Pfizer Inc and BioNTech's vaccine was 73.2% effective in preventing COVID-19 among children aged 6 months through 4 years, new data from the companies showed on Tuesday, two months after the U.S. rollout of the shots began for that age group.

The Pfizer-BioNTech vaccine was authorized for children under 5 years of age in June, based on data that showed the vaccine generated a similar immune response as in older age groups.

An early analysis based on 10 symptomatic COVID-19 cases in the study had suggested a vaccine efficacy of 80.3%. But experts had warned that the data was preliminary due to a low number of symptomatic cases.

The updated data released on Tuesday showed 13 children had COVID-19, at least seven days after receiving a third dose of the Pfizer-BioNTech COVID-19 vaccine, compared with 21 cases among those who received a placebo.

Most cases were caused by the Omicron BA.2 variant that was dominant in March and April, when the study was conducted.

Pfizer and BioNTech also said they were preparing an application seeking U.S. authorization for a so-called bivalent vaccine that targets the BA.4/BA.5 subvariants of Omicron for children under 12.

The companies on Monday applied for U.S. authorization of the bivalent vaccine as a booster in those aged 12 and above. 

Pfizer said a pediatric trial for the Omicron-adapted vaccine would begin in the fall, and it would work with regulators to determine what data was needed for authorization.