What Will COVID Booster Shot Side Effects Be Like?

person at vaccination site sitting with nurse while others wait

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Key Takeaways

  • Studies about the side effects associated with COVID-19 booster shots are still in the works, and additional doses have not yet been cleared for most vaccinated Americans.
  • Side effects are expected to be mild or moderate, with most people experiencing symptoms similar to after the initial doses of mRNA vaccine.
  • The most commonly reported side effects include injection-site pain and swelling, fatigue, headaches, chills and fever.

If COVID-19 booster shots are approved by the Food and Drug Administration (FDA) and recommended by the Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices, most people will be eligible for a third dose of an mRNA vaccine eight months after their second shot. Already, some people are worried about the possible side effects of a booster shot, especially after experiencing fatigue, muscle pain and fever from the initial doses.

Clinical studies about the safety and efficacy of additional doses are still ongoing and only a few countries have begun administering extra doses, meaning there is little real-world data about the effects of boosters yet.

But the data gathered so far suggests that people should not expect severe side effects.

In its recommendation for immunocompromised people, the CDC says the safety, efficacy and benefits of an additional dose is still being tested.

“So far, reactions reported after the third mRNA dose were similar to that of the two-dose series: fatigue and pain at [the] injection site were the most commonly reported side effects, and overall, most symptoms were mild to moderate,” the CDC says.

What the Data Shows So Far

In their application for Emergency Use Authorization for filing a third dose, Pfizer-BioNTech cited a study on additional doses for organ transplant patients. The side effects after a third dose were similar to those following a second dose, and none of the recipients reported serious effects one month later.

In a press release about its application for a booster dose, Pfizer says the frequency of side effects was “similar to or better than” after the second dose.

The most common side effects for people receiving an mRNA vaccine include:

  • Redness and swelling around the injection
  • Tiredness
  • Headache
  • Muscle and joint pain
  • Chills

Moderna is asking the FDA to authorize a 50-microgram dose of its COVID-19 vaccine for booster shots, which is half the dose given in the first two shots, The Wall Street Journal reported. People familiar with the booster trials say that both doses produce a strong immune response, according to the story.

But a lower dose doesn't necessarily mean fewer side effects. In its application for authorization of a booster shot, announced on September 1, the company says that the side effects after a third dose are comparable to those reported after the primary series.

“The incidence of these immune-mediated responses—which actually mean that your immune system is doing a good thing, but which are unpleasant—wasn’t much higher than what we’re seeing with a second dose of the vaccine,” Kate Mullane, DO, PharmD, a professor of medicine and infectious disease specialist at the University of Chicago, tells Verywell.

Israel has started administering booster doses to people 12 years and older who were fully vaccinated at least five months earlier. A group of Israeli researchers looked at the efficacy of a third dose of the Pfizer vaccine in people 60 years and older, but did not share data on side effects after the booster in its preprint study.

Vaccine Hesitancy

Throughout the pandemic, people have cited worries about vaccine side effects as a major reason for foregoing the shot.

In a Kaiser Family Foundation survey from June, half of the polled unvaccinated adults said they were concerned about side effects of the first- and second-dose COVID-19 vaccines, and one in ten say that is the primary reason for not being vaccinated. In an earlier poll, half of respondents expressed concerns about having to take time off work if the vaccine side effects make them feel sick.

In most cases, feeling a bit sick after vaccination is a sign that the vaccine is working. The immune system must build an antibody response to best combat the virus if it enters the body, and feeling injection-site pain, fever, fatigue and headache can indicate that this process is happening.

Data is still needed on the likelihood of rarer, more serious outcomes from booster shots, like heart inflammation called myocarditis and pericarditis.

What This Means For You

Early studies suggest that while side effects will vary by person, most people will experience only minor or moderate discomfort after the third dose of an mRNA vaccine. Any side effects are expected to diminish within 48 hours. In some places, like New York state, employers are required to grant workers paid time off to recover from vaccine side effects. Talk to your doctor and employer if you’re concerned about the side effects of a booster shot and how to plan for them.

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.

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Article Sources
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  1. Centers for Disease Control and Prevention. COVID-19 vaccines for moderately to severely immunocompromised people. Updated August 27, 2021.

  2. Centers for Disease Control and Prevention. Possible side effects after getting a COVID-19 vaccine. Updated August 6, 2021.

  3. Bar-On YM, Goldberg Y, Mandel M, et al. BNT162b2 vaccine booster dose protection: A nationwide study from Israel. MedRxiv. Preprint posted online August 31, 2021. doi:10.1101/2021.08.27.21262679

  4. Kaiser Family Foundation. KFF COVID-19 vaccine monitor: June 2021. Published June 30, 2021.

  5. Kaiser Family Foundation. KFF COVID-19 vaccine monitor: April 2021. Published May 6, 2021.