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Your Guide To Adult Vaccines

8 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

A nurse marks a coronavirus vaccination card with a third "booster" dose of the Pfizer COVID-19 vaccine in Pasadena, Calif.

ROBYN BECK / Getty Images

A nurse marks a vaccination card with a third dose of the Pfizer COVID-19 vaccine.

En español | Tens of millions of Americans who have been vaccinated against COVID-19 are now eligible for a booster shot, which is meant to wake up the immune system so it stays sharp if confronted with the coronavirus. 

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

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

Nearly 190 million Americans are fully vaccinated, according to the Centers for Disease Control and Prevention (CDC), but not everybody can get a booster shot at this time. Moderna and Pfizer vaccine recipients are only eligible for a booster if they are at least six months out from the original vaccine and are 65 and older, or 18 and older with an underlying health condition that puts them at high risk for severe COVID-19. Individuals 18 and up who live or work in high-risk settings also qualify for an mRNA booster.

Young and otherwise healthy adults whose jobs or living situations don't put them at risk are encouraged to hold off on a booster shot for now. Data shows the mRNA vaccines are still highly protective against severe illness in most people. 

When it comes to J&J’s booster, eligibility requirements are more straightforward: Anyone 18 or older who had their vaccine at least two months ago can get a booster. 

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.


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3. Booster shots could rev up waning protection in some  

While the coronavirus vaccines can help to thwart infection (unvaccinated people are eight times more likely to get COVID-19 than vaccinated individuals, according to the CDC), their primary function is to prevent 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. Very few fully vaccinated Americans have been hospitalized or have died from COVID-19 — about 0.009 percent, according to the most recent data.

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.

Israel started administering booster shots to its 60-plus population this summer and has seen a significant reduction in the risk of infection and severe disease, Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases, said in a recent briefing.

Many experts argue the data isn’t strong enough to support booster shots for younger populations who are showing a slower decline in immune protection — at least right now. “And we shouldn't be giving a valuable resource just because we can,” Durbin says about booster doses for everybody. “It really needs to be indicated, I think, by the epidemiology of the disease and of the illness,” she notes, adding that the science could change.

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

Let’s start with Pfizer’s booster: Data reviewed by the FDA shows that Pfizer’s booster shot does not appear to have different safety risks than the first two vaccine doses, Mark McClellan, M.D., a former FDA commissioner and director of the Duke-Margolis Center for Health Policy at Duke University, told AARP in a statement. And evidence on boosters from Israel and elsewhere “seems to show benefits in people who are older or are at higher risk of serious complications because of health conditions, without unusual side effects,” McClellan adds.

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.

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? ​

If you had a Pfizer or Moderna vaccine, you are eligible for a booster if:

  • You completed your primary series at least six months ago 
  • You are 65 or older 
  • You are 18+ and have an underlying medical condition
  • You are 18+ and work or live in a high-risk settings

If you had J&J, you are eligible for a booster if: 

  • You had your initial vaccine at least two months ago
  • You are 18 or older 

Source: CDC

5. 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. 

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. 

6. 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 will be 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.

“The bottom line is that we are prepared for boosters and we will hit the ground running,” said Jeffrey Zients, White House coronavirus response coordinator. 

7. 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.”


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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.”

8. 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., recently 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.

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.

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