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9 Things to Know About COVID-19 Booster Shots

Extra vaccine doses are now available to millions of Americans. Here's the latest on what to expect

spinner image A nurse marks a coronavirus vaccination card with a third "booster" dose of the Pfizer COVID-19 vaccine in Pasadena, Calif.
A nurse marks a vaccination card with a third dose of the Pfizer COVID-19 vaccine.
ROBYN BECK / Getty Images

Millions of Americans who have been vaccinated against COVID-19 are eligible for a booster shot, which is meant to wake up the immune system so it stays sharp if confronted with the coronavirus. Experts say the enhanced protection the booster affords is especially needed in the wake of the highly contagious omicron variant, which is driving up cases of infection and illness throughout the country.

Boosters from Pfizer-BioNTech, Moderna and Johnson & Johnson (J&J) are available in pharmacies, health clinics and doctor’s offices across the country. Here’s what you need to know about these additional doses:

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1. Boosters are available, but not everyone qualifies for one 

More than 206 million Americans are fully vaccinated, according to the Centers for Disease Control and Prevention (CDC), but not everybody who is eligible for a COVID vaccine can get a booster shot at this time. Boosters from Moderna and Johnson & Johnson (J&J) are only available for adults 18 and older; people 12 and older who had Pfizer's vaccine are eligible for its booster. Pfizer and Moderna vaccine recipients can get theirs if they are at least five months out from the original vaccine; J&J vaccine recipients qualify for a booster two months after their initial shot.

2. Moderna’s booster is a tad different 

The boosters from Pfizer and J&J are the same formulation and dosage as the initial vaccines, but Moderna’s booster is half the dose — 50 micrograms — of the first two shots. Clinical trial data show that the smaller dose still generates a strong immune response, and the company says the lower dose helps to increase worldwide supply.

3. CDC recommends Pfizer and Moderna over J&J

When planning for your booster, know that the CDC now recommends Pfizer and Moderna’s vaccines over J&J’s product. The guidance, issued Dec. 16, 2021, came after a panel of experts that advises the agency reviewed data of a rare but serious blood -clotting disorder linked to J&J’s vaccine. Fifty-four cases of the condition were confirmed as of August out of about 14 million doses administered; nine people have died from it. Young women in their 30s and 40s are most at risk, according to the data presented. The CDC said in a statement that J&J’s vaccine will still be available to those who are “unable or unwilling to receive an mRNA vaccine.” 

4. Booster shots enhance protection

While the coronavirus vaccines can help to thwart infection (unvaccinated people are about five times more likely to test positive for COVID-19 than vaccinated individuals, according to data from the CDC), their real strength is preventing serious illness, explains Anna Durbin, M.D., a vaccine and infectious disease expert at Johns Hopkins Bloomberg School of Public Health. And the COVID-19 vaccines are still highly effective at doing just that — they’re keeping people out of the hospital and preventing them from succumbing to the disease. Unvaccinated people are about 14 times more likely to die from the disease than their vaccinated peers, data shows. 

That said, multiple studies show that some populations are starting to see protection against disease dwindle, including older adults, who account for the majority of the severe breakthrough infections. And top public health experts have said that the current protection could continue to diminish in the months ahead, “especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout.”  

It’s not unusual to see this waning response. “Even highly effective vaccines become less effective over time,” U.S. Surgeon General Vivek Murthy, M.D., explained in a White House COVID-19 task force briefing. And other vaccines require booster shots to reinvigorate the immune system, like the tetanus, diphtheria and pertussis (Tdap) vaccine. What's more, the new omicron variant is better able to side step some of the vaccine's defenses than previous coronavirus variants, which is why experts are encouraging anyone who is eligible for the booster shot to get one as omicron spreads throughout the U.S.

