Pfizer's ongoing phase 3 trial studying its COVID-19 vaccine among children between 6 months and 4 years old continued to prove efficacy but showed it has slipped from about 80 percent to 73.2 percent.
The updated efficacy rate results from 34 study participants, according to Pfizer.
Four infant cases showed a vaccine efficacy rate of 75.8 percent and nine cases among children aged between 2 and 4 found a 71.8 percent efficacy rate.
The FDA and CDC initiated emergency use authorization for Pfizer's vaccine for the youngest population June 17, and Moderna's vaccine for children 6 months to 5 years old was authorized the same day. Pfizer's data at that time — before omicron subvariants BA.4 and BA.5 surged — showed an 80 percent efficacy rate. Moderna reported a 50.6 percent efficacy rate for infants 6 to 23 months old and a 36.8 percent efficacy rate for 2- to 5-year-olds.
As omicron subvariant BA.5 accounts for a high proportion of national cases, Pfizer is steaming ahead to gain federal approval of its tweaked vaccine booster aimed at BA.5.