The development of messenger RNA (mRNA)–mediated vaccines, useful in the prevention of severe COVID-19 disease, offers the potential for novel preventive vaccines for pathogens such as cytomegalovirus, Epstein-Barr virus, and respiratory syncytial virus, as well as vaccines to treat cancer.1 Some of these vaccines may become available soon. At present, Medicare covers some vaccines under Medicare Part B, without cost sharing, although others are covered under Medicare Part D, with cost sharing for many beneficiaries. Why is this the case, and how should Medicare cover vaccines in the future?