Objectives: To investigate specific immunoglobulin A (sIgA), specific immunoglobulin G (sIgG), and neutralizing antibodies (NAbs) against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in breast milk and compare immunity in mothers with hybrid immunity (infection and vaccination) versus those solely vaccinated (coronavirus disease [COVID]-naïve). Methods: A longitudinal study was conducted among lactating mothers who received at least two doses of the coronavirus disease 2019 (COVID-19) vaccine or tested positive for SARS-CoV-2. Details of vaccination and infection were collected through questionnaires and interviews. Fifteen milliliters of breast milk samples, self-collected at 1, 3, and 6 months postvaccination or infection, were sent to analysis for sIgA, sIgG, and NAbs using enzyme-linked immunosorbent assay. Results: In total, 119 lactating mothers (202 milk samples) were enrolled; 82 participants had hybrid immunity, and 32 were COVID-19-naïve. Two-thirds received a combination of different vaccines and booster shots. Breast milk retained sIgA, sIgG, and NAbs for up to 6 months post-COVID vaccination or infection. At 3 months, mothers with hybrid immunity had significantly higher sIgA and NAbs compared with COVID-naïve mothers (geometric mean [95% confidence interval (CI)] of sIgA 2.72 [1.94-3.8] vs. 1.44 [0.83-2.48]; NAbs 86.83 [84.9-88.8] vs. 81.28 [76.02-86.9]). No differences in sIgA, sIgG, and NAbs were observed between lactating mothers receiving two, three, or more than or equal to three doses, regardless of hybrid immunity or COVID-naïve status. Conclusion: sIgA, sIgG, and NAbs against SARS-CoV-2 in breast milk sustained for up to 6 months postimmunization and infection. Higher immunity was found in mothers with hybrid immunity. These transferred immunities confirm in vitro protection, supporting the safety of breastfeeding during and after COVID-19 vaccination or infection.
Keywords: COVID-19; SARS-CoV-2; breast milk; neutralizing antibodies; specific IgA; specific IgG.