Half of Moderna Booster Recipients Have These 4 Side Effects, CDC Says

Side effects from the third dose were more common after Moderna than after Pfizer.

At the end of September, the U.S Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) officially approved a booster shot of the Pfizer COVID vaccine for select groups of people in the U.S. The FDA is planning to discuss booster shots of the Moderna and Johnson & Johnson vaccines on Oct. 14. That means for people who didn't get Pfizer, there's little to do but wait. But even if you can't get a Moderna booster yet, you still probably want to know what vaccine reactions you should prepare for. Thankfully, the CDC has released recent trial data that reveals the most common side effects of the Moderna booster shot.

RELATED: If You Got Moderna, Dr. Fauci Warns Against Doing This.

In the Oct. 1 issue of the CDC's Morbidity and Mortality Weekly Report, the agency documented the responses of 212,191 participants who used the agency's v-safe smartphone surveillance system to report their side effects after getting a booster shot of any of the three vaccines between Aug. 12 and Sept. 19. Of those participants, 10,453 got three doses of Moderna. The data showed that local reactions—that is, reactions around the injection site—were more commonly reported after the third dose of Moderna than after the second. But systemic reactions, which occur throughout the body, were reported less frequently after the third dose of Moderna than the second.

According to the CDC's study, four side effects happened in at least half of the people who got three doses of Moderna. The most common side effect was pain at the injection site, with 75.9 percent of people reporting it. The second-most common side effect was fatigue, which 61.8 percent of people had. Muscle pain and headache had a similar number of reports, with 49.8 and 49 percent of participants reporting these side effects, respectively.

There were fewer side effects associated with the Pfizer vaccine's third shot than Moderna's booster, which is consistent with trends of the first and second shots. According to the data, only two side effects were reported in more than half of people who got three doses of Pfizer. The most common side effect after the third dose of Pfizer was pain at the injection site, which 66.6 percent of participants reported. The runner-up was fatigue, which 51 percent of recipients experienced.

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When the FDA publishes its report after approving or denying the authorization of Moderna booster shots, the common side effects will be further outlined. Some experts say Moderna recipients may not need boosters as urgently as those who got the Pfizer or Johnson & Johnson shots, because Moderna's protection is more durable. Per CDC data, after four months, Moderna's vaccine remained 92 percent effective at preventing hospitalization, while Pfizer's was 77 percent effective and Johnson & Johnson's was 68 percent, Business Insider reported.

Robert Atmar, MD, who is leading a COVID booster study at Baylor College of Medicine, told Business Insider that while he wouldn't be surprised if Johnson & Johnson recipients were recommended to get a booster shot soon, "for the Moderna [shot], it is an open question." The FDA's decision will come after its Oct. 14 meeting.

RELATED: The CDC Director Says If You Want a Booster, Don't Do This Right Now.

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