Immunogenicity and Gut Microbiota Profiles After SARS-CoV-2 mRNA Vaccination

Understanding the immune response to SARS-CoV-2 vaccination in cancer patients remains a critical priority given their immunocompromised status. In this prospective observational study, we evaluated humoral and cellular immunity across three timepoints—baseline, post-second dose, and post-booster—in 23 breast cancer patients undergoing active treatment. IgG antibody levels showed a significant increase following vaccination, with a 300-fold rise after the second dose and a 2200-fold increase post-booster, indicating a strong humoral response. CD19? B cells also increased significantly, supporting B cell-mediated activation. Although overall T cell frequencies remained stable, we observed a shift toward memory phenotypes, with decreased naïve CD4? and CD8? T cells and increased central and peripheral memory subsets after the booster. Notably, CD8? TEMRA cells expanded significantly, suggesting cytotoxic memory formation. Correlation analyses linked peripheral memory CD4? T cells with anti_SARS_CoV2 IgG titers, while CD8? TEMRA cells showed an inverse association. Microbiota profiling via sPLS-DA suggested weak but distinct microbial signatures associated with immune responsiveness, particularly enrichment of taxa like Alistipes and Romoutsia among high antibody responders. These findings emphasize that SARS-CoV-2 vaccination is immunogenic and well tolerated in breast cancer patients under therapy and highlight the need to further explore microbiota-immune interactions to optimize vaccination strategies in oncology.