What Are COVID Booster Shot Side Effects Like?

COVID-19 booster shots are now authorized for all individuals in the U.S. who are 5 years and older.

On November 19, the Food and Drug Administration (FDA) amended its emergency use authorization (EUA) for both the Pfizer and Moderna COVID-19 vaccines, approving the use of a single a booster dose for all persons 18 years and older who completed the primary series. The agency had previously authorized a booster for all adults who received one shot of the Johnson & Johnson vaccine and for specific groups of people at increased risk for COVID-19 exposure or severe illness.

On December 9, the FDA expanded eligibility for the Pfizer booster to 16- and 17-year-olds. Eligibility was later expanded to include children and adolescents 5 through 15 years of age.

According to the CDC, eligible adults are able to choose any authorized COVID-19 booster. However, it is now recommended that individuals get the Pfizer or Moderna mRNA COVID-19 vaccine over the Johnson & Johnson vaccine, following concerns about blood-clotting side effects. The Johnson & Johnson shot still remains an option for those who are not able or willing to get a different vaccine.

Children and adolescents ages 5 to 17 who completed Pfizer's primary series are only eligible for the Pfizer booster.

The CDC is also recommending a second booster dose (of either the Pfizer or Moderna COVID-19 vaccines) for the following people who received their initial booster shot at least four months prior:

• Adults ages 50 years and older
• Individuals who are moderately or severely immunocompromised
• Those who received the Johnson & Johnson COVID-19 vaccine for their first shot and booster

The Pfizer and Johnson & Johnson boosters will be administered with the same dosage as the initial vaccine, whereas Moderna’s will be a half dose (50 micrograms).

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.

What the Data Shows So Far

A recent study published by the CDC, which examined data collected between August 12 and September 19 from 22,191 people who received an additional dose of the mRNA COVID-19 vaccine, found no unexpected patterns of adverse reactions.

Of those who received a third dose of the mRNA COVID-19 vaccine, 79.4% and 74.1% of recipients reported local or systemic reactions, respectively—most of which were mild to moderate. 77.6% and 76.5% reported local or systemic reactions after the second dose, respectively.

The most commonly reported reactions were injection site pain, fatigue, and headache.

According to the CDC, the side effects reported after the third dose of the mRNA vaccine are similar to those reported after the second dose.

Similarly, clinical trial data showed that side effects following a booster dose of the Johnson & Johnson vaccine are comparable to those experienced after the first dose. The most commonly reported side effects were injection site pain, fatigue, headache, muscle pain, and nausea.

Other limited studies have reported similar findings.

In their application for Emergency Use Authorization for 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.

Although Moderna's booster is half the dose of the initial vaccine, a lower dose doesn’t necessarily mean fewer side effects. In Moderna's application for authorization of a booster shot (announced on September 1), the company said that the side effects after a third shot are comparable to those reported after the primary series.

Additionally, no new safety concerns have been reported following a second booster dose of either mRNA COVID-19 vaccine.

While current data shows that COVID-19 boosters are relatively safe, rare and serious adverse events have been reported after vaccination.

Myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining outside the heart) have occurred following vaccination with the Moderna and Pfizer COVID-19 vaccines—particularly after the second dose. Additionally, there has been an observed increased risk of thrombocytopenia syndrome and Guillain-Barré syndrome following vaccination with the Johnson & Johnson COVID-19 shot.

As both the FDA and CDC continue to monitor such cases, they emphasize that the potential benefits of the vaccine continue to outweigh this very low risk.

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, more than 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, almost 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 myocarditis and pericarditis.

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.