Diet modulates vaccine fever risk

A longitudinal human cohort (µHEAT) plus organoid and gnotobiotic‑mouse experiments link a gut microbiome rich in bacterial flagellins to higher risk of fever after SARS‑CoV‑2 mRNA vaccination; flagellin expression can be increased by Western‑style dietary additives in vitro and worsens reactogenicity in mice. The signal was present before vaccination, suggesting baseline diet and microbiome state prime inflammatory vaccine responses. — If causal, this creates a low‑tech lever (dietary guidance or pre‑vaccine interventions) to reduce vaccine reactogenicity and could change public‑health messaging, contraindication screening, and debate over food additives.