5. Don’t expect any new or unusual side effects

Pfizer’s booster trial reported symptoms similar to what some people experienced after their first and second doses: temporary pain at the injection site, fever, chills, headache, fatigue, vomiting, diarrhea, and joint and muscle pain. And data presented on Sept. 22 by a vaccine safety group within the CDC’s advisory committee found that a third dose of the mRNA vaccines brought on fewer side effects than the second shot.

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An FDA review of Moderna’s booster data details similar findings. The booster caused side effects like those seen after the second dose of the vaccine. The most common among adults 65 and older were injection site pain, fatigue, headache, muscle aches and joint pain. No serious adverse events or serious safety concerns were reported. 

When it comes to J&J booster shots, about 40 percent of clinical trial participants 60 and older reported pain at the injection site after the shot, an FDA review of the data shows. Roughly 29 percent experienced headache and fatigue, about 26 percent noted muscle pain, 12.4 percent had nausea, and 2.3 percent had a fever.

Who is eligible for a booster shot? ​

  • People 12 and older who completed the two-shot vaccine series from Pfizer at least five months ago
  • Adults 18 and older who completed the two-shot vaccine series from Moderna at least five months ago
  • Adults 18 and older who recieved the one-shot Johnson & Johnson vaccine at least two months ago

Source: CDC

6. Boosters can be mixed and matched

Federal officials have given their blessing for mixing and matching booster shots, making it possible for people to get boosted with a different vaccine than the one they originally had. “Some people may have a preference for the vaccine type that they originally received and others may prefer to get a different booster,” the CDC said in its news release announcing the decision. 

Mixing and matching also makes it easier for people to get a booster if they don’t have access to their original vaccine. 

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Preliminary results from a new study funded by the National Institutes of Health (NIH) found that people who were boosted with a shot different from their original vaccine saw a spike in antibody levels, which is one measure of immune response. In particular, J&J vaccine recipients who were boosted with either the Pfizer or the Moderna vaccine saw an increase in antibody levels much higher than those boosted with the J&J vaccine. What’s more, no safety concerns were identified. 

If you are considering mixing and matching and have questions and concerns, talk to your doctor, says Mohammad Sobhanie, M.D., an infectious disease expert at the Ohio State University Wexner Medical Center. “I think it's incredibly important that you have these conversations with your primary care physician so that they can give you the best advice out there based on your medical conditions,” he says. 

7. Booster doses should be widely available   

Wondering how you can get your booster shot? The same way you got the first shot: Health clinics, pharmacies and other official vaccination sites will continue to administer the COVID-19 vaccines, as well as the boosters. You may need an appointment, so it’s good to check ahead of time.

Health officials have confirmed that the government has adequate supplies, so no shortages are expected. And just like the initial series, the booster shots are free in the U.S. — no ID or insurance card is required. It is, however, a good idea to bring your paper vaccination record so the date of your booster shot can be added.

8. It’s unclear whether boosters will be needed annually

Experts aren’t sure if the COVID-19 vaccine will be needed on a regular basis, like the flu shot. One thing that could make that scenario more likely, Durbin says, is “if we are unable to control this pandemic — if we continue to see surges that are requiring hospitalization and really taxing health care systems.”

However, if we can control the spread of COVID-19 and bring down the levels of severe disease we’re seeing, “we may not need booster shots every year,” Durbin adds. “But a lot of that is going to depend on the epidemiology of the pandemic.”

9. There could be a new standard for 'fully vaccinated'

Now that boosters are available, our definition of “fully vaccinated” could change. Currently, people are considered fully vaccinated if they have had two doses of the Pfizer or Moderna vaccine or a single dose of the J&J vaccine.   

“I anticipate, over time, that may be updated,” CDC Director Rochelle Walensky, M.D., said in a COVID-19 news briefing. “But we will leave that to our [CDC Advisory Committee on Immunization Practices] to give us some recommendations.”

So stay tuned.

​Editor's Note: This story has been updated to reflect new information.

Video: COVID Vaccine: Potential Side Effects Explained

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