Side effects after receiving a COVID-19 booster shot appear similar to those after receiving a second dose of the vaccine, described as "mild to moderate," according to the U.S. Centers for Disease Control and Prevention.
Findings from a study were published Tuesday in the CDC's Morbidity and Mortality Weekly Report, sharing an analysis of responses from more than 12,500 Americans who received an additional dose of either the Pfizer or Moderna vaccine.
At the time of the study, which encompassed data from Aug. 12 to Sept. 19, only a third dose of Pfizer or Moderna’s vaccine was authorized in the U.S. for people with severely weakened immune systems. On Sept. 23, the CDC endorsed just Pfizer’s booster shot for millions of older and otherwise vulnerable Americans.
The agency looked at responses from 12,591 Americans who had completed the CDC's "V-safe" survey after receiving the third dose. Most (97.6%) reported receiving a third dose of the same vaccine administered previously.
Reactions at the injection site, like redness or swelling, were reported by 79.4% of people after the booster dose, compared to 77.6% after a second dose. Systemic reactions, such as fever, fatigue or headache, were reported by 74.1% of people following their third dose compared to 76.5% after the second.
"Most reported local and systemic reactions were mild to moderate, transient, and most frequently reported the day after vaccination. Most registrants who received an additional dose reported a primary mRNA vaccination series followed by a third dose from the same manufacturer," the study authors wrote.
The CDC’s study results followed findings from Pfizer-BioNTech’s small clinical trial of booster doses, which were reviewed by advisers to the U.S. Food and Drug Administration and CDC last week. The trial, which included 306 people ages 18 to 55, also found adverse reactions were similar following the second and third doses.
The CDC study authors noted that although the timeframe covered only when immunocompromised people were eligible for booster shots, the results indicated that other people were also likely included — such as health care workers and older adults who were prioritized during the initial rollout of COVID-19 vaccines
More than 2.2 million people in the U.S. had received a COVID-19 booster shot by the end of the CDC’s study period on Sept. 19, the authors wrote.
President Joe Biden, who is 78, received his booster dose on Monday live on camera in an effort to encourage others who are eligible to do the same. The president said he didn’t have any side effects from the first two shots.
Vice President Kamala Harris, 56, received the Moderna vaccine, for which federal regulators have not yet authorized boosters beyond those who are immunocompromised — but they are expected to in the coming weeks.
Data on Johnson & Johnson's boosters are also under review.
This story was reported from Cincinnati.