A Study to Learn About Variant-Adapted COVID-19 RNA Vaccine Candidate(s) in Healthy Children

The purpose of this clinical trial is to learn about the safety, extent of the side effects, and immune responses of the study vaccine (called variant-adapted BNT162b2 RNA-based vaccine) in healthy children. The trial is divided into 5 individual studies or substudies based on age group and prior history of COVID-19 vaccinations. All participants in each of the 5 sub-studies will receive study vaccine as a shot depending on what group they are in. * Substudy A design: Phase 1 includes participants 6 months through less than 4 years 3 months of age who have not received a previous coronavirus vaccination (COVID-19 vaccine naïve) and will receive 3 doses of study vaccine as their initial series, followed by a fourth dose of study vaccine. Phase 2/3 includes participants 6 months through less than 5 years of age who have not received a previous coronavirus vaccination (COVID-19 vaccine naive) and will receive 1, 2, or 3 doses of study vaccine, depending on what group they are in. * Substudy B design: includes participants 6 months through less than 5 years of age who have either received 2 or 3 prior doses of BNT162b2 and will receive study vaccine as their third or fourth dose. * Substudy C design: Phase 1 includes participants 6 months through less than 5 years of age who have received 3 prior doses of BNT162b2 and will receive study vaccine as their fourth dose. * Substudy D design: includes participants 5 through less than12 years of age who have received 2 or 3 prior doses of BNT162b2 and will receive study vaccine as their third or fourth dose. * Substudy E design: includes participants 5 through less than 12 years of age who have not received a previous coronavirus vaccination (COVID-19 vaccine naive) and will receive a single dose of study vaccine.

Substudy A Inclusion Criteria: * Phase 1: Healthy male or female participants ≥6 months to \