The purpose of this study is to learn about the safety, tolerability, and immunogenicity of an updated vaccine against COVID-19, called BNT162b2 (2025/2026 formulation). This study is seeking participants 5 through 11 years of age who: * have at least 1 underlying condition that puts them at high risk for severe outcomes from COVID-19, * and are medically stable. All participants in this study will receive 1 vaccine dose given in the muscle of their arm of a BNT162b2 (2025/2026 formulation) vaccine which targets the COVID-19 virus, specifically the strain selected for the 2025-2026 COVID-19 viral respiratory season. Participants will take part in this study for about 6 months and will need to visit the clinical study site at least 2 times.