In an interview Wednesday, Dr. Paul Burton, Moderna’s chief medical officer, said that “there is an unmet medical need” for the youngest children.
“The data that we have now, that will go in today, should be sufficient for F.D.A. to begin the review,” he said. He said he hoped that the agency would “carefully and appropriately assess the data and not hold it up.” He added: “They’ll do the right thing. They always have.”
Moderna’s clinical trial data showed that the antibody response of the youngest children compared favorably with that of adults ages 18 to 25, meeting the trial’s primary criterion for success. Although the trial was not big enough to measure vaccine effectiveness, Moderna said Thursday the vaccine appeared to be 51 percent effective against symptomatic infection among those younger than 2, and 37 percent effective among those 2 to 5.
Those results were slightly better than the ones Moderna previously released for children under 2. The company said that was because the second time, the firm excluded infections that had not been confirmed with a P.C.R. test analyzed in a laboratory.
Dr. Burton said the new results came from a more rigorous analysis, but did not differ significantly from the earlier ones. He said the Omicron variant, which proved adept at skirting the body’s first line of defense, accounted for about 80 percent of infections in the study group.
Omicron has scrambled the calculus for evaluating vaccines because it has proved far more adept than previous versions of the virus at evading the vaccines’ shield against infection, although their protection remains strong against severe illness and death. Both Moderna and Pfizer found that, compared with other, earlier trials, their vaccines’ effectiveness against infection plunged in clinical trials for young children, which were carried out largely during the winter Omicron surge.
At the same time, Omicron helped build up the nation’s immunity. As of February, 60 percent of Americans, including 75 percent of children, had been infected with the coronavirus at some point, according to research released this week by the Centers for Disease Control and Prevention. Dr. Sean O’Leary, an infectious disease specialist at Children’s Hospital Colorado, said the C.D.C.’s findings did not lessen the need to vaccinate young children. “A lot of kids have been infected,” he said, but “there is certainly additional benefit of getting vaccinated.”