June 28, 2022 11:00 AM EDT

COVID-19 vaccines for some of the youngest children in the U.S. are now rolling out, and parents are faced with a new question: Moderna or Pfizer-BioNTech?

Both of the mRNA shots—which are now authorized for kids ages 6 months and up—are effective at preventing severe illness, and they both help prevent symptomatic infections. They offer vital protection in this age group, even for kids who have already recovered from COVID-19. The vaccines “provide protection against a broader variety of variants than [a previous] infection, which generally provides protection mostly against the variant that you had,” says Dr. Alissa Kahn, a pediatric hematologist and oncologist in Paterson, N.J. That’s particularly important as the virus evolves to overcome prior immunity. Each time an unvaccinated child is exposed to SARS-CoV-2, “you’re sort of rolling the dice—and most kids will do well, but sometimes they don’t.” Since the start of the pandemic, more than 440 children under 5 in the U.S. have died.

But unlike with the adult shots—where the best shot was the one you could get quickly—there are some key differences between the two under-5 vaccines.

“The science behind how they work is the same,” says Dr. Mona Amin, a pediatrician in Fort Lauderdale, Fla. “The difference is the schedule and the dose.”

Moderna’s vaccine for kids ages 6 months through 5 years is 25 micrograms (mcg)—one-quarter of the dose that adults get. Pfizer’s shot for kids 6 months through 4 years is 3 mcg—one-tenth the adult dose. Likely because of the higher dose, mild side effects (such as a fever) are more common with the Moderna shot, according to trial data.

The timeline for vaccination is also different. Moderna’s vaccine involves two shots four weeks apart, while Pfizer’s requires three shots over 11 weeks. So while Pfizer has a lower dose and possibly fewer side effects, it takes one more shot than Moderna and several more weeks until kids are fully vaccinated.

If parents want kids to be protected as soon as possible—a particular concern with most schools starting in a couple of months—they might want to go with Moderna. That’s what Kahn is choosing for her 4-year old son. The faster Moderna timeline “is really going to allow parents to be able to send their kids to school in the fall fully vaccinated, which is important for a lot of families,” she says.

Moderna is also the “pretty clear” choice for Dr. Alpa P. Shah, a pediatrician in Milwaukee. Her 2-year-old daughter attends preschool at a school with elementary-aged kids, and Shah is worried about increased chances of exposure this fall as more children return to the classroom. “The timeline is much more friendly for the fall with Moderna,” she says. Moderna might also be more appealing for families who prefer two initial shots over three because of transportation issues or time spent away from work.

But if parents are concerned about side effects, and they’re not in a particular hurry, Pfizer might be a better option. Having the option for a lower dose with a decreased risk for side effects could also help some families overcome hesitancy about getting vaccinated. The third Pfizer shot also seems to elicit broader antibody responses, which likely help against Omicron.

That’s what Amin is choosing for her 2-year-old son. He already had COVID-19 twice, and while that doesn’t offer complete protection against reinfection, it means his existing immune response could be strong even before he gets a third shot.

Amin also likes the fact that the Pfizer vaccine has already been given to millions of children ages 5 and up, while Moderna was only available for U.S. adults until June 24, when the U.S. Centers for Disease Control and Prevention recommended its use in kids. That means there are more real-world data on Pfizer’s side effects among older children, including very rare side effects like myocarditis.

Dr. Roby Bhattacharyya, an infectious disease doctor at Massachusetts General Hospital, went back and forth on which vaccine to schedule for his 4-year-old son. “The data for either one looks great, far above my threshold for where the expected benefits exceed the risks,” he writes in an email. “If only one or the other were available, I’d take either in a heartbeat for my kid.”

Ultimately, he decided the most important thing was to get a vaccine quickly. “Our kids are starting camp, and I think the cost of waiting another 10 days where they’re hanging out in close quarters with a bunch of kids at a time when there’s still a decent amount of circulating virus is more risk than any difference I see between these two, both of which I see as good options,” Bhattacharyya writes. A few hours later, his son received the first vaccine they could schedule: Pfizer’s.

That’s fairly typical right now; some pediatricians and health systems are only carrying the Pfizer shot, at least at first, because they already have a system set up for giving Pfizer to older kids. As of mid-June, providers had ordered 2.5 million doses of Pfizer and 1.3 million doses of Moderna. But if you only have access to one type of vaccine, even if it’s not your preference, you shouldn’t feel like you’re getting an inferior vaccine, the experts agreed.

And, more generally, the under-5 vaccines are not available in as many places as vaccines for other ages, which were often administered at mass vaccination sites or schools. Instead, most families are going to their pediatrician or general practitioner, pharmacies (which by law can only give COVID-19 vaccines to kids ages 3 and up), state or local health departments, and federally qualified health centers. (If you’re looking to set up an appointment, you can search by vaccine type at vaccines.gov.)

“I would just love to see both vaccines available in all states and distributed in an equitable fashion,” Shah says, “so families can really choose what makes sense to them.”

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