Here are the side effects you can expect from a COVID-19 vaccine booster shot

The topic is increasingly relevant as more people become eligible for booster shots. So far, more than 30 million people have received one.

Johnson & Johnson vaccine recipients are eligible for a booster two months after their first dose. Adults who received Pfizer-BioNTech or Moderna are eligible six months after their second dose if they are 65 or older, have an underlying health condition, or work or live in a high-risk environment.

Although the FDA and CDC have yet to approve booster eligibility for all adults, the answer could come before Thanksgiving. In the meantime, the commissioner of the NYC Department of Health and Mental Hygiene, Dr. Dave Chokshi, said this Monday that all NYC adults are eligible for a booster shot, especially as the holiday season approaches. Colorado, California, and New Mexico similarly allow all adults to get boosters.

The side effects of boosters are similar across all three vaccine brands. They include muscle and joint aches, swelling or soreness at the injection site, fatigue, headache, chills or fever, and swollen lymph nodes, according to Johns Hopkins Medicine. Underarm lymph node swelling is more likely to stem from the Pfizer booster than from the others, the FDA says. 

Drug trial data released on Sept. 28 (excluding patients who received two doses of the J&J vaccine) show that 74.9% of people who received a third dose of the COVID-19 vaccine reported local reactions such as pain or swelling at the injection site. Meanwhile, nearly 70% reported systemic reactions like fever or loss of appetite, mostly on the day following their vaccination. The most common specific reactions were pain at the injection site (71%), fatigue (56%), and headache (43.4%). For most people, those side effects were fleeting, and mild or moderate in severity.

Of the 6,200 participants in the test who received three doses of Moderna, local reactions were more common after the booster than after the second dose (from 83.5% to 84.7%), and systemic reactions were less common (from 79% to 81.3%).

Among the 6,300 three-time Pfizer dose recipients in the trial, a similar trend was reported. Local reactions increased slightly (from 71.7% after the second dose to 74.1% after the booster), and systemic reactions modestly declined (from 71.7% after the second dose to 69.2% after the booster). 

In its Phase III drug trial data released on Sept. 21, Johnson & Johnson noted that recipients of its booster reported fewer local and systemic side effects than participants in its Phase I/II study in January 2021. Those who were initially vaccinated with the J&J vaccine benefit the most from a booster shot, because the first dose is less effective than the Pfizer and Moderna vaccines. While all three boosters are successful, the CDC lists J&J’s “vaccine effectiveness” or “VE” at 71%, while Pfizer’s rate is 88% and Moderna’s 93%. “Now we’re seeing what was approved as a one-dose vaccine being converted to a two-dose vaccine,” John Moore, professor of microbiology and immunology at Weill Cornell Medicine, said of the J&J vaccine.

For people who get boosters, feeling under the weather in the days following is the most likely outcome, the CDC says. Most reactions are merely a sign that your body is “building protection,” and the negative impact usually disappears in a few days, the CDC notes.

Although unlikely, serious reactions are still possible:

So far, with the Johnson & Johnson vaccine, anaphylaxis (a severe allergic reaction like a skin rash or difficulty breathing) and thrombosis (blood clotting) occurred in only seven cases per 1 million vaccinated women, 18 to 49 years of age. The American Society of Hematology confirms that the likelihood of vaccine-induced thrombosis is extremely low. Also linked to the J&J vaccine is the neurological disorder Guillain-Barré syndrome (GBS), although about 100 in 12.8 million cases is not enough to confirm causation, the FDA said in July. Blood clots have also been reported on rare occasions, mostly in women ages 18 to 49 a week or two after vaccination, J&J says.

A preliminary report on a National Institutes of Health study that hasn’t been peer-reviewed found that people who received the Moderna booster were slightly more likely to experience an adverse reaction such as vomiting, fatigue, or insomnia at 15.6% than those who received Pfizer (14.3%) or J&J (12%). Most described the severity as a two out of five. A small number of cases of myocarditis (heart muscle inflammation) and pericarditis (inflammation of the heart’s outer lining) have also been reported in relation to both the Pfizer and Moderna vaccines, mostly in young men. In a recent Israeli study of 5.4 million people over the age of 16, both vaccinated and unvaccinated, 283 people reported “probable or definite myocarditis.” Of those, only 142 cases were related to the Pfizer vaccine, and 95% were “mild.” After medical treatment, most who experienced inflammation recovered quickly, the CDC reports.

If you do experience side effects from COVID booster shots, Yale New Haven Health recommends over-the-counter pain treatment, an ice pack for swelling, and a cool bath. If symptoms persist for more than 72 hours, people should contact their doctor.

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