In a striking reversal, federal regulators and Pfizer-BioNTech said on Friday that they would wait for data on whether three doses of the companies’ coronavirus vaccine are effective in children younger than five before deciding whether to authorize a vaccine for the age group.

The Food and Drug Administration will postpone a meeting of outside experts that was scheduled for Tuesday; the experts were to weigh the evidence and make a recommendation on whether to authorize two doses of the vaccine in young children, as Pfizer had requested.