In December 2020, 2 lipid nanoparticle-formulated, nucleoside-modified messenger RNA–based vaccines received emergency use authorization by the US Food and Drug Administration, after their trials demonstrated 94% to 95% efficacy in preventing coronavirus disease 2019 (COVID-19). Although no lactating people were included in the vaccine trials, national organizations support vaccination of this population, suggesting potential infant protection by passive transfer of maternal antibodies. , However, there are no published data to support this theoretical benefit. We sought to characterize breast milk levels of anti–severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies in lactating people undergoing COVID-19 vaccination.
Participants were prospectively recruited during phase IA rollout of the COVID-19 vaccine at a tertiary care center, after institutional review board approval. Inclusion criteria included lactation and planned vaccination with the Pfizer-BioNTech (Pfizer, Inc, New York, NY)/BNT162b2 vaccine (BioNTech SE, Mainz, Germany). After obtaining informed consent, participants provided frozen breast milk samples at the following time points of vaccination: before, within the first 24 hours, and the following week. Samples were assessed for SARS-CoV-2 RNA by quantitative real-time polymerase chain reaction and antispike immunoglobulin (Ig) G and IgA by an enzyme-linked immunosorbent assay.
A total of 5 subjects and 29 human milk samples were included in the analysis. Subject characteristics are reported in Figure 1 , A. All prevaccine milk samples tested negative for SARS-CoV-2 RNA, as defined by the cycle threshold value of >40 for the N1 target ( Figure 1 , B). Antispike IgG and IgA levels were significantly elevated relative to the prevaccine baseline at all time points. Antispike protein IgG remained sustained at a significant elevation beginning at 20 days after the first dose compared with the prevaccine baseline ( P =.0061), through the final milk sample (day 30–39 P =.0095, >40 days P =.0040; ( Figure 1 , C). Levels of antispike protein IgA were significantly elevated from baseline, starting 2 weeks after the first dose ( P =.0286) through to the final sample (day 20–29 P =.0121, day 30–39 P =.0095, >40 days P =.0040); however, individual level data suggest a possible gradual decline in antispike IgA in human milk over time after the second dose ( Figure 1 , D).