With COVID-19 booster shots rolling out, parents have plenty of questions: Why are booster shots necessary? Will 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.

Previously, everyone 16 and up was eligible for an additional dose of the coronavirus vaccine, at least 5 months after their primary shot series. Now the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) recommend boosters for 12- to 15-year-olds. CDC Director Dr. Rochelle Walensky signed off on the decision January 6, which allows the boosters to be distributed immediately to this age group.

"This booster dose will provide optimized protection against COVID-19 and the Omicron variant," said Dr. Walensky in a statement. "I encourage all parents to keep their children up to date with CDC's COVID-19 vaccine recommendations."

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 Delta variant and 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.

It's possible that booster shots will be a "one and done" situation. It's also possible that people will need them 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; adults 18 years and older should get it. For both age groups, the booster is recommended at least 5 months after completion of the primary vaccination series. The CDC also authorized a third shot, given 28 days after the second, for immunocompromised children ages 5 through 11.

Moderna: Everyone 18 and up who received Moderna is eligible for a booster. It should be given at least 6 months after completion of the primary vaccination series.

Johnson & Johnson's Janssen: Adults 18 and older who received the Johnson & Johnson vaccine can get a booster shot, which is recommended at least 2 months after the initial vaccine.

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 Pfizer offers some protection against Omicron

Indeed, according to FDA, "peer-reviewed data from multiple laboratories indicate that a booster dose of the Pfizer-BioNTech COVID-19 Vaccine greatly improves an individual's antibody response to be able to counter the omicron variant."

Data has suggested that two doses of an mRNA vaccine are about 35 percent effective against Omicron infection. A third shot increases the effectiveness to about 75 percent.

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. 

The 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!) 

The FDA also studied the real-world data from Israel—"including safety data from more than 6,300 individuals 12 through 15 years of age who received a booster dose of the vaccine at least 5 months following completion of the primary two-dose vaccination series," says the organization. They found no new safety concerns; myocarditis and pericarditis weren't reported.

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).

Part of the reason boosters haven't approved for younger kids years old yet, according to Dr. Amin, is that they got the go-ahead 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, but there's no confirmation of that. The CDC currently suggests that "primary series and additional primary doses should be with the same vaccine product" if possible.