Medical Examiner

Should Your Kids Get Moderna or Pfizer?

Yes, both vaccines for kids under 5 are great. But there are some differences between the two.

A nurse high-fiving a small child sitting on a woman's lap in an exam room
David Ryder/Getty Images

Finally, kids under 5 are eligible for a COVID-19 vaccine. Last Wednesday, a Food and Drug Administration panel unanimously recommended the authorization of Moderna and Pfizer-BioNTech’s COVID-19 vaccines for children under 5.* It wasn’t long before the Centers for Disease Control and Prevention officially recommended the vaccines for young children. Parents are already excitedly making appointments (or at least trying to).

One important question remains: Is one of the available vaccines better than the other? You probably remember from the rollout of adult shots that the best vaccine is the one that you can get into your arm as soon as possible. The bottom line is that this is true of kids’ vaccines too.

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But, as with their adult counterparts, the kids’ editions of the Pfizer and Moderna vaccines are not identical. Here, we break down everything we know about the options, including some pros and cons you might want to consider if you have a choice of which kind of vaccine appointment to book.

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What are the major differences between the Pfizer and Moderna kids’ vaccines? 

Both are mRNA vaccines that are, most importantly, safe. However, they differ in terms of their dose size and number, side effects, and efficacy in clinical trials.

Here come a bunch of numbers: Moderna is recommended for children 6 months through 5 years (previously, it was available just to people over 18). It’s delivered in two doses, one month apart.* Each dose is 25 micrograms of mRNA (one-fourth the strength of an adult dose). The clinical trials, which started in December during the initial omicron wave, showed 50.6 percent efficacy against symptomatic illness in children ages 6 to 23 months old, and 36.8 percent efficacy in kids 2 to 5 (the efficacy was measured after two doses).

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Pfizer’s shot is 3 micrograms (one-tenth the strength of an adult dose) for children 6 months through 4 years (that is, kids about to turn 5). It’s a three-shot regimen, with the first two shots three weeks apart, and the last one at least two months after the second. In clinical trials, the vaccine efficacy against symptomatic illness was 75.5 percent in the younger cohort, children ages 6 to 23 months, and 82.3 percent in the older one, kids 2 to 4 (the efficacy was calculated after three doses).

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Pfizer sounds like the obvious choice efficacy-wise!

Not necessarily. “I wouldn’t put any stock into the actual numbers,” Jake Scott, a physician who specializes in infectious diseases, said. “It’s so hard to compare because the clinical trials were different, the doses were different, the populations were different.” To get a little technical, the Pfizer data in particular had wide confidence intervals—basically, the efficacies are more like pretty good estimates than numbers that the scientists are really certain of. Also, the virus is evolving and looks a little different from how it did when the trials started. “We don’t know how well they’re going to protect against infection from BA.4 and BA.5, which are probably going to dominate in the coming months,” Scott said.

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One more question about efficacy: How well do these vaccines work against severe disease and death?

It’s hard to tell for sure. There’s no data available on those outcomes from the vaccine trials as they are relatively rare outcomes for kids in the first place (nationally, there have been 442 deaths from COVID in children 0 to 4). But based on data from adult populations, experts are very hopeful that the shots will help protect against severe outcomes from COVID-19 in kids.

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Why are the Pfizer and Moderna vaccine dose sizes—and number of shots required— different? 

It really has to do with how much mRNA vaccine is needed to reach the appropriate level of antibodies. Additionally, the two manufacturers were pursuing different strategies when developing their respective shots. As an article in Stat News explains, Pfizer started developing the vaccine with a small dose to start but found that two doses of the Pfizer vaccine 21 days apart did not create a strong enough immune response (the FDA required that vaccination in young children had a similar level of antibodies as in young adults). The third Pfizer shot provides that extra boost. Moderna developers, on the other hand, decided to start with a larger dose initially and found that they only needed two shots to get to the desired immune response.

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Well, two shots sounds better than three—right? Fewer shots!

Well, in theory, sure. But while the Moderna has a larger amount of mRNA, some predict that there will need to be a third shot anyway. “We know for adults and teenagers that you need three doses against omicron, so I wouldn’t be surprised and fully expect that there would be a booster for the Moderna vaccine to get that efficacy up a little higher against omicron,” said Dr. Kawsar Talaat, an associate professor in the Department of International Health at Johns Hopkins University who was a principal investigator for the Pfizer vaccine trials in children.

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So the message is don’t stress too much over the exact efficacy of either vaccine—but also, don’t be surprised if the Moderna efficacy does need a boost. 

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Exactly. Another thing to keep in mind: The Moderna side effects might be worse.

Really?

Yep. One in 4 children who received the Moderna shot had a fever (in comparison to 1 in 20 with Pfizer), and more children in the Moderna group experienced irritability, drowsiness, and fatigue. However, the observed side effects—or as Dr. Scott calls them, the “intended effects”—are likely a result of the higher Moderna dosage.

“It’s really important that people understand this is a sign of your immune response. It’s a sign your immune response is doing what it’s supposed to do, which is making antibodies to block the virus,” Scott said. “I don’t think it’s anything to worry about, I think it’s a good thing. You have to earn your immune protection.”

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What about myocarditis?

Many parents are worried about the risk of myocarditis, or inflammation of the heart muscle that results in chest pain, shortness of breath, or arrhythmias. Although there have been some cases of myocarditis after vaccine administration, there were none observed in the clinical trials.

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“The worry was going to be that if you went to younger kids, the lower the age, the risk would go up. The good news is, so far, it does not seem to be the case, although the clinical trials on younger kids are still fairly small,” said Dr. Peter Hotez, co-director of Texas Children’s Hospital Center for Vaccine Development.

OK, what’s the bottom line here? Doctors must suggest one over the other. 

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Not really!

“There’s not really enough information to be able to discriminate [between] the two at this point,” Hotez explained. “The advantage of Pfizer is we already know that this is going to be a three-dose vaccine so you don’t have to wait around and find out if you need another dose. The disadvantage may be that you require those three doses to get a strong immune response. The advantage of Moderna is you might be able to get a higher immune response after two doses, but then you have to worry about whether you need the third dose. The bottom line is you can argue either way and I treat the two as more or less equivalent in terms of overall safety and effectiveness.”

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All three of the experts I spoke to agreed that both would do the trick of preventing severe illness and death—the goal of vaccines. At the end of the day, it’s just important to get those kids vaccinated.

“Because most cases of COVID-19 in children are mild, people have this impression that COVID-19 is no big deal in kids,” Talaat said. “But COVID-19 has killed kids more than any other infectious disease. It’s really important that we vaccinate our kids. Doesn’t matter which one.”

Correction, June 23, 2022: This piece originally misstated that the two doses of the Moderna vaccine are given 2 months apart. They are given one month apart.

Correction, June 22, 2022: This piece originally misidentified the Food and Drug Administration as the Federal Drug Administration.

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