The Summary
Researchers tracked 171 healthy adults receiving COVID-19 mRNA vaccines to investigate why vaccine-induced fevers vary. Participants who spiked fevers showed pre-existing low-grade intestinal inflammation and elevated levels of flagellated Lachnospiraceae bacteria expressing flagellin before vaccination. In laboratory follow-ups, transplanting microbiomes from fever-prone donors provoked inflammatory pathways in human intestinal organoids and triggered flagellin-dependent reactions in gnotobiotic mice. Notably, these bacterial traits shifted with diet in animal models and tracked with dietary habits in humans, demonstrating that resident microbes tune systemic immune responses to vaccines.
Why this is interesting
Post-vaccine fevers are typically blamed solely on individual genetics or the vaccine formula itself. This discovery reveals that our internal microbial ecosystem actively sets our baseline immune tone, dictating how intensely the body reacts to immunization. Because the culprit bacterial traits were tied to diet and metabolic markers, future nutritional interventions or targeted therapies could potentially reduce uncomfortable vaccine side effects without compromising protective immunity, helping alleviate vaccine hesitancy.