With the emergence of the rapidly spreading omicron variant, COVID-19 booster shots are now strongly recommended by the Center for Disease Control and Prevention.
"Everyone ages 18 and older should get a booster shot ... when they are six months after their initial Pfizer or Moderna series," CDC Director Dr. Rochelle Walensky said in November.
"The recent emergence of the omicron variant (B.1.1.529) further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against COVID-19," she added.
The World Health Organization echoes that sentiment, saying the omicron variant is highly contagious, and that "preliminary evidence suggests there may be an increased risk of reinfection."
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Here's what the CDC says about side effects of each booster shot currently available:
What are the most common side effects of the Pfizer or Moderna COVID booster shot?
The most common side effects reported after getting a third shot of an mRNA vaccine, the type made by Moderna and Pfizer, were pain at the injection site, fatigue, muscle pain, headache and fever, followed by chills and nausea, according to data from the CDC.
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"With the Pfizer booster, and I think this has been confirmed, there appears to be a fair amount of swelling of the lymph nodes in the underarm area on the side of the vaccine," Dr. Shira Doron of Tufts Medical Center said. "I'm certainly hearing a lot about that, you know, and it may be more than the first and second shot but it is harmless."
What are the most common side effects of the Johnson & Johnson booster shot?
The data available for Johnson & Johnson was more limited, but people reported fever, fatigue and headache after receiving a second dose of that vaccine, according to the agency.
How strong are side effects from COVID booster shots?
Overall, the CDC says that so far, reactions reported after getting a booster shot were similar to those after the two-dose or single-dose primary series.
Fever, headache, fatigue and pain at the injection site were the most commonly reported side effects, and overall, most side effects were mild to moderate. However, as with the two-dose or single-dose primary series, serious side effects are rare, but may occur.
"The side effects of the booster are exactly the same as the side effects of the first and second shot," Dr. Daniel Kuritzkes of Brigham and Women's Hospital said. "It's kind of feeling crummy for a day or so."
Which booster shot should you get, based on your first COVID vaccine dose?
Federal regulators have recommended getting the same shot as your first dose for booster doses, and that applies particularly to those who got a Moderna or Pfizer vaccine.
Some advisers have said they would prefer that Johnson & Johnson recipients receive a competitor's booster, citing preliminary data from an ongoing government study that suggested a bigger boost in virus-fighting antibodies from that combination.
What's the difference between the Moderna and Pfizer booster shot?
According to the CDC, Pfizer and Johnson & Johnson COVID boosters are the same dosage as the first round of shots.
Moderna, however, is half the dose of the vaccine used in the initial series.
The reason why Moderna is a half dose is because Moderna had a higher dose of the mRNA the only active part of the vaccine to start with. So it's part of why the side effects are sometimes a little higher and the folks who have Moderna.