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Your Guide To Adult Vaccines

What Are the Side Effects of Booster Shots?

These are the most common symptoms experienced after receiving an extra dose

a healthcare worker administers a third dose of the Pfizer-BioNTech Covid-19 vaccine

Bloomberg / Getty Images

En español | Tens of millions of Americans can now get their COVID-19 booster shots from Pfizer, Moderna and Johnson & Johnson. As with the first round of vaccines, some people may experience a few mild to moderate flu-like symptoms after the shot. 

Here’s what we know so far about the side effects from the boosters: 

Pfizer

So much about this third shot, which is meant to rev up the immune system so that it stays sharp in the fight against COVID-19, will be just like the previous two. Pfizer’s third booster dose is the same formulation and the same strength as shots one and two. And data collected to this point suggests the side effects brought on by the booster are very similar to the symptoms some people experienced after the initial set — possibly even milder.

Pain at the injection site was the most commonly reported reaction after receiving the booster, according to the clinical trial data Pfizer and BioNTech submitted to the U.S. Food and Drug Administration (FDA). About 83 percent of the trial’s booster recipients reported it, followed by fatigue (63.7 percent) and headache (48.4 percent), most of which were mild to moderate. These findings closely mirror the side effect data collected from Pfizer’s second vaccine shot. Other side effects recorded in the booster trial also fall in line with symptoms documented after the primary Pfizer series. They include muscle and joint pain, chills, diarrhea, vomiting and fever. And Pfizer’s trial found that, compared to adults ages 18 to 55, adults 65-plus were less likely to experience these fatigue or flu-like symptoms after receiving the booster.

5 Common Side Effects After Pfizer's Booster

According to clinical trial data collected by Pfizer-BioNTech, the most commonly reported side effects after the booster shot were:

  • Injection site pain
  • Fatigue
  • Headache
  • Muscle pain
  • Chills

Source: Pfizer-BioNTech/FDA

That’s not surprising, says Melanie Swift, M.D., cochair of Mayo Clinic’s COVID-19 Vaccine Allocation and Distribution Workgroup. Most of the side effects are not a result of the vaccine, directly, but rather “an indication of your immune system reacting” to the vaccine, she notes. Meaning, the more robust your immune response is, “the more side effects you’re going to have.”

As people age, their immune responses typically dwindle, Swift says. This is why older adults are at the top of the list of people who should get a booster. New research suggests that while the vaccines continue to provide a strong defense against hospitalization and death from COVID-19, that protection wanes faster over time in older adults.

There were no reported cases of myocarditis, pericarditis, anaphylaxis, appendicitis or Bell’s palsy in the booster trial population (around 300 adults) during the study period. One symptom that the FDA highlighted, however: Swollen lymph nodes in the underarm were observed more frequently following the booster dose than after the primary two-dose series.

Moderna

5 Common Side Effects After Moderna Booster

According to clinical trial data collected by Moderna, the most commonly reported side effects after the booster shot were: 

  • Injection site pain 
  • Fatigue 
  • Headache 
  • Muscle pain 
  • Joint pain 

Source: Moderna/FDA

Unlike Pfizer, Moderna's booster shot is half the dose (50 micrograms) of the primary series. Even still, the side effects from this smaller-dose booster are similar to those reported after shot number two, the company says. For adults 65 and older, pain at the injection site was the most commonly reported symptom, affecting 76 percent of booster recipients, followed by fatigue (47.4 percent), muscle aches (47.4 percent), headache (42.1 percent) and joint pain (39.5 percent), an FDA review of Moderna’s clinical trial data shows. Chills, nausea and vomiting were also recorded.

Overall, older adults were less affected by side effects from Moderna’s booster shot than people ages 18 to 64.  No serious adverse events were reported during the window when the booster data was captured, and no new safety concerns were flagged during the trial.

Johnson & Johnson

5 Common Side Effects After J&J Booster

According to clinical trial data collected by J&J, the most commonly reported side effects after the booster shot were:

  • Injection site pain 
  • Fatigue 
  • Headache 
  • Muscle pain 
  • Nausea

Source: Johnson & Johnson /FDA

All 15 million Americans inoculated with the one-shot J&J vaccine are eligible for a second “booster” dose, at least two months after the first shot. 

Clinical trial data suggests that adults 60 and older who get the J&J booster two to three months after the first shot may experience pain at the injection site — about 40 percent of the participants 60-plus reported it. Roughly 29 percent experienced headache and fatigue, about 26 percent noted muscle pain, 12.4 percent had nausea, and 2.3 percent had a fever. The frequency of these side effects was similar to the first shot's side effects, albeit slightly less. 

