WASHINGTON: Sixteen children in the trial who had received a placebo got Covid-19, compared with three who were vaccinated, Pfizer said in briefing documents submitted to the US Food and Drug Administration.Because more than twice as many children in the 2,268-participant trial were given the vaccine than placebo, that equates to better than 90% efficacy. Pfizer’s clinical trial in those 5 to 11 years old was not primarily designed to measure efficacy against the virus. Instead, it compared the amount of neutralizing antibodies induced by the vaccine in the children to the response of older recipients in their adult trial.Based on those results, Pfizer and BioNTech said last month that their Covid-19 vaccine induced a robust immune response in the children. The 5 to 11 year olds were given two shots of a 10-microgram dose of the vaccine, a third of the dose size given to people 12 and older. Outside advisors to the FDA are scheduled to meet on Tuesday to vote on whether to recommend that the agency authorize the vaccine for that age group. The FDA’s staff is expected to publish their review of the evidence submitted by Pfizer later on Friday.