With COVID-19 booster shots rolling out, parents have plenty of questions: Why are booster shots necessary? Will younger children be eligible in the future? Can you mix and match booster shots? We spoke with experts to learn more.
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Last year, Americans were rushing to pharmacies and clinics to get their COVID-19 vaccines. Now they're rolling up their sleeves for another reason: booster shots.

The Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) currently recommend Pfizer boosters for everyone 12 and up, given at least five months after the primary vaccine series. On May 17, the FDA approved boosters for those ages 5 to 11; now the CDC must sign off on the decision, which is expected to happen this week. Data from Pfizer showed a third dose of the vaccine 36-fold increase in antibodies against the Omicron variant in younger kids, when compared to two doses.

Boosters protect against waning immunity from the vaccines over time, and they're considered safe and effective. Still, parents have plenty of questions. Will younger kids be eligible for a COVID-19 booster shot in the future? How often will my family need a booster shot? Can you "mix and match" COVID vaccines? Here, we explore these questions and more. 

Why Do People Need COVID-19 Booster Shots?

It's important to realize that booster shots don't indicate the COVID-19 vaccines aren't working. All approved vaccines still protect against severe illness, hospitalization, and death—even against the Omicron variant. Rather, the boosters offer increased protection against waning immunity and breakthrough infections (cases that occur in fully vaccinated individuals, which are usually mild or moderate), said Mona Amin, D.O., a board-certified pediatrician based in Fort Lauderdale, Florida, in an Instagram Live interview with Parents.

People will likely need booster shots regularly, like with the influenza vaccine, to ensure optimal protection, says Dr. Amin. Experts are currently researching the specifics, and recommendations will likely change as they learn more.

So how long after the COVID booster are you immune? Studies show that increased protection kicks in within days of receiving the booster. The shots may reach their full potential within a week or two; experts are currently learning more.

Who Is Eligible for Booster Shots? 

Are you wondering, "When can I get a COVID booster?" Eligibility varies based on age and the primary vaccine you received (Pfizer, Moderna, or Johnson & Johnson). Here's the latest guidance, according to the CDC.

Pfizer: For Pfizer, anyone 12 and up can get a booster shot at least five months after the final dose in their primary vaccine series; moderately or severely immunocompromised people can get it at least three months afterwards. In addition, certain immunocompromised people ages 12 and older, as well as adults age 50 years and older, can get a second booster at least four months after the first one.

Moderna: Everyone 18 and up who received Moderna is eligible for a booster. It should be given at least five months after completion of the primary vaccination series—though moderately or severely immunocompromised can get a booster is eligible at least three months afterwards. Adults ages 50 and older, as well as certain immunocompromised people, can get a second booster at least four months after the first one.

Johnson & Johnson's Janssen: Adults 18 and older who received the Johnson & Johnson vaccine can get a Pfizer or Moderna booster shot, which is recommended at least two months after the initial vaccine. People who are moderately or severely immunocompromised can get a booster at least two months after the additional dose of Pfizer or Moderna, says the CDC.

The following people can get a second booster of Pfizer or Moderna at least four months after the first one: anyone who received the Johnson & Johnson vaccine for their primary dose and booster, adults ages 50 and up, people who are moderately or severely immunocompromised.

An image of A healthcare worker preparing a Dose of COVID-19 Vaccine.
Credit: Getty Images.

Will Booster Shots Protect Against Coronavirus Variants?

Researchers are studying whether booster shots are effective against different strains of COVID-19, such as the Omicron variant that's been spreading around the world, says Purvi Parikh, M.D., an allergist and immunologist with the Allergy and Asthma Network and co-investigator on the vaccine trials. But so far, data looks promising, and preliminary research shows that boosters offer some protection against Omicron. 

Take an April 2022 study published in JAMA, which evaluated 11,690 adults admitted to U.S. hospitals from March 2021 to January 2022; about half had confirmed COVID-19 cases. During the Omicron surge, two doses of the vaccine provided 65% protection against hospitalization. Three doses brought the effectiveness against hospitalization to 86%

It's also important to note that scientists should be able to tweak the vaccine over time so it's more effective against future variants that might emerge, reports The New York Times

What Are the COVID-19 Booster Shot Side Effects?

In general, COVID-19 booster shots have the same formulation as the primary vaccines. The exception is Moderna, whose booster shot comprises half the dose. So far, side effects appear similar to those of the primary vaccine series, and the most common reactions are pain at the injection site, fatigue, headache, and fever. 

A CDC studied 22,191 people who received a booster shot from August 12 to September 19, 2021. The majority (97.6 percent) received the same booster shot as their primary vaccines. After analyzing self-reported health check-ins, the CDC found that 71 percent experienced injection site pain, 56 percent reported fatigue, and 43.4 had a headache. Also, approximately 28.3 percent were unable to perform daily activities, "most commonly on the day after vaccination," says the CDC. (Be prepared for a possible sick day!) 

Serious side effects from COVID vaccines are extremely rare. Learn more about them on the CDC website

Will Kids Need COVID-19 Booster Shots?

Everyone 12 and older can currently get a booster shot. It's possible that younger children will become eligible for them in the future (kids 5 and up can currently receive the primary vaccine series, and they may get the go-aheads for booster shots this week.)

Part of the reason boosters haven't approved for younger kids years old yet, according to Dr. Amin, is that they got the green light for vaccination more recently than adults and older adolescents. "Everything has to happen based on when the clinical trials happened," says Dr. Amin. 

Experts are currently analyzing whether immunity wears off over time in kids, as it does with adults. They're also determining if and when booster shots might benefit younger children.

Can You "Mix and Match" Booster Shots?

You might be wondering if you can mix different COVID shots. For example, if you received Pfizer or Johnson & Johnson for your initial immunization, is it possible to get a Moderna booster? As it turns out, the answer is yes—as long as you're 18 or up.

"If you are 18 years or older you may choose which COVID-19 vaccine you receive as a booster shot," according to the CDC. "Some people may prefer the vaccine type that they originally received, and others may prefer to get a different booster. CDC's recommendations now allow for this type of mix and match dosing for booster shots."

If you're under 18, however, you need to get a Pfizer booster shot. That's because Pfizer is the only vaccine with emergency use authorization for that age group. 

Are there any benefits of mixing and matching? That's still up for debate. Some experts say that the strategy can result in fewer doses being wasted. It's also possible that mixing booster shots could increase antibody response. According to the National Institutes of Health, "Mixing vaccines may enhance the immune response, and it increases flexibility for when people need a booster dose but doses of the vaccine they first received are not available."