In view of data scarcity to guide decision-making in breastfeeding women, we evaluated how mRNA vaccines impact immune response of lactating health care workers (HCW) and the effector profile of breast milk transferred immune protection. We show that upon BNT162b2 vaccination, immune transfer via milk to suckling infants occurs through secretory IgA (SIgA) and T cells. Functionally, spike-SIgA was non-neutralizing and its titers were unaffected by vaccine boosting, indicating that spike-SIgA is produced in a T-cell independent manner by mammary gland. Even though their milk was devoid of neutralizing antibodies, we found that lactating women had higher frequencies of RBD-reactive circulating memory B cells and more RBD-IgG antibodies, when compared to controls. Nonetheless, blood neutralization titers in lactating and non-lactating HCW were similar. Further studies are required to determine transferred antibodies and spike-T cells complete functional profile and whether they can mediate protection in the suckling infant.HighlightsMilk and blood responses to BNT162b2 vaccine are initially isotype discordantImmune transfer via milk to suckling infants occurs by spike-reactive SIgA and T cellsSpike-reactive SIgA in the breastmilk is non-neutralizing and T-cell independentLactating vs non-lactating HCW had distinct cellular responses, despite similar NT50