Background Pregnant and lactating women were excluded from initial COVID-19 vaccine trials; thus, data to guide vaccine decision-making are lacking . Objectives To evaluate the immunogenicity and reactogenicity of COVID-19 mRNA vaccination in pregnant and lactating women compared to: (1) non-pregnant controls and (2) natural COVID-19 infection in pregnancy . Study Design 131 reproductive-age vaccine recipients (84 pregnant, 31 lactating, and 16 non-pregnant) were enrolled in a prospective cohort study at two academic medical centers . Titers of SARS-CoV-2 Spike and RBD IgG, IgA and IgM were quantified in participant sera (N=131) and breastmilk (N=31) at baseline, second vaccine dose , 2-6 weeks post second vaccine, and at delivery by Luminex . Umbilical cord sera (N=10) titers were assessed at delivery . Titers were compared to those of pregnant women 4-12 weeks from natural infection (N=37) by ELISA . A pseudovirus neutralization assay was used to quantify neutralizing antibody titers for the subset of women who delivered during the study period . Post-vaccination symptoms were assessed via questionnaire . Kruskal-Wallis tests and a mixed effects model, with correction for multiple comparisons, were used to assess differences between groups . Results Vaccine-induced antibody titers were equivalent in pregnant and lactating compared to non-pregnant women (median [IQR] 5.59 [4.68-5.89] pregnant , 5.74 [5.06-6.22] lactating , 5.62 [4.77-5.98] non-pregnant, p = 0.24). All titers were significantly higher than those induced by SARS-CoV-2 infection during pregnancy (p <0.0001). Vaccine-generated antibodies were present in all umbilical cord blood and breastmilk samples . Neutralizing antibody titers were lower in umbilical cord compared to maternal sera, although this finding did not achieve statistical significance (median [IQR] 104.7 [61.2-188.2] maternal sera , 52.3 [11.7-69.6] cord sera, p=0.05). The second vaccine dose (boost dose) increased SARS-CoV-2-specific IgG, but not IgA, in maternal blood and breastmilk . No differences were noted in reactogenicity across the groups . Conclusions COVID-19 mRNA vaccines generated robust humoral immunity in pregnant and lactating women, with immunogenicity and reactogenicity similar to that observed in non-pregnant women . Vaccine-induced immune responses were significantly greater than the response to natural infection . Immune transfer to neonates occurred via placenta and breastmilk.