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En español | ​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 some of the more recently circulating coronavirus variants. ​

The revised boosters are similar to their now-retired predecessors, which were first made available about a year ago. Only, 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 omicron subvariants BA.4 and BA.5. ​

Health officials are recommending that fully vaccinated individuals 5 years and older roll up their sleeves for the added dose of protection against the illness that has taken more than 1.08 million American lives. Children as young as 6 months old are also eligible for an updated booster if they were vaccinated with Moderna.​

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, also known as bivalent vaccines, a bit differently this time around, since clinical trials testing the BA.4- and BA.5-specific shots were still happening at the time of their authorization. (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 to 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 the 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 shows. 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.

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, is currently testing an omicron version of its vaccine.

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.

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.

Another tip: If your schedule allows, clear your calendar the day after your booster. “If you work during the week, you might want to get vaccinated on Friday or Saturday so that you don’t have to work when you’re not feeling great,” said Kawsar Talaat, M.D., an associate professor in the Department of International Health at the Johns Hopkins Bloomberg School of Public Health. “Just know that one day of not feeling well from the vaccine is far better than several days of not feeling well with COVID,” she added.

Not everyone experiences side effects after a COVID-19 vaccine or booster — some people even find that the side effects from the bivalent boosters are milder than those from previous shots in the primary series, Talaat said. 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. ​

Boosters are key to preventing severe illness ​

COVID-19 is still sending nearly 5,000 Americans to the hospital each day, and experts say the new boosters can help prevent that number from swelling even more this winter, especially as other respiratory viruses rip through the country.

“With flu and RSV co-circulating, there’s just so much respiratory virus out there right now … and the more you can do to protect yourself by getting the COVID booster, by getting the flu vaccine and wearing masks in large gatherings, will help you stay healthy,” Talaat said. “We also know that you can get more than one virus at the same time and that people who do get multiple viruses tend to be sicker. And so again, anything you can do to prevent infection is good.” ​​​

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, 2021, has been updated to reflect new information.

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