A Science study reported that gut bacteria—particularly flagellin-producing Lachnospiraceae—along with diet influenced whether individuals developed fever after mRNA vaccination, without changing antibody responses. The work points to a disconnect between innate inflammatory side effects and adaptive humoral immunity. The observation suggests that post-vaccination reactogenicity may be biologically modulated by intestinal microbial and dietary states, with potential implications for predicting tolerability in clinical practice. It also adds a mechanistic layer to vaccine immunology by shifting attention toward gut-immune signalling. For vaccine developers and clinical researchers, the key next step will be whether these microbiome signals can be used to forecast fever risk in larger, more diverse cohorts and whether interventions could safely modulate outcomes.