Booster dose side effects were similar to second dose, with brief, mild-to-moderate effects, study says

New data suggests booster doses of Covid-19 vaccines were well-tolerated, US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said Tuesday.

“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,” Walensky said during a White House Covid-19 briefing on Tuesday, when she described a new study published in CDC’s Morbidity and Mortality Weekly Report.

As of September 19, 2.21 million people in the United States have received additional doses of Covid-19 vaccine after their primary series, according to the new report. Among them, 22,191 made reports to CDC’s v-safe system, a voluntary, smartphone-based app that lets people report how they feel after they’ve been vaccinated. The new study includes reports from August 12, when the US Food and Drug Administration OK’d additional doses for certain immunocompromised people, and September 19.

President Joe Biden receives a COVID-19 booster shot during an event in the South Court Auditorium on the White House campus, Monday, Sept. 27, 2021, in Washington. (AP Photo/Evan Vucci)

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Among those 22,191 who made reports, about 7,000 – nearly 32% – reported any health impacts. More than 6,200 – about 28% – reported they were unable to perform normal daily activities, mostly commonly on the day after vaccination.

The most common complaints were injection site pain (71%), fatigue (56%) and a headache (43.4%). Of those who reported general pain, only about 7% described it as “severe.” Severe was defined as pain that makes “daily activities difficult or impossible.”

Nearly 2% said they sought medical care and 13 people were hospitalized, but it was not clear from the v-safe reports why these people sought medical care or were hospitalized. Those who sought medical attention are contacted by staff members from the Vaccine Adverse Event Reporting System and encouraged to make a report, it said.

Of the 12,591 people who tracked how they felt after each dose of an mRNA vaccine, 79.4% reported a local reaction to the third shot and 74.1% reported a systemic reaction. That’s similar to what people reported after a second dose, when 77.6% reported local reactions and 76.5% reported systemic reactions.

No unexpected patterns of adverse reactions were identified, the report said.

“The third Covid-19 shots appear to be safe and people experienced similar mild to moderate reactions, if any to the Covid-19 vaccine,” according to the report.

Some people reported getting a booster from different company than their original vaccine or getting a second dose of the single-dose Johnson & Johnson vaccine, but the report’s authors said the numbers in both cases were too small to draw any conclusions.