The Covid-19 vaccine booster shot's side effects appear to largely mirror how people felt after their second dose, according to a study published Tuesday by the Centers for Disease Control and Prevention.
The report found that the side effects were mostly considered mild or moderate, and arm pain, fatigue and headache were the most commonly reported symptoms after the third shot.
The side effects kicked in generally the day after the injection, the report found, and 28 percent of people said they were unable to perform normal daily activities because of them.
Nearly 2.8 million people in the United States have received a booster shot since mid-August, when additional doses of the mRNA vaccines, both from Pfizer-BioNTech and Moderna, were first authorized for people with compromised immune systems.
Last week, a booster shot was recommended for an even larger segment of the population: People originally inoculated with the Pfizer vaccine either ages 65 or older or at higher risk of Covid due to underlying conditions or their occupation.
The new report looked at data from 22,191 people who received a booster dose and responded to questions on a CDC-run smartphone app called v-safe.
The vast majority of respondents reported initially getting the Pfizer or the Moderna vaccine, and nearly all got the same booster vaccine as their initial vaccine.
Arm pain was slightly more common after the third shot than the second, the report found, and systemic reactions, such as headache or fatigue, were slightly less common after the third shot compared with the second.
"This latest report includes some of the data of our early experience with third doses," CDC Director Dr. Rochelle Walensky said Tuesday during a White House Covid task force briefing.
"The frequency and types of side effects were similar to those seen after the second vaccine doses, and were mostly mild or moderate and short-lived," she added.
Dr. Helen Keipp Talbot, an associate professor of medicine at Vanderbilt University Medical Center in Nashville, said she was not surprised by the apparent safety of the third dose.
Talbot is a member of the CDC's independent panel, called the Advisory Committee on Immunization Practices, which convened last week to discuss a Pfizer booster dose. She noted the group had seen and discussed the newly published data during its two-day meeting.
"We are very comfortable with the safety of these vaccines. They’ve been given to millions and millions and millions and millions of people," she said.
"But that doesn’t mean we’re going to stop monitoring and stop looking" for adverse events, she said. "We’re always going to be cautious and careful."