Breastfeeding is strongly associated with reduced infection risk and infant mortality. However, data on breast milk antibody responses following the Pfizer-BioNTech BNT162b2 COVID-19 mRNA vaccine is limited, as pregnant and lactating women were excluded from initial trials. This study aimed to address this knowledge gap by investigating the temporal dynamics of the vaccine-specific antibody response in breast milk and serum of lactating women after receiving two doses of the mRNA vaccine. Understanding this response is crucial for informing global health authorities on vaccination recommendations during lactation and assessing potential neonatal protection through passive antibody transfer via breast milk. Previous studies have shown the presence of antibodies in the breast milk of women with COVID-19, and a few reports have begun to examine the antibody response in breast milk following mRNA vaccination. However, key information regarding the neutralizing capacity of these antibodies, the dynamics of the response compared to serum, and the comparison to antibody levels elicited by other vaccines (e.g., TDaP) is lacking. This study directly addresses these gaps to better understand the efficacy of the vaccine in lactating women and its potential benefits in protecting newborns.

Existing literature highlights the significant protective role of breastfeeding in reducing infant infections and mortality. Studies have demonstrated the presence of SARS-CoV-2-specific antibodies in breast milk of women with COVID-19 infections, showcasing the potential for passive immunity transfer. However, limited research exists specifically on the antibody response in breast milk following mRNA vaccination. While a couple of studies have examined antibody presence in breast milk after mRNA vaccination, critical details remain unclear. This includes the neutralization capacity of the antibodies, the comparison between breast milk and serum antibody dynamics, and a comparison with established vaccine responses like those induced by the TDaP vaccine during pregnancy. The absence of comprehensive data hindered global health recommendations regarding COVID-19 vaccination during breastfeeding.

This prospective cohort study enrolled ten lactating healthcare providers (mean age 34.6 years) who received two doses of the BNT162b2 mRNA vaccine approximately five months postpartum. Blood and breast milk samples were collected at four time points: 7 and 14 days after the first and second vaccine doses. Enzyme-linked immunosorbent assays (ELISAs) were used to quantify spike and receptor-binding domain (RBD)-specific IgG and IgA antibody endpoint titers in serum and breast milk. The neutralization capacity of breast milk antibodies was evaluated using a spike-bearing pseudovirus neutralization assay. Breast milk IgG and IgA were purified and separately tested for neutralization capacity. The vaccine-specific IgG:IgA molar ratio was determined in both serum and breast milk. The results were compared to antibody titers elicited by the TDaP vaccine, which all participants had received during their third trimester of pregnancy. Statistical analysis was performed using Mann-Whitney U-tests and Wilcoxon signed-rank tests where appropriate. Control serum and breast milk samples from healthy women pre-dating the COVID-19 pandemic were used for comparison.

The study found a rapid and highly synchronized antibody response to the mRNA vaccine in both serum and breast milk. A significant increase in spike-specific IgG and IgA was observed 14 days after the first dose, peaking 7 days after the second dose. Both IgG and IgA in breast milk exhibited neutralizing activity, with all samples at 2D7 (7 days after the second dose) showing inhibition. The neutralizing capacity was predominantly attributed to IgG rather than IgA, which was lower overall but more abundant in concentration within the milk. The IgG:IgA molar ratio was significantly higher in serum than breast milk. The levels of vaccine-specific antibodies in breast milk were comparable to those elicited by the TDaP vaccine. The antibody production stabilized 14 days post-second dose, indicating the peak of the immune response. A pseudovirus neutralization assay revealed neutralization capacity in breast milk from all participants at 2D7. While purified IgG showed higher neutralization activity than IgA at the concentration tested (50 µg/ml), the significantly higher concentration of IgA in breast milk suggests its contribution to neutralization. The study also found no significant cytotoxicity in filtered breastmilk samples in the XTT assay. Analysis of fold-change in antibody titers demonstrated the primary and boost effects of each vaccine dose, with antibody levels peaking seven days following each dose and then stabilizing 14 days after the second dose. The IgA antibody production in breastmilk showed a decline 14 days following the second dose.

The findings demonstrate that the BNT162b2 mRNA vaccine induces a robust and rapid antibody response in both serum and breast milk of lactating women. The presence of neutralizing antibodies in breast milk suggests the potential for passive immunity transfer to infants, which could provide protection against COVID-19 during early infancy. The comparable antibody levels to those elicited by the TDaP vaccine further strengthens this hypothesis. However, it is important to note that although IgG showed greater neutralizing activity in the assay, the higher concentration of IgA may play a significant role in providing protection. This protective effect is further enhanced by the continuous consumption of breastmilk which acts as a renewable source of antibodies for the infant. The synchronized antibody response between serum and breast milk highlights the efficient transfer of maternal antibodies into breast milk following vaccination. The study results provide valuable evidence supporting the safety and potential benefits of COVID-19 mRNA vaccination during lactation. Future studies with larger cohorts and longer follow-up periods are needed to confirm these findings and evaluate the long-term persistence of breast milk antibodies and their effectiveness in preventing COVID-19 infection in infants.

This study provides compelling evidence of a robust, rapid, and synchronized antibody response in both serum and breast milk following the administration of the BNT162b2 mRNA vaccine to lactating women. The presence of neutralizing antibodies in breast milk, comparable to TDaP vaccine levels, indicates the potential for passive immunity to breastfed infants. This supports the safety and potential benefits of mRNA vaccination during lactation, though larger, longer-term studies are necessary to assess long-term antibody persistence and clinical efficacy in protecting infants from COVID-19. Future research should focus on larger cohorts, more extended follow-up periods to monitor antibody persistence, and real-world surveillance of COVID-19 incidence in infants of vaccinated mothers to determine the level of protection conferred.

The limited sample size (n=10) restricts the generalizability of the findings. The neutralization assay for purified IgG and IgA was performed at a single concentration (50 µg/ml), limiting the interpretation of IC50 values and preventing a detailed assessment of IgA's neutralizing capacity. Further research is needed to confirm these results in a larger and more diverse population. The study's focus on short-term antibody dynamics requires further investigation into the long-term persistence of antibodies in breast milk and their effectiveness in protecting infants against COVID-19.