| Summary: | Background: Initially, there were no data on the safety of COVID-19 vaccines in lactating women. The aim of our study was to evaluate the immune response to COVID-19 vaccinations in breastfeeding women. Methods: The study included 32 breastfeeding women who, regardless of the study, had decided to be vaccinated. Maternal serum and breast milk samples were simultaneously collected on days 8 ± 1, 22 ± 2, 29 ± 3, and 43 ± 4 after the first dose of the vaccine. The immune response was assessed by determining the presence of anti-SARS-CoV-2 IgG and IgA. Results: The breast milk IgG level was detectable (6.50 ± 6.74, median 4.7, and maximum 34.2 BAU/mL) and highly correlated to serum IgG level (r<sub>S</sub> 0.89; <i>p</i> < 0.001). The breast milk ratio of IgA to the cut-off value was higher in serum IgA-positive (4.18 ± 3.26, median 2.8, and maximum >10) than in serum IgA-negative women (0.56 ± 0.37, median 0.5, and maximum 1.6; <i>p</i> < 0.001). The highest concentrations of serum and breast milk antibodies were observed on day 29 ± 3 with a decrease on day 43 ± 4. Conclusion: The immune response to the vaccination against SARS-CoV-2 is strongest 7 ± 3 days after the second dose of the vaccine. Lactating mothers breastfeeding their children after vaccination against SARS-CoV-2 may transfer antibodies to their infant.
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