Pfizer Inc said on Tuesday it will begin testing its Covid-19 vaccine in a larger group of children under age 12 after selecting a lower dose of the shot in an earlier stage of the trial.

The study will enroll up to 4,500 children at more than 90 clinical sites in the United States, Finland, Poland and Spain, the company said.

Based on safety, tolerability and the immune response generated by 144 children in a phase I study of the two-dose shot, Pfizer said it will test a dose of 10 micrograms in children between 5 and 11 years of age, and 3 micrograms for the age group of 6 months to 5.