Insights into Microbiota–Vaccine Crosstalk in Humans: Mechanisms, Modulators, and Translational Horizons

Vaccine responses differ substantially among individuals and across populations. Although factors such as age, genetics, and vaccine type are recognized contributors, they do not fully explain this heterogeneity. Emerging evidence suggests that the human microbiota, particularly the gut microbiota, may modulate immune responses to vaccination and represents a potentially modifiable component of immunity. This review integrates data from human studies, microbiota-targeted clinical trials, and experiments using germ-free and humanized models to clarify the mechanisms underlying microbiota–immune system interactions during vaccination. Identified mechanisms include pattern-recognition receptor signaling, modulation of innate immune activation, regulation of germinal-center responses, maintenance of mucosal barrier integrity, and the influence of microbial metabolites on T and B lymphocytes. The relevance of these pathways varies by age, developmental stage, and vaccine platform, including live-attenuated, inactivated, subunit, viral-vector, and mRNA vaccines. Additional factors such as diet, antibiotic exposure, infections, medications, and environmental or social determinants also affect both the microbiota and vaccine outcomes. Current research explores approaches to improve vaccine potency through microbiota modulation using probiotics, prebiotics, synbiotics, postbiotics, and engineered microbes, though clinical results remain inconsistent. A greater understanding of microbiota–vaccine interactions may enable personalized immunization strategies; however, further research is required to establish causality and identify actionable microbial targets. Longitudinal studies using multi-omics, advanced cellular analyses, robust clinical trials, and in silico modeling are essential to determine whether microbiome-based interventions can improve vaccine efficacy and durability.