If you’re one of the many people who now qualify for a COVID-19 booster shot, you’ve probably got some questions about what to expect after you get the third vaccine dose. Thankfully, the most common side effects after a booster shot appear to be very similar to the side effects seen after other doses of the COVID-19 vaccine, according to new data from the Centers for Disease Control and Prevention (CDC).
That new data comes from a study, published this week in the CDC’s Morbidity and Mortality Weekly Report, involving nearly 12,600 people who reported on their side effects after receiving the initial mRNA COVID-19 vaccine doses as well as the Pfizer booster shot. Specifically, these participants all used V-Safe, a voluntary CDC survey completed via smartphone, between August 12 and September 19, 2021. If someone indicates that they required medical attention after the vaccine, an employee from the CDC will call to get more information about what happened.
On August 12, the Food and Drug Administration (FDA) updated the emergency use authorization for the Moderna and Pfizer vaccines to allow immunocompromised people to get a third dose. And on September 23, the FDA updated Pfizer’s emergency use authorization to allow even more people to get boosters. Although the boosters were authorized only for immunocompromised people during the study period, the authors note that it’s likely other people also got them during this time.
In this study, about half of the participants received three doses of the Pfizer/BioNTech vaccine and the other half received three doses of the Moderna vaccine. There were a few people who got two doses of one vaccine initially and then got a different vaccine for their third dose, but the vast majority of participants got the same vaccine for all three doses.
Here are the most common side effects after COVID-19 booster shots that participants reported in this study:
- Pain at the injection site (66.6% in participants who received three Pfizer doses, 75.9% in participants who received three Moderna doses)
- Fatigue (51% for Pfizer, 61.8% for Moderna)
- Headache (38.4% for Pfizer, 49% for Moderna)
- Muscle aches (36.3% for Pfizer, 49.8% for Moderna)
- Joint pain (23% for Pfizer, 33% for Moderna)
- Fever (22.2% for Pfizer, 36.4% for Moderna)
Other less frequently reported side effects include chills, swelling at the injection site, nausea, vomiting, diarrhea, abdominal pain, rash, and redness at the injection site.
More people reported having reactions at the injection site after their third dose than their second, according to the data. But they also reported having fewer systemic reactions (such as fatigue and headaches) after their third dose than the second. About 22% of Pfizer recipients and 35% of Moderna recipients reported side effects so severe they couldn’t perform their daily activities, which was slightly less than the number of people who reported that after the second dose.