This research is studying a vaccine against Mpox (formerly known as monkeypox) that is FDA approved for adults but not yet approved for children. The goal of the study is to determine if the vaccine protects children just as well as it protects adults. The study is also looking at the safety of the vaccine. This study is open to healthy adolescents, ages 12-17. In addition, adolescents with certain immunodeficiency diseases or other conditions, that are well controlled, are eligible to participate.