The side effects experienced by people who received a third dose of either the Pfizer-BioNTech or Moderna Covid-19 vaccine were typically mild and similar to those experienced after a second dose, according to a study released Tuesday by CDC.

Are you ready for booster shots? Start thinking about these 6 factors now.

The side effects of booster shots

For the study, 12,591 people who received a third dose filled out surveys through a voluntary safety monitoring system. The study was conducted between Aug. 12 and Sept. 19, when only those with compromised immune systems were eligible for an extra shot. However, the study authors note it's possible others outside of this population also sought a booster shot during the study period.

'The Never Again Plan': Moderna CEO Stéphane Bancel wants to stop the next Covid-19—before it happens

According to CDC, there weren't enough people who reported receiving a booster shot of the Johnson & Johnson vaccine to study any potential side effects.

The study found the most commonly reported side effect involved reactions at the injection site, such as pain or swelling. This was reported by 79.4% of people following their third shot, compared to 77.6% following a second dose.

Fever or headache was reported by 74.1% of people following their booster shot, compared to 76.5% after a second dose.

"Most reported local and systemic reactions were mild to moderate, transient, and most frequently reported the day after vaccination," the study said.

CDC Director Rochelle Walensky said the study was a positive development and showed that third shots are "well tolerated."

"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.

More than 400,000 people received Covid-19 booster shots over the weekend

As many as 25 million Americans now qualify for a third, booster dose of the Pfizer-BioNTech vaccine. FDA authorized the shots last week for those 65 and older, those at high risk of severe Covid-19, and those facing an elevated risk of Covid-19 because of their jobs.

White House Covid-19 response coordinator Jeff Zients on Tuesday announced that more than 400,000 people received booster shots this past weekend. Roughly one million more Americans are currently signed up to receive a booster dose within the coming weeks.

"We're off to a very strong start with the booster campaign," Zients said.

Even as the country rolls out its booster shot campaign, health officials have said their main focus is getting the roughly 25% of unvaccinated Americans their first shot.

A particular priority, according to Walensky, is to increase the vaccination rate among pregnant women, which is currently 30% nationwide and at 15% among Black women. Walensky encouraged pregnant unvaccinated women to get their shots, saying research shows they're safe for both the mother and baby. (Pietsch/Suliman, Washington Post, 9/29; Cohen, Roll Call, 9/28; Mueller, New York Times, 9/28; AHA News, 9/28; Sullivan, The Hill, 9/28; AP/ABC4, 9/28)