A Phase 1, Open-Label Dose-Finding Study To Evaluate Safety, Tolerability, And Immunogenicity And Phase 2/3 Placebo-Controlled, Observer-Blinded Safety, Tolerability, And Immunogenicity Study Of A Sars-Cov-2 Rna Vaccine Candidate Against Covid-19 In Healthy Children and Among Young Adults (Pfizer C4591007)