Does Getting a COVID Booster and a Flu Shot Together Make Side Effects Worse?

A recent CDC study showed that getting a flu shot and an mRNA COVID-19 booster at the same time was associated with an 8%-11% increased chance of side effects, compared to receiving a COVID-19 booster alone.

Doubling up on the flu shot and COVID-19 vaccine can protect against both viruses. But is it still safe to get them on the same day?

Anne Hause, PhD, MSPH, a CDC researcher who led the study, wrote in an email to Verywell that getting two vaccines at once introduces a higher amount of antigens than a single shot, likely causing the immune system to be more reactive.

However, the side effects from flu shots and COVID-19 vaccines tend to be mild and resolve quickly, Hause said, and the CDC continues to recommend getting the shots together if eligible.

“CDC recommends people stay up-to-date with their COVID-19 vaccination and receive a booster dose or second booster dose as soon as they are eligible,” Hause said. “Likewise, people should receive an influenza vaccine at the beginning of the influenza season. These two vaccines may be given together if a person is eligible to receive them.”

Do Two Shots Hurt More Than One?

Edward Belongia, MD, director of the Center for Clinical Epidemiology and Population Health at Marshfield Clinic Research Institute, told Verywell that he doubts the statistical significance of the CDC review translates to an increased risk in real life.

“A statistical difference is not the same as a meaningful difference,” Belongia said. “Their study was so large, they had hundreds of thousands of people, so even very, very tiny differences can be statistically significant, but they’re not meaningful.”

Belongia said the data showed similar outcomes for both vaccine regimens. For example, nearly the same percentage of respondents who simultaneously received a flu shot and a Pfizer booster reported having “any systemic reaction” as those who received only the Pfizer booster.

“This study just emphasizes that getting the vaccines together doesn’t pose any meaningful increased risk of having either local or systemic side effects,” Belongia said. “People should not have any concern about getting them together, it’s fine to get them together.”

He encourages people to get both shots at once, especially if they are struggling to schedule appointments.

“There are important reasons not to delay the vaccine,” he said. “If you have an opportunity to get both at the same time, we would really strongly encourage people to do that.”

People who are getting two shots at once could ask to be injected in different arms, he added, although that might be up to personal preference.

“If I ever get a choice, I choose to get them in different arms,” Belongia said. “I’d rather have a slight discomfort in two arms than a lot of discomfort in one.”

An ongoing clinical trial from the CDC is looking to further answer questions about vaccine side effects. Researchers are comparing side effects from flu and COVID-19 vaccinations given on the same day versus given two weeks apart.

While this trial is still recruiting participants, it should be able to eventually examine how vaccine side effects differ and help guide recommendations on future booster rollouts.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.