The recent approval of vaccines against COVID-19 has generated great concern among breastfeeding women, since these patients were excluded from vaccination clinical trials. The present study aimed to analyze the levels of specific SARS-CoV-2 antibodies in breast milk of mRNA-vaccinated women across time and their correlation with serum antibody levels. Methodsprospective study including lactating women aged over 18 who were vaccinated against SARS-CoV-2 with the Pfizer-BioNTech(R) COVID-19 vaccine (BNT162b2). Paired serum and breast milk samples were simultaneously taken from each participant at three timepoints after receiving the vaccine: 2 weeks after 1st dose, 2 weeks after 2nd dose and 4 weeks after 2nd dose (Timepoints 1, 2 and 3, respectively). Levels of IgG antibodies against the spike protein (S1 subunit) were determined for each sample (Architect, Abbott(R)). Resultswe collected and analyzed 52 serum and 52 milk samples from the first 18 study participants. Median (interquartile range) IgG(S1) levels for serum - milk pairs at each timepoint were 410 (208-606) - 1.7 (0-2.9) AU/ml at Timepoint 1, 11505 (8933 - 21184) - 52.2 (34.1-113) at Timepoint 2 and 8311 (5578-17419) - 41.7 (24.8-75.3) at Timepoint 3. Pearsons correlation coefficient between breast milk and serum IgG(S1) levels was 0.71. No major adverse reactions were observed in mothers or infants. ConclusionsBreast milk from women vaccinated with mRNA-based Pfizer-BioNTech(R) vaccine contains specific anti-SARS-CoV-2 IgG(S1) antibodies, with levels increasing considerably after second dose. IgG(S1) levels in breast milk are positively correlated with corresponding serum levels.