Anti-SARS-CoV-2
antibodies have been found in human-milk after
COVID-19 infection and vaccination. However, little is known about their persistence in milk after booster vaccination and
breakthrough infection. In this study, human-milk, saliva and blood samples were collected from 33 lactating individuals before and after
mRNA-based vaccination and
COVID-19 breakthrough infections. Antibody levels were measured using ELISA and symptoms were assessed using questionnaires. Evaluation of maternal and infant symptomatology revealed that infected mothers reported more symptoms than vaccinated mothers. We found that after vaccination, human-milk anti-SARS-CoV-2
antibodies persisted for up to 8 months. In addition, distinct patterns of human milk
IgA and
IgG production we observed after
breakthrough infection compared to 3-dose vaccination series alone, indicating a differential central and mucosal immune profiles in hybrid compared with
vaccine-induced immunity. To investigate passively-derived milk antibody protection in infants, we examined the persistence of these
antibodies in infant saliva after breastfeeding. We found that
IgA was more abundant in infant saliva compared to
IgG and persist in infant saliva longer after feeding. Our results delineate the differences in milk antibody response to vaccination as compared to
breakthrough infection and emphasize the importance of improving the secretion of
IgA antibodies to human milk after vaccination to improve the protection of breastfeeding infants.