This study investigated the impact of the SARS-CoV-2 mRNA vaccine (BNT162b2) on breast milk microbiota composition and its association with vaccine-induced antibody levels. 49 lactating mothers received two doses of the vaccine, and breast milk samples were collected at four time points. SARS-CoV-2 spike-specific IgA and IgG levels peaked one week post-second dose, then waned. Breast milk microbiota richness and composition changed dynamically post-vaccination, but beneficial microbes like Bifidobacterium species were unaffected. Baseline breast milk bacterial composition predicted spike-specific IgA levels (AUC 0.72). The study suggests specific microbiota markers associated with high IgA levels and that the vaccine did not significantly reduce probiotic species.

Shilin Zhao, Kris Y. W. Lok, Zhen Y. Sin, Ye Peng, Heidi S. L. Fan, Nitya Nagesh, Martha S. L. Choi, Jojo Y. Y. Kwok, Edmond P. H. Choi, Xi Zhang, Hogan Kok-Fung Wai, Leo C. H. Tsang, Samuel S. M. Cheng, Matthew K. L. Wong, Jie Zhu, Chris K. P. Mok, Siew C. Ng, Francis K. L. Chan, Malik Peiris, Leo L. M. Poon, Hein M. Tun