When Rachel Adams-Kaplan heard that the Covid-19 vaccines would soon be available for her children, she knew she would get her 7-year-old son vaccinated right away. She wasn’t so sure about her 11-year-old daughter.

Children between 5 and 11 years of age began getting Covid-19 shots last week, after the Centers for Disease Control and Prevention recommended Pfizer Inc. and BioNTech SE’s vaccine for that age group. Some 900,000 children in that age group had received a first dose through Wednesday, White House Covid-19 coordinator...

When Rachel Adams-Kaplan heard that the Covid-19 vaccines would soon be available for her children, she knew she would get her 7-year-old son vaccinated right away. She wasn’t so sure about her 11-year-old daughter.

Children between 5 and 11 years of age began getting Covid-19 shots last week, after the Centers for Disease Control and Prevention recommended Pfizer Inc. and BioNTech SE’s vaccine for that age group. Some 900,000 children in that age group had received a first dose through Wednesday, White House Covid-19 coordinator Jeff Zients said.

The vaccine will be given in two shots, spaced three weeks apart, as with adults. But the amount per dose for younger children and people 12 and older differs significantly. People who are between the ages of 5 and 11 will get 10 micrograms of the vaccine per dose, while those 12 and older will get 30 micrograms—three times as much.

Ms. Adams-Kaplan, a kindergarten teacher in Philadelphia, said she thought about waiting to get her daughter the larger, 30-microgram dose after she turns 12 on Nov. 26, thinking it might better protect her against Covid-19.

“She’s not a little itty-bitty 11-year-old,” she said. “I wasn’t sure what to do.”

Younger children will get 10 micrograms of the Covid-19 vaccine per dose, while those who are 12 and older will get 30 micrograms.

Photo: Maddie McGarvey for The Wall Street Journal

Pediatricians across the country said they have received similar inquiries from parents. If their child is approaching the age of 12, should they wait to get them the bigger dose? The answer from most pediatric and infectious-disease experts is no.

“Don’t wait until you’re 12,” said Paul Offit, an infectious-disease specialist and director of the Vaccine Education Center at the Children’s Hospital of Philadelphia who also sits on the Food and Drug Administration’s advisory panel on vaccines.

A lower dose doesn’t necessarily render the vaccine less effective, said Dr. Offit. The goal of any vaccine is to induce a protective immune response with the lowest dose possible, he said. Lower doses minimize side effects, he said.

The 12-year-age cutoff for the larger dose reflects the way Pfizer designed trials for the vaccine. Dr. Offit said 12-year-olds should get the larger dose because Pfizer’s studies demonstrate the safety and efficacy of the larger dose at that age, he said.

Ten-, 20- and 30-microgram doses of the Pfizer-BioNTech vaccine were tested in young children, said Stanley Perlman, a pediatric infectious-disease specialist who is also a member of the panel advising the FDA on Covid-19 vaccines. “The immune responses were the same,” he said.

Families waiting with their children after receiving the Pfizer-BioNTech Covid-19 vaccine at a high school clinic in Hazel Park, Mich.

Photo: Emily Elconin/Bloomberg News

The Covid-19 vaccine was safe and elicited robust neutralizing antibody responses in a mid- to late-stage trial of children age 5 to 11, Pfizer said. The antibody responses in participants who got 10-microgram doses were comparable to those in a previous Pfizer-BioNTech study of people age 16 to 25 who got the larger, 30-microgram doses, Pfizer said.

Pediatric and infectious-disease experts said dosing is different for vaccines than for medicines like antibiotics because they work in different ways. With antibiotics, a person’s weight matters because the patient needs to maintain a certain level of the medication in their bloodstream, Dr. Offit said. That isn’t the case with a vaccine, which works by stimulating the immune system.

Children aged 5 to 11 started getting their first doses of Covid-19 vaccines last week after the CDC recommended use of the Pfizer-BioNTech shot for that age group. Some parents said they were eager for kids to get vaccinated and return to normal life. Photo: Maddie McGarvey/WSJ The Wall Street Journal Interactive Edition

The FDA authorization allows children who turn 12 between their first and second dose to get either the smaller dose meant for 11-year-olds or the larger dose for those 12 and older. The CDC says children should receive the vaccine dosage based on their age on the day of vaccination of that particular dose. For instance, if a child turns 12 between doses, they should receive the larger, 30-microgram formulation for their second dose, the CDC said.

Tina Tan,

a pediatric infectious-disease specialist at Ann & Robert H. Lurie Children’s Hospital of Chicago, said people shouldn’t wait to get their children vaccinated until they age into the larger dosage range at 12 years old.

“The coronavirus is still circulating in the community, there are still a lot of people who are unvaccinated, and Delta is highly transmissible,” she said.

Ms. Adams-Kaplan said she spoke with her children’s pediatrician, who echoed that advice. She decided not to wait.

“I will get the vaccine for her as soon as I can get it,” she said. “We’ve been waiting for so long.”

Write to Sarah Toy at sarah.toy@wsj.com