The best way for new mothers to protect their nursing babies from COVID-19 is to get vaccinated themselves. Getting vaccinated during pregnancy or during the postpartum period can help protect you and your baby after birth, as some of a mother’s immunity against COVID from a vaccine can pass through breast milk to your baby.
When a person gets vaccinated while breastfeeding, their immune system develops antibodies that protect against COVID. These antibodies are passed through breast milk to the baby. Newborns of vaccinated mothers who breastfeed can benefit from these antibodies against COVID.
SOURCE: https://www.health.harvard.edu/blog/wondering-about-covid-19-vaccines-if-youre-breastfeeding-202109032584 (Harvard Medical School)