The FDA analysis of J&J’s booster study shows that no new safety concerns were identified following the booster dose.

Who is eligible for a booster shot?

If you had a Pfizer or Moderna vaccine, you are eligible for a booster if:

  • You completed your primary series at least 6 months ago
  • You are 65 or older
  • You are 18+ and have an underlying medical condition
  • You are 18+ and work or live in high-risk settings 

If you had J&J, you are eligible for a booster if: 

  • You had your initial vaccine at least 2 months ago
  • You are 18 or older 

Source: CDC

Symptoms could be less severe after booster

If your last shot was a doozy, that doesn’t necessarily mean the booster dose will be equally as unpleasant. Real-world data from Israel’s booster program and from vaccine safety monitoring in the U.S. shows that reports of side effects have been “substantially lower” after the third dose of Pfizer’s COVID-19 vaccine than after dose one and two, according to a presentation given at the Sept. 22 Advisory Committee on Immunization Practices meeting — although this could be due to underreporting, researchers note.

And data from J&J’s booster trial shows that side effects were reported less frequently after the second shot, compared to the first. 

Robert Weber, administrator for pharmacy services at the Ohio State University Wexner Medical Center and assistant dean for medical center affairs at the Ohio State University College of Pharmacy, has been administering third doses of the Pfizer and Moderna vaccines to adults with certain immune-compromising conditions and says “many, if not all of them, are experiencing zero to very minor mild-to-moderate side effects,” mostly pain at the injection site and some fatigue.  


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He expects that the majority of the population eligible for the Pfizer and Moderna boosters — people 65 and older and those at greatest risk for COVID-19 due to underlying health conditions — will have similar experiences.

It helps, he says, that booster recipients are already familiar with the coronavirus vaccines: They know the symptoms to expect and are more prepared to take care of themselves after the shot. “Once you get the vaccine, that anxiety and that fear is lessened … and so when you get future shots, you're going to be more comfortable,” Weber says.

“I think people also understand how to manage the side effects better in terms of drinking plenty of water prior to getting a vaccine, drinking plenty of water after you get the vaccine, taking it easy the next day” and, if needed, taking an over-the-counter pain reliever after the vaccine to help manage any discomfort, he adds.

The time lapse between shot two and three for the mRNA vaccines, which the FDA says should be a minimum of six months, can also explain the milder side effects that some have experienced with the Pfizer and Moderna boosters, says Swift, who explains that “the farther away from the original series the booster is given, the less we expect side effects.”  

What's more, health officials have said it's OK for people to get a booster shot that's different from their original vaccine. So if your experience with side effects was rough the first time around, ask your doctor if switching booster brands could make a difference.

3 things to keep in mind

Whether your symptoms after the booster are similar to the previous shots or are more mellow, Swift says it’s important for people to know that side effects are “nothing to be alarmed about.” They are expected and they are temporary. To date, no long-term side effects related to the COVID-19 vaccines have been detected, the CDC says. Serious adverse events after vaccination have happened, but are rare.

Another sticking point boosters: Stay flexible. This booster could be the end of the COVID-19 vaccine series or it could be the start of a more regular schedule, like the annual flu shot. “This is all still such an evolving landscape,” Swift says.

And finally, while boosters have been the center of attention these last few weeks, it’s crucial to remember that “the most important shot is not shot three or four, it's shot one,” Swift says.

“We still have people out there who haven't had shot one, and that's much more critical to ending the pandemic than boosters. Boosters are icing on the cake. And I like icing on my cake, but we need the cake,” Swift adds.

What to Know About Myocarditis

While rare, cases of myocarditis — or inflammation of the heart muscle — and pericarditis —inflammation in the sac that surrounds the heart — have been reported in adolescents and young adults who have had the Pfizer or Moderna vaccines. Cases typically resolve on their own with care, and health officials say the benefits of the vaccine far outweigh the known risks, especially since a viral infection like COVID-19 can also cause myocarditis.

Myocarditis and pericarditis have the following symptoms:

  • Chest pain
  • Shortness of breath
  • Feelings of having a fast-beating, fluttering or pounding heart

Seek medical care if you or your child has any of these symptoms. Patients can usually return to their normal daily activities after their symptoms improve, the CDC says.

Source: CDC

Rachel Nania writes about health care and health policy for AARP. Previously she was a reporter and editor for WTOP Radio in Washington, D.C. A recipient of a Gracie Award and a regional Edward R. Murrow Award, she also participated in a dementia fellowship with the National Press Foundation.

Editor's Note: This story, originally published Sept. 24, has been updated to reflect new information.

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