New Omicron Booster Side Effects: What to Expect From the Bivalent Vaccines

Experts say the side effects from the new booster doses are expected to be very similar to other COVID-19 shots.

The data is still rolling in, but experts say people can expect to have generally the same reaction to the new bivalent vaccines as they did the first generation COVID-19 vaccines.

“There’s been nothing that has been brought up that is any different from the side effects people experienced with previous boosters,” Andrew Pekosz, PhD, a virologist at Johns Hopkins Bloomberg School of Public Health, told Health.

The new boosters—made by Pfizer and Moderna—are bivalent mRNA vaccines, meaning they contain two different components of SARS-CoV-2, the virus that causes COVID. One helps the body recognize the original virus while the second one is a mutation found in the Omicron variants. The total vaccine dose, however, is the same as the old boosters, said Pekosz.

The boosters are not live vaccines and do not contain any amount of infectious virus. Intead, they contain a sort of road map that allows the immune system to recognize and fight off SARS-CoV-2 if it comes into contact with the virus. The temporary side effects people have after being vaccinated—including chills, fever, headache and body aches—are an immune response rather than an illness.

“Your body is being told that it needs to learn how to respond if it comes across this virus. It requires tickling your body’s own immune system to get it to respond to that little bit of a tickle, and that often feels like getting sick,” William Messer, MD, PhD, an associate professor in Oregon Health & Science University’s Department of Molecular Microbiology and Immunology, told Health.

Better Protection Against New Variants

Since the new boosters contain parts of the most recently circulating version of the virus, they provide the most up-to-date protection against severe illness and death in practice.

“This is the ideal situation where you are vaccinating with almost the identical virus as what is circulating,” Pekosz said.

Although Pekosz said there is a chance the well-matched vaccines will reduce risk of infection, the primary goal of vaccination is to protect against death and severe illness that requires hospitalization and overwhelms the healthcare system.

“It’s important to note that there are still 400 people a day dying of COVID-19 in the U.S.,” said Pekosz. “The virus is here, so if you are eligible for the vaccine, you should go get it.”

Side Effects of the COVID Boosters

Pekosz said that although it’s not a perfect predictor, people can generally gauge how they will react to the booster based on how they reacted to past doses of COVID vaccines.

“For the most part, people have said that if you had a response previously, those people tend to have a response again for a booster,” he said.

Dr. Messer said that very few people have no reaction at all, but that “you shouldn’t count it as being exactly the same as the last one.”

Pekosz advises people who are worried about feeling down for a day or two after getting a booster to try to schedule it when they have a couple of days off from work.

The most common side effects in adults include:

  • Fever
  • Headache
  • Fatigue
  • Pain at the injection site

People also report:

  • Muscle pain
  • Chills
  • Nausea

The response usually lasts 24 to 48 hours and will be gone within three days maximum, according to Paul Offit, MD, director of the Vaccine Education Center and an attending physician in the Division of Infectious Diseases at Children’s Hospital of Philadelphia.

“This vaccine is very good at inducing an immune response, when you see people have swelling of the lymph node in the armpit, that’s a sign that it’s inducing a vigorous immune response,” Dr. Offit told Health.

Early data from Moderna showed reactions to the bivlant boosters to be slightly less intense than the previous monovalent vaccines, though not significantly so. Pfizer’s data showed the routine side effects to be the same.

Serious Adverse Reactions Are Still Being Monitored

Serious side effects are very rare and the safety profile of the new vaccines should be the same as the monovalent first generation COVID vaccines, said Dr. Offit.

“The Pfizer and Moderna mRNA vaccines have now been in billions of people. We have a very good idea of serious side effects but we don’t have specific data on the bivalent vaccines,” he said. “I don’t expect the side effects to be different than the previous vaccines.”

A January 2022 study published in JAMA reported that of the more than 192 million people in the U.S. who were vaccinated against COVID with an mRNA vaccine, 1,626 developed a heart condition called myocarditis within seven days of getting their shot. Most of the cases were in adolescent and young men in their early 20s.

The CDC is continuing to monitor serious adverse reactions.

“It’s important that we not let the safety data we’ve seen so far be the only data we ever see,” said Dr. Messer. “There always will be rare adverse effects and it’s important for us to know what they are so we can make vaccines safer.”

When to Get Your COVID Booster

Moderna’s bivalent booster is approved for people ages 18 and older who do not have a health condition preventing them from safely getting vaccinated, such as a previous allergic reaction, which is very rare. Pfizer’s bivalent booster is approved for those ages 12 and older.

The CDC advises people who had a rare condition called multisystem inflammatory syndrome (MIS), in the past three months to wait to get a booster and talk to their doctor about the risks for them.

People who have recently had a confirmed COVID infection can wait three months after testing negative to get boosted. That’s because an infection also produces antibodies that protect against another infection, at least for a little while.

Immunity takes two to four weeks after inoculation to build.

“If I’m looking at my calendar thinking about when we will be going back inside again, I want about 3 weeks before that,” said Dr. Messer, noting that it’s likely the U.S. will experience another wave during the upcoming fall and winter months, when people are spending more time indoors.

“Now,” he said, “is a good time to start thinking about getting boosted.”


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