This Phase 1 clinical study will assess the safety and immunogenicity of 2 dose levels of mRNA-1653, a combined human metapneumovirus and human parainfluenza virus type 3 vaccine, in healthy adults (18 to 49 years of age) and 3 dose levels in children (12 to 59 months of age) with serologic evidence of prior exposure.

The safety profile of the adult cohort will permit enrollment of the pediatric cohort.