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What Are the Side Effects of Booster Shots?

What symptoms to expect after getting boosted with the new omicron vaccines ​

A new batch of COVID-19 booster shots is now available, and these latest vaccines — from Pfizer-BioNTech and Moderna — have been reformulated to better target the coronavirus variants that are currently circulating and will likely keep spreading this fall and winter. 

The revised boosters are similar to their now-retired predecessors, which were first made available about a year ago. However, instead of delivering a single set of instructions (called mRNA) that the body uses to build its own defense against the virus that causes COVID-19, they pass along two sets: one specific to the original strain of the coronavirus and one specific to subvariants of omicron (BA.4 and BA.5) that are behind nearly all new infections in the U.S.

Health officials are recommending that everyone ages 12 and older roll up their sleeves this fall for the added dose of protection against the illness that has taken more than a million American lives. Still, some people have questions about the new omicron boosters, including the side effects they can cause. Here’s what to expect if you go in for the jab. 

No surprises from common side effects

Health officials approached the approvals for the retooled omicron boosters a bit differently this time around, since clinical trials testing the BA.4 and BA.5-specific shots are ongoing. (This is not an uncommon process for vaccines that need a strain update, much like the annual flu shot.)  

Instead, they relied on data from the first round of booster shots and from a very similar bivalent COVID-19 vaccine — one that targets the original coronavirus strain and also BA.1, an omicron subvariant that “only differs slightly from BA.4 and BA.5,” said Peter Marks, M.D., director of the Center for Biologics Evaluation and Research at the U.S. Food and Drug Administration (FDA). Experts don’t expect these minor mutations will have any effect on the vaccine’s safety profile. 

Looking at the side effects of boosters in these trials, there were no surprises. “All of the side effects from a bivalent booster were very similar to what we saw with the regular booster, and even going back to the initial vaccination,” said Andrew Pekosz, a virologist at Johns Hopkins Bloomberg School of Public Health. 

Moderna booster side effects

Pain at the injection site was the most commonly reported side effect among people vaccinated with Moderna’s bivalent omicron booster, CDC data show. About 80 percent of trial participants reported it, followed by fatigue, headache, muscle and joint pain, chills, nausea and vomiting, and fever. No severe adverse events were seen. 

For comparison’s sake, the five most commonly reported side effects following a dose of Moderna’s original COVID-19 booster were injection site pain, fatigue, headache, muscle pain and joint pain. 

Pfizer booster side effects

Pain at the injection site was also the most common complaint among people boosted with Pfizer’s updated vaccine; about 60 percent of trial participants reported it. Other typical side effects included fatigue, headache, muscle pain, chills, joint pain, diarrhea, fever and vomiting — all were seen with Pfizer’s original COVID-19 booster too. 

And just like with Moderna’s bivalent booster trial, no adverse events were reported in the Pfizer trial, though experts say they will continue to keep an eye on the rare risk of myocarditis in both Pfizer and Moderna booster recipients. This condition — an inflammation of the heart muscle that can weaken the heart — has occurred mostly in teens and younger adults.

One thing to note: Pfizer and Moderna are the only COVID-19 vaccine makers in the U.S. with these new bivalent boosters, however, not everyone is eligible for them at this time. Currently, Pfizer's booster is authorized for people 12 and older; adults 18 and up can get the Moderna booster. Health officials have said that they will work quickly to evaluate the data on the updated boosters for younger populations as it comes in. 

Johnson & Johnson’s booster hasn’t been retooled to target omicron, and the CDC continues to recommend the Pfizer and Moderna options over a J&J vaccine, due to a rare but serious complication involving a blood clotting disorder. 

Novavax, another company that brought its two-shot COVID-19 vaccine to the market in June, has said it’s in the process of testing an omicron version of its vaccine; initial results could be available this fall. But for now, the Pfizer and Moderna boosters are the only updated booster options available, and it’s recommended that all fully vaccinated adults get one, as long as it’s been at least two months since your last COVID-19 vaccine or booster. (And it doesn't matter what brand you had before.)

COVID-19 is still sending more than 4,000 Americans to the hospital each day, and experts expect the new boosters will help prevent that number from spiking during a time of year when respiratory illnesses tend to thrive. In fact, projections show that if the same number of people who get the annual flu shot get an updated COVID-19 booster this fall, it could prevent as many as 100,000 hospitalizations and 9,000 deaths. 

Alleviating booster side effects  

The good news: If you’ve been vaccinated and boosted with an mRNA vaccine (and millions of Americans have), you likely already know what to expect and how your body may react. 

“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,” 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, told AARP in a previous interview.

Taking an over-the-counter pain reliever after the vaccine can help manage any discomfort, he added. Just know it’s not advised to take these medicines ahead of time to try to prevent potential side effects. And applying a clean, cool, wet washcloth to the injection site can help to relieve pain or swelling on the arm where you got the shot, the CDC says.

Not everyone experiences side effects after a COVID-19 vaccine or booster, and for most who do, symptoms tend to be mild to moderate and go away in a few days. That said, if your side effects are worrying or linger longer, it’s important to call your doctor. And while allergic reactions are rare, they can occur, and should be treated immediately.

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