NEW YORK — Pfizer Inc. and BioNTech will begin conducting a coronavirus vaccine trial among children under the age of12 in a late-stage study seeking to extend the protection against the virus across the entire range of ages, the vaccine producers announced plans on Tuesday. As many as 4,500 participants will be enrolled at more than 90 sites in the United States, Finland, Poland and Spain for the trial which is expected to begin within weeks. 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. — Developing