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)

ImportanceTo our knowledge, this is the first study to analyze long–term passage (6 months after immunization) of specific antibodies induced by BNT162b2 COVID-19 vaccine through breast milk.ObjectivesMain objective: to determine SARS-CoV-2 vaccine induced antibody levels in the breast milk of lactating women 4 weeks after mRNA BNT162b2 Pfizer-BioNTech COVID-19 complete vaccination.Secondary objectives: to analyze SARS-CoV-2 antibody levels (breast milk and serum) at different time-points after vaccination, examine the correlation of SARS-CoV-2 antibody levels between serum and breast milk, describe adverse events related to vaccination (AErV) in both mothers and infants and determine the rate of COVID-19 infections.DesignProspective cohort study between February and September 2021.SettingParc Sanitari Sant Joan de Déu, an urban hospital in Spain.ParticipantsDuring our health worker vaccination campaign at our hospital between January and March 2, we recruited 33 lactating women vaccinated with BNT162b2 Pfizer-BioNTech COVID-19.ResultsA total of 33 volunteers were included in the study. The median (IQR) age of mothers was 38 (36-39) years and 15 (10-22) months for the infants.Primary end-point: at 4 w after second dose median (IQR) IgG-S1 levels for serum–milk pairs were 12,478 (6,870-20,801) to 50.4 (24.3-104) arbitrary units (AU) per mL.Secondary end-pointsSARS-CoV-2 antibody levels at different time-points were (serum-milk): 519 (234-937) to 1 (0-2.9) AU/mL at 2w after first dose, 18,644 (9,923-29,264) to 78 (33.7-128) AU/mL at 2w, 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 found a positive correlation of SARS-CoV-2 antibody levels between serum and breast milk (Pearson correlation coefficient 0.68).No serious AErV were observed.We found two (6%) COVID-19 vaccine breakthrough infections.ConclusionsPfizer-BioNTech COVID-19 vaccination is safe during breastfeeding and it transmits antibodies into breast milk with a positive correlation with serum levels, and both decrease over time in a 6-month follow-up. Infants of breastfeeding vaccinated women could be protected for at least six months after vaccination and serum determination of SARS-CoV-2 IgG-S1 could indicate the breastmilk levels of antibodies during this period.