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In a major milestone in the race to get children vaccinated against COVID-19, the Pfizer-BioNTech vaccine was found to be safe and effective in a trial of children ages 5 to 11 years old, the companies announced Monday.

The drugmakers said they would use data from the trial to apply for emergency use authorization from the US Food and Drug Administration, which would need to be granted before kids could be inoculated.

Here’s what we know so far about the children’s vaccine and when it might be available.

What kind of vaccine did Pfizer and BioNTech test in children?

If authorized, children would not receive the exact same Pfizer-BioNTech vaccine that the FDA approved for adults and adolescents. In the trial of children ages 5 to 11 years old, Pfizer gave participants a two-dose vaccine that had 10 micrograms of active ingredient, or messenger RNA, whereas the shots for people 12 and older have 30 micrograms each.

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This is a typical approach. According to an article about vaccine dosing on the Children’s Hospital of Philadelphia website, “in some cases, children and adults need different quantities of a vaccine because our immune system weakens with age.” (This is different from dosing regimens for medications, where body weight is a factor in how well a drug works).

Similar to the dosing schedule for adults and adolescents who got the Pfizer-BioNTech vaccine, the two doses for children were given three weeks apart.

How soon might kids be able to get the vaccine in Massachusetts?

Pfizer and BioNTech did not announce a timeline for when they expect the FDA to issue a decision on the children’s vaccine. The companies have not yet submitted their data to the agency, but said they plan to do so by the end of the month, according to the New York Times.

If Pfizer submits an application to the FDA by the end of the week, it is possible that the FDA could make a decision before Halloween, based on previous COVID-19 vaccine timelines. That would give the agency more than four weeks to review the data.

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After Pfizer submitted an FDA application last November, it took three weeks for the FDA to clear the COVID-19 vaccine for adults in December. It took about four weeks for the FDA to authorize the expansion of the original EUA to cover adolescents 12 to 15 years of age in May.

How did Pfizer and BioNTech measure safety and efficacy?

Unlike adults, children rarely became severely ill from COVID-19. Therefore, Pfizer and BioNTech could not draw meaningful conclusions about their vaccine’s ability to prevent COVID or hospitalization in children. Instead, researchers relied on measurements of immune response from the shots, on the assumption that the protective levels of antibodies seen in older people would be as protective in younger children, according to the New York Times.

“The safety profile and immunogenicity data in children aged 5 to 11 years vaccinated at a lower dose are consistent with those we have observed with our vaccine in other older populations at a higher dose,” said Dr. Ugur Sahin, the CEO of BioNTech, in a press release.

The companies said the vaccine was “well tolerated” in children and compared the side effects to what was observed in participants 16 to 25 years old.

I have children under 5 years old — what about them?

Pfizer and BioNTech said that the same kind of results announced for children ages 5 to 11 on Monday could be available for younger children as soon as October. Additional age groups being tested include a 2- to 5-year-old cohort and a 6-month to 2-year-old cohort.

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How many children need to be vaccinated?

Massachusetts officials say there are 884,000 children under the age of 12 in the state, including 515,000 who are ages 5 to 11.

Will there be a vaccine for kids from Moderna or Johnson and Johnson?

Both companies have vaccines available for adults in the US, but they are behind Pfizer in getting FDA authorization for adolescents and children.

On June 10, Moderna announced that it requested FDA emergency use authorization for its vaccine in adolescents, following the release of data that suggested it worked in a study of 12- to 17-year-olds. The company is still awaiting a decision. Moderna has a separate study of children between 6 months and 11 years old that is ongoing.

Johnson and Johnson said in an e-mail to the Globe in August that it expects to begin a study in 12- to 17-year-olds in the fall. Based on the results of that trial, additional studies to test the vaccine in children as young as a few months old would begin. In April, the company expanded its ongoing vaccine trial to include adolescents, beginning with those 16 and older. Johnson & Johnson has not released data from that study.




Anissa Gardizy can be reached at anissa.gardizy@globe.com. Follow her on Twitter @anissagardizy8.