Pfizer’s COVID-19 vaccine was 73% effective in protecting children younger than 5 as omicron spread in the spring, the company announced today.

Vaccinations for babies, toddlers and preschoolers opened in the U.S. in June after months of delay. Only about 6% of youngsters ages 6 months through 4 years had gotten at least one dose of a COVID-19 vaccine by mid-August, according to the American Academy of Pediatrics.

Health authorities authorized tot-sized vaccine doses made by Pfizer and its partner BioNTech based on a study showing they were safe and produced high levels of virus-fighting antibodies. But there was only preliminary data on how that translated into effectiveness against symptomatic COVID-19.

The new update analyzed COVID-19 diagnoses between March and June in Pfizer’s ongoing study of the three-dose vaccine. There were 21 COVID-19 cases among the 351 tots who got dummy shots — compared to just 13 among the 794 youngsters given three vaccine doses.

The child cases primarily were caused by the BA.2 omicron version that was circulating at the time. Today, another omicron relative, BA.5, is causing most COVID-19 cases in the U.S. and much of the world.

In older children and adults, the COVID-19 vaccines have been used long enough to prove that they remain strongly protective against severe disease and death even as the coronavirus mutates — while early protection against infection wanes. Still, scientists track that initial effectiveness rate as extra evidence of vaccine performance — and to look for signs of how they initially hold up against new mutants.

Pfizer this week asked U.S. regulators to authorize modified vaccine doses that better match the newest omicron variants for people 12 and older as boosters this fall. The company said it also is developing updated shots for kids under 12.