Among persons who gave birth between August 9, 2020, and April 25, 2021, at Pennsylvania Hospital, IgG antibodies to the SARS-CoV-2 spike protein were present in higher concentrations after vaccination with mRNA vaccines compared with the antibody levels present after symptomatic or asymptomatic infection and were associated with higher cord blood antibody levels after maternal vaccination compared with maternal infection. Here, PROS1 is linked to infection.