ABSTRACT The bacterium Rhodococcus equi is a common cause of severe pneumonia in foals worldwide. Foals are exposed to R. equi from birth and are most susceptible in the first weeks of life. In North America, control measures are limited to transfusion to newborn foals of hyperimmune plasma whose potency is assessed by surpassing a threshold of IgG against the virulence- associated protein A (VapA) of R. equi. Here we report results of vaccination of pregnant mares with mRNA encoding a GPI-anchored virulence-associated protein A (VapA) of R. equi induced VapA-specific IgG subisotypes transferred to their foals during the 63-day period after birth. Pregnant mares were randomly assigned to receive either VapA mRNA vaccine (n=10) at 8 and 10 months of gestation or no vaccine (n=10). Serum was collected from mares at 8 months of gestation (before vaccination for immunized mares) and ≈24 hours after foaling and from foals at ages 1, 35, and 63 days. Mean relative concentrations of were low before immunization in all mares and did not change over time in unvaccinated mares; however, mean relative concentrations of anti-VapA IgG 1, IgG 4/7, and IgG 3/5 of mares were significantly increased after VapA mRNA immunization and strongly correlated with the corresponding concentrations in their foals’ sera at age ≈24 hours. Concentrations of anti-VapA IgGs remained increased in foals of vaccinated mares compared with control foals for at least 35 days. IMPORTANCE Results of our study indicate that maternal vaccination of pregnant mares with a GPI-anchored VapA mRNA vaccine provides humoral immunity to foals when they are most susceptible to infection. These findings indicate that further evaluation of the efficacy of this vaccine to prevent pneumonia in foals infected with R. equi is warranted.