vaccine. The first protein subunit vaccine in the U.S. was Hepatitis B, approved 30 years ago. Studies of NovoVax are ongoing, with no serious adverse effects found to date.

The risk of blood clotting from a COVID-19 infection among unvaccinated people was still greater than the risk of getting blood clots after the Johnson & Johnson vaccination. People who received the Johnson & Johnson vaccination in 2020-2021 are not at higher risk of getting blood clots today.

Blood clotting is helpful for stopping bleeding, but if clots form in the leg, lungs, or brain they can lead to a heart attack or stroke.

Scientists continue to study the effectiveness and side effects associated with the different types of COVID-19 vaccines, and additional information will be published on the CDC website as it becomes available.