Difference in neutralizing antibodies percentages against SARS-CoV-2 in vaccinated breastfeeding women's milk and blood.

The aim is to compare the percentage of antibodies with neutralizing potential to Wuhan and Omicron variants in the milk and blood of lactating women immunized for SARS-CoV-2 based on the number of doses, vaccine technologies used. Cross-sectional study conducted in 2022 with 214 lactating women vaccinated for COVID-19 with mRNA, non-replicating viral vector or inactivated whole virus vaccines. Data gathered via standardized questionnaire, and samples of breast milk and blood. Antibodies detected by the ELISA method. A significance level of 5% was considered. Blood had higher neutralizing antibodies percentage than milk for Wuhan (blood-91·3%/milk-23·1%) and Omicron (blood-83·7%/milk-15·2%) variants. Women receiving three or more doses had higher blood antibody percentages for Wuhan and Omicron variants. In milk, no disparity in the percentage of antibodies was found, but the probability of antibodies' presence was higher than in blood for both variants. Vaccine technologies led to significant differences in the presence of antibodies in milk but not in blood over time for Omicron variant. No difference was observed for Wuhan variant. Protection conferred by antibodies present in milk lasted longer than in blood.