If You Get a Moderna Booster, Expect These Side Effects, FDA Report Says

·4 min read

Millions of people in the U.S. are already receiving booster shots, after the Food and Drug Administration (FDA) approved an additional dose of Pfizer for certain groups in late September. Now, the agency is scheduled to meet on Oct. 14 to discuss and vote on a third Moderna shot. Two days ahead of that meeting, the FDA published a report evaluating the safety and efficacy of Moderna's booster dose. The report utilizes data on booster shot side effects from a trial of nearly 200 recipients who were given an additional half-dose of the Moderna vaccine after two full doses.

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According to the FDA's report, injection site pain was the most common local adverse reaction, reported by 86 percent of those ages 18 to 65 and 76.3 percent of those 65 years and older. Lymphadenopathy, or swollen lymph nodes, was the second most common, but among participants 18 to 65 years old this side effect was significantly higher after the booster dose than after the second dose at 24.8 percent. Among those 65 and older, 5.3 percent reported swollen lymph nodes after their boost.

For people 65 years and older, "there were no notable differences or trends in the overall frequency of other solicited local reactions following the booster dose compared to after the second primary series dose," the report stated.

Other local reactions included swelling at the injection site for for 6.2 percent of those ages 18 to 65 and 2.6 percent of those 65 and older, and redness for 5.4 percent of the younger group and 2.6 percent of the older group.

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In terms of systemic reactions, those in the younger age group appeared to experience these side effects more frequently than people 65 and older. But for both age groups, fatigue was the most commonly reported systemic reaction, at 62 percent for those 18 to 65 and 47.4 percent for those 65 and older.

In the younger group, other systemic reactions included headache for 58.9 percent, muscle pain for 49.6 percent, joint pain for 41.9 percent, chills for 40.3 percent, nausea or vomiting for 12.4 percent, fever for 7 percent, and rash for 2.3 percent.

"Severe reactions after the booster dose were uncommon and generally reported by a numerically lower proportion of participants after the booster dose compared to after the second primary series dose. There were no notable differences or trends in the overall frequency of systemic reactions following the booster dose compared to after the second primary series dose in this age group," the FDA noted.

For the older group, muscle pain and joint pain were significantly more common after the booster dose than after the second dose, with 47.4 percent having muscle pain and 39.5 percent experiencing joint pain. There were no other notable differences in the frequency of systemic reactions for this age group. These other reactions included headache for 42.1 percent, chills for 18.4 percent, nausea or vomiting for 7.9 percent, and fever for 5.4 percent.

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Despite a largely similar side effect experience following the second dose and booster shot of Moderna, the FDA has not yet concluded if the benefit of the booster outweighs the risk, as Moderna's vaccine "still [affords] protection against severe COVID-19 disease and death in the United States."

"It is currently not known if there will be an increased risk of myocarditis/pericarditis or other adverse reactions after a booster dose of the Moderna COVID-19 vaccine. These risks and associated uncertainties have to be considered when assessing benefit and risk," the FDA wrote in its report.

Two senior FDA officials and the World Health Organization (WHO) released a study in The Lancet on Sept. 13, warning that getting a booster too soon may lead to the potential for more vaccine-induced side effects, like myocarditis, which is a rare heart inflammation condition that was more common after second doses of the mRNA vaccines than first doses. "If unnecessary boosting causes significant adverse reactions, there could be implications for vaccine acceptance that go beyond COVID-19 vaccines," the study authors warned. "Thus, widespread boosting should be undertaken only if there is clear evidence that it is appropriate."

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