Three doses of Pfizer’s COVID-19 vaccine provide excellent protection for children under the age of five, the company announced on Monday. Pfizer intends to provide the data to US regulators later this week, a step toward allowing the shots to be administered to the youngest children.

The announcement comes after months of anxious waiting by parents desperate to vaccinate their babies, toddlers, and preschoolers, especially with COVID-19 cases on the rise. The 18 million children under the age of five are the only group in the United States who are not yet eligible for COVID-19 vaccination.

The FDA has begun reviewing data from rival Moderna, which hopes to begin offering two kid-sized shots by summer.

Pfizer has had a more difficult time determining its strategy. It aims to give preschoolers an even lower dose — one-tenth of what adults receive — but discovered during its trial that two shots weren’t quite strong enough. During the winter surge of the omicron variant, researchers gave a third shot to over 1,600 children ranging in age from 6 months to 4 years.

Pfizer and its partner BioNTech said in a press release that the extra shot did the trick, increasing tots’ levels of virus-fighting antibodies enough to meet FDA criteria for emergency use of the vaccine with no safety issues.

The companies stated that preliminary data suggested the three-dose series is 80 percent effective in preventing symptomatic COVID-19, but they cautioned that the calculation is based on only 10 cases diagnosed among study participants by the end of April. According to the study rules, at least 21 cases are required to formally determine effectiveness, and Pfizer promised an update as soon as more data became available.

The companies had already submitted data to the FDA on the first two doses, and BioNTech’s CEO, Dr. Ugur Sahin, stated that the final third-shot data would be submitted this week.

According to Dr. Jesse Goodman of Georgetown University, a former FDA vaccine chief, Pfizer’s conclusion that the tots produce antibodies similar to levels that protect young adults was encouraging.

If the data is confirmed by the FDA, the vaccine could “be an important tool to help parents protect their children,” according to Goodman. However, he cautioned that it is critical to track how long protection lasts, particularly against serious disease.

What comes next? Dr. Peter Marks, the FDA’s vaccine chief, has promised that the agency will “move quickly without sacrificing our standards” in evaluating tot-sized doses from both Pfizer and Moderna.

Moderna wants to be the first to vaccinate the youngest children. It reported to the FDA that tots develop high levels of virus-fighting antibodies after two shots containing one-quarter the dose given to adults. The Moderna study found that during the omicron surge, effectiveness against symptomatic COVID-19 was 40% to 50%, similar to adults who had only two vaccine doses.

To complicate Moderna’s progress, the FDA has only allowed its vaccine to be used in adults so far. Other countries allow it to be given to children as young as six, and the company is also seeking FDA approval for teens and elementary-aged children.

In June, the FDA set tentative dates for its scientific advisers to debate Moderna and Pfizer’s data publicly. The most recent deadline is June 14 for deciding whether to allow Moderna vaccine for older children and June 15 for debating vaccinations for children under the age of five from either or both companies, depending on the status of the applications.

If either vaccine is approved for the youngest children, the Centers for Disease Control and Prevention must decide whether all children under the age of five should receive it or only those at high risk.

While COVID-19 is not as dangerous to children as it is to adults, some children do become seriously ill or die as a result of it. The omicron variant was especially hard on children, with those under the age of five being hospitalized at higher rates than at the peak of the previous delta surge.

It’s unclear how much demand there will be for vaccines for the youngest children. Pfizer shots for children aged 5 to 11 were introduced in November, but only about 30% of that age group received the recommended initial two doses. For the best protection against the latest coronavirus variants, US health officials said last week that elementary-age children should get a booster shot, just like everyone 12 and older.