The human gut microbiome primes fever after vaccination.

Fever risk after SARS-CoV-2 mRNA vaccination is linked to low-grade intestinal inflammation and increased flagellated Lachnospiraceae bacteria in the gut microbiome.

• Why it matters: Understanding the factors that contribute to vaccine side effects like fever can help reduce vaccine hesitancy and improve uptake. The gut microbiome's role in immune responses remains underexplored, especially in relation to vaccine reactions.
• What they did: The study profiled fecal microbiota, oral temperature, and serological markers in 171 healthy adults before vaccination, and examined microbiome effects on inflammation and vaccine reactions using human organoids and gnotobiotic mice, also exploring diet influences.
• The result: Microbiomes from fever-high donors caused stronger inflammation and flagellin-dependent vaccine reactions, which could be modulated by diet, indicating the microbiome's potential as a target to reduce vaccine side effects.