Quantification and progress over time of specific antibodies against SARS-CoV-2 in breast milk of lactating women vaccinated with BNT162b2 Pfizer-BioNTech COVID-19 vaccine (LacCOVID)

Abstract Background Transfer of passive and active immunity through human milk is a key aspect in infant protection against infections. Several observational studies demonstrated the passage of postvaccine antibodies through breast milk in women vaccinated against COVID-19, mostly with mRNA-based vaccines, but lacked long-term data. Methods A six-month prospective cohort study was performed to determine SARS-CoV-2 vaccine-induced antibody levels in the breast milk of 33 lactating healthcare workers at different time-points after mRNA BNT162b2 Pfizer-BioNTech COVID-19 vaccination. Moreover, we examined the correlation of SARS-CoV-2 antibody levels between serum and breast milk, adverse events related to vaccination (AErV) and rate of SARS-CoV-2 infections. Results Mothers’ median (IQR) age was 38(36-39) years and 15(10-22) months for infants. SARS-CoV-2 IgG-S1 vaccine-induced levels at different time-points for serum–milk pairs, median (IQR), were: 519(234-937) to 1(0-2.9) arbitrary units (AU)/mL at 2w after first dose, 18,644(9,923-29,264) to 78(33.7-128) AU/mL at 2w, 12,478(6,870-20,801) to 50.4(24.3-104) AU/mL at 4w, 4,094(2,413-8,480) to 19.9(10.8-51.9) AU/mL at 12w, and 1,350(831-2,298) to 8.9(7.8-31.5) at 24w after second dose. We observed a positive correlation of antibody levels between serum and breast milk (Pearson correlation coefficient 0.68), no serious AErV and 2(6%) COVID-19 vaccine breakthrough infections. Conclusions Women vaccinated with Pfizer-BioNTech transmit antibodies into breast milk with a positive correlation with serum levels. Both decreased over time in a 6-month follow-up. Finally, Infants of breastfeeding vaccinated women could be acquiring vaccine antibodies for at least six months after vaccination and serum determination of SARS-CoV-2 IgG-S1 could indicate breastmilk antibody levels.