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COVID-19 vaccines for children under 5 are imminent but so are a lot of questions. Experts have some answers.

A 5-year-old boy receives his first dose of a coronavirus vaccine at Texas Children's Hospital in Houston on Nov. 3, 2021.MERIDITH KOHUT/NYT

It’s enough to make even the most level-headed parent’s head spin.

The Food and Drug Administration on Friday authorized emergency use of the Moderna and Pfizer COVID-19 vaccines for the nation’s youngest children, some 18 million under the age of 5, including those as young as 6 months. But many questions remain.

When an expert advisory panel unanimously voted Wednesday to recommend the authorization, several experts expressed concern that Pfizer’s three-dose regimen, with the third dose coming at least two months after the second, would be confusing for parents who may not understand that protection only appears to kick in after the third shot.

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And there were questions about whether squeezing COVID vaccines into the schedule of shots babies already receive for other illnesses might have an unforeseen impact.

With the Centers for Disease Control and Prevention expected to give its green light to the vaccines this weekend, the shots could be available at physician’s offices next week.

So what’s the best advice for parents on how to proceed?

Which shot is preferable, Moderna or Pfizer?

The vaccines were not studied in a head-to-head trial, making it challenging to directly compare them. Moderna’s shots appeared to more commonly produce fevers in children than Pfizer’s, but also seemed to be more effective in protecting against illness after two doses. Moderna’s was more than 40 percent effective after its second dose, compared to Pfizer’s 28 percent.

But there is a big caveat here. Moderna’s is designed as a two-dose regimen, with each shot given a month apart, whereas Pfizer’s is a three-dose series, with the first two doses administered three weeks apart, and the third at least two months later.

Pfizer’s data showed the third dose was required before the vaccine appeared effective. That third dose indicated an 80 percent effectiveness, but it was based on a small number of children in the study who had received all three shots.

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Experts said they are concerned parents may not understand that their children are still highly vulnerable to infection during that two-month gap.

“Getting a vaccine is an inconvenient thing to schedule,” said Dr. Andrew Siesennop, a pediatrician at Tufts Children’s Hospital, and a primary investigator for Moderna’s vaccine trial at Tufts. He said concerns about convenience may prompt more parents to prefer the two-shot regimen.

Siesennop and other physicians said that for older children and adults many vaccine sites have offered only Moderna or only Pfizer, but not both. So parents who are eager to just start the process as soon as possible might want to pick Moderna. But Siesennop and other experts said that as long as parents understand the three-dose Pfizer regimen, they should feel comfortable with whichever shot is available to them because both appear safe.

What about side effects?

Both Moderna and Pfizer created doses for these youngest children that are a fraction of the size of adult portions after observing too many kids in early trials with a fever, which may have been caused by too large of a dose.

“There is some impression that Moderna may produce more frequent fevers, but that’s not definitive, it’s just an impression from the data,” said Dr. Ofer Levy, director of the Precision Vaccines Program at Boston Children’s Hospital and a voting member of the FDA’s vaccine advisory panel.

Otherwise, the side effects from both shots appeared similar to those from other childhood vaccines, including pain and redness at the injection site, headaches, and fatigue.

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When will the shots be available?

After the CDC signs off on the shots, as it is expected to do this weekend, shipments should be ready to go. A recent notice from the Massachusetts health department to physicians said doses will begin being delivered on Thursday. The department said there are about 320,000 children in the state who are between 6 months and 4 years old.

“They are the forgotten families,” said Dr. Shannon Scott-Vernaglia, a pediatrician at Mass General for Children. “There is a large chunk of our population that hasn’t had a choice yet to get vaccinated. We have been waiting with bated breath.”

She said parents have been struggling to keep masks on very young children in public places, or have continued to avoid travel until the shots were available.

That, she said, has meant major disruptions to everyday life to avoid infections and serious illness.

“It’s canceling vacations and not seeing your parents or grandparents,” she said.

Will the COVID vaccine impact other childhood shots?

Probably not. Although this concern was not explicitly studied in the vaccine trials, physicians say babies already receive multiple shots in their first year of life without one affecting the other, and the COVID vaccines should be no different.

“There is no reason to think that having another vaccine will take away this one’s ability to work,” Siesennop said. “It’s totally fine to give children multiple vaccines at the same time.”

And Siesennop, who has a 2-year-old son, added, “Once it’s available, I would love to be able to vaccinate my littlest one.”

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Serious illness from COVID is rare in young children. So why should they get this vaccine?

Federal data indicate that while young children are still far less likely to be hospitalized from COVID than adults, there was a significant jump in hospitalizations among children under 5 during the recent Omicron surge — outpacing hospitalizations of older children. The data show that of the 535 children in this age group nationwide hospitalized from mid-December through March, nearly a quarter ended up in intensive care units.

“It’s small numbers, but it’s numbers that are similar to other disease that we immunize against,” said Levy, from Boston Children’s Hospital.

The data show COVID hospitalizations among children under 5 in the first year and half of the pandemic were less than those for the flu. But since October, COVID hospitalization rates for such young children were as high or higher than for flu.

“This is a rapidly evolving pandemic, and we don’t know if the next variant will be worse, or hits children worse, but at least this [vaccine for young children] is a tool,” Levy said. “And having another tool against this pandemic is important.”


Kay Lazar can be reached at kay.lazar@globe.com Follow her on Twitter @GlobeKayLazar.