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Albert Bourla, Pfizer’s president and CEO, said in a statement, “Ultimately, we believe that three doses of the vaccine will be needed for children aged 6 months to 4 years to achieve a high level of protection against current and possible future variants. But he said that if two doses are allowed in the meantime, “parents will have the opportunity to launch a Covid-19 vaccine series for their children while pending potential authorization for a third dose.”

Pfizer and BioNTech’s study of young children aimed to measure immune responses, not the vaccine’s effectiveness in preventing infections or serious cases of Covid-19. But two people familiar with the study said that because some of the children in the experiment were infected with the virus, the researchers got indications of how well the vaccine was working to fend off the virus. A similar development occurred in the companies’ vaccine study on 5- to 11-year-olds, which coincided with an overwhelming wave of the Delta variant.

One person familiar with the data, speaking on condition of anonymity, said that children ages 2 to 4 who received two injections were 57 percent less likely to be infected than children in the placebo group. Children aged 6 months to 2 years who were vaccinated became infected at a 50 percent lower rate than the placebo group. There were fewer than 100 cases of symptomatic infection – a small proportion of respondents overall – and that person said the margins for error were wide.

The data also suggested that the vaccine protected children better against Delta infection than Omicron. Omicron is better than Delta at avoiding the protection of both Pfizer-BioNTech and Moderna vaccines in adults.

The regulators’ strategy was based in part on the possibility that Pfizer-BioNTech’s three-dose trial would be successful. Professor of pediatric infectious diseases at Stanford, Dr. Many experts, including Yvonne Maldonado, said the history of vaccines suggests that a third dose will actually boost the immune response.

D., principal investigator at the Stanford field of Pfizer-BioNTech’s pediatric vaccine trials. “There is almost no hypothesis that a third dose would be worse off,” Maldonado said. “At worst it can’t be different. So the best dose you’ll ever take is two doses.”

Another member of the FDA advisory committee and head of the pediatric infectious diseases division at Tufts Children’s Hospital, Dr. H. Cody Meissner said he has an open mind about the FDA’s approach. But he said he worries that side effects, which are rare in small trials, might not be overlooked. The Pfizer-BioNTech trial included approximately 1,200 children younger than 2 years old and approximately 1,500 children 2 to 4 years old, according to people familiar with the study’s design.