Study to Evaluate Safety and Immunogenicity of COVID-19 Vaccine in Children 6 Months to < 12 Years

This is a Phase 2/3 randomized, observer-blinded, placebo-controlled, age de-escalation trial to evaluate the safety and immunogenicity of 2 primary doses of SARS-CoV-2 rS with Matrix-M™ adjuvant (NVX-CoV2373) given 21 days apart and NVX CoV2373 or a variant-based vaccine given as a booster dose or at crossover in pediatric participants (3 age cohorts; 6 to \< 12 years, 2 to \< 6 years, and 6 to \< 24 months of age). Each age cohort will be conducted in 2 parts starting with the oldest age cohort (6 to \< 12 years of age). Part 1 will enroll approximately 120 healthy or medically stable sentinel participants per age cohort (10% of the intended enrollment population per age cohort, for a total of 360 sentinel participants overall) who will be randomized in a 1:1 ratio to receive 2 doses of NVX-CoV2373 or placebo with doses given 21 days apart. Part 2 will enroll a larger number of healthy or medically stable participants (N= approximately 1,080 per age cohort), for a total of approximately 3,240 pediatric participants enrolled in Part 2, and a total of approximately 3,600 participants enrolled in the entire trial). Initial randomization in Part 2 will be in a 2:1 ratio, and the safety and effectiveness of 2 doses of NVX-CoV2373 given 21 days apart will be assessed.