Pfizer said Monday its COVID-19 vaccine works for children ages 5 to 11 and that it will seek U.S. authorization for this age group soon — a key step toward beginning vaccinations for youngsters.

The vaccine made by Pfizer and its German partner BioNTech already is available for anyone 12 and older. But with kids now back in school and the extra-contagious delta variant causing a huge jump in pediatric infections, many parents are anxiously awaiting vaccinations for their younger children.

For elementary school-aged kids, Pfizer tested a much lower dose — a third of the amount that’s in each shot given now. Yet after their second dose, children ages 5 to 11 developed coronavirus-fighting antibody levels just as strong as teenagers and young adults getting the regular-strength shots, Dr. Bill Gruber, a Pfizer senior vice president, told The Associated Press.

Read: US assures Covid cooperation to continue as 1-mn doses of Pfizer's vaccine received

The kid dosage also proved safe, with similar or fewer temporary side effects — such as sore arms, fever or achiness — that teens experience, he said.

“I think we really hit the sweet spot,” said Gruber, who’s also a pediatrician.

Gruber said the companies aim to apply to the Food and Drug Administration by the end of the month for emergency use in this age group, followed shortly afterward with applications to European and British regulators.

Earlier this month, FDA chief Dr. Peter Marks told the AP that once Pfizer turns over its study results, his agency would evaluate the data “hopefully in a matter of weeks” to decide if the shots are safe and effective enough for younger kids.

An outside expert said scientists want to see more details but called the report encouraging.

“These topline results are very good news,” said Dr. Jesse Goodman of Georgetown University, a former FDA vaccine chief. The level of immune response Pfizer reported “appears likely to be protective.”

Read: US regulators give full approval to Pfizer COVID-19 vaccine

Many Western countries so far have vaccinated no younger than age 12, awaiting evidence of what’s the right dose and that it works safely. Cuba last week began immunizing children as young as 2 with its homegrown vaccines and Chinese regulators have cleared two of its brands down to age 3.

While kids are at lower risk of severe illness or death than older people, more than 5 million children in the U.S. have tested positive for COVID-19 since the pandemic began and at least 460 have died, according to the American Academy of Pediatrics. Cases in children have risen as the delta variant swept through the country.