Three doses of Pfizer’s COVID-19 vaccine offer strong protection for children younger than 5, the company announced Monday, another step toward shots for the littlest kids possibly beginning in early summer.

Pfizer plans to submit the findings to U.S. regulators later this week. The Food and Drug Administration already is evaluating an application by rival Moderna to offer two-dose vaccinations to tots — and set June 15 as a tentative date for its independent scientific advisers to publicly debate the data from one or both companies.

The news comes after months of anxious waiting by parents desperate to vaccinate their babies, toddlers and preschoolers, especially as COVID-19 cases once again are rising. The 18 million youngsters under 5 are the only group in the U.S. not yet eligible for COVID-19 vaccination.

Pfizer has had a bumpy time figuring out its approach. It aims to give tots an extra low dose — just one-tenth of the amount adults receive — but discovered during its trial that two shots didn’t seem quite strong enough for preschoolers. So researchers gave a third shot to more than 1,600 youngsters — from age 6 months to 4 years — during the winter surge of the omicron variant.

In a press release, Pfizer and its partner BioNTech said the extra shot did the trick, revving up the children’s levels of virus-fighting antibodies enough to meet FDA criteria for emergency use of the vaccine with no safety problems.

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

While the vaccine effectiveness likely could change somewhat, “all of this is very positive for those parents who are looking forward to having a vaccine for their younger children in the coming months,” said Dr. William Moss of the Johns Hopkins Bloomberg School of Public Health, who was not part of the study.

If FDA confirms the data, the vaccine could “be an important tool to help parents protect their children,” agreed Dr. Jesse Goodman of Georgetown University, a former FDA vaccine chief. But he cautioned that it’s essential to track how long protection lasts, especially against serious disease.

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