Some 22,191 booster recipients registered with a federal monitoring system starting Aug. 12, when the doses were first authorized on an emergency basis by U.S. regulators, through Sept. 19, the CDC said in a report on Tuesday. 

Overall, 71% reported pain at the injection site, while 56% reported fatigue and 43% said they experienced a headache after getting a third dose of a messenger RNA vaccine. The data were reported by recipients 0 to 7 days after a third dose. 

Regulators opted to make third doses of the shots made by Pfizer Inc.-BioNTech SE and Moderna Inc. available last month after data suggested that transplant recipients and other people with immune-system deficiencies weren’t being given adequate protection by the conventional two-shot regimen.

Last week, the Food and Drug Administration also authorized a third dose of the Pfizer vaccine for a broader group. Additional shots for recipients of the Moderna and Johnson & Johnson vaccines could be cleared soon, health officials have indicated.

Among those who reported their reactions to the monitoring system, about 1.8% said that they sought medical care, while 13 people said they were hospitalized. Specific reasons for receiving care weren’t reported in the survey.

“The frequency and type of side effects were similar to those seen after the second vaccine doses, and were mostly mild or moderate, and short-lived,” CDC Director Rochelle Walensky said at a briefing by the White House Coronavirus Task Force on Tuesday.

Little Mixing

The patterns of adverse reactions after a third dose for both the Moderna and Pfizer-BioNTech shots were consistent with those that have been described after a second dose. The so-called primary series for both of the messenger RNA vaccines involves two doses administered several weeks apart.

Debate over the merits of taking vaccine doses from more than one drugmaker has been growing since last week’s Pfizer clearance, but there weren’t enough data to determine if mixing causes adverse reactions. Nearly all of those who reported to the monitoring system said they stuck with the same manufacturer for all three doses. 

The findings are limited to people who self-reported. A majority identified themselves as White and non-Hispanic. 

Overall, approximately 2.21 million people in the U.S. had received additional vaccine doses as of Sept. 19, according to the CDC.

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