Tech & Science
Study links gut bacteria flagellin to post-vaccine fever risk
New research reveals that specific gut bacteria producing flagellin may trigger vaccine-induced fevers, offering dietary strategies to mitigate side effects without reducing immunity.
Specific intestinal microbes appear to drive the likelihood of developing a fever after vaccination, according to recent findings published by the American Association for the Advancement of Science (AAAS). The study indicates that targeting the microbiome and diet could potentially lower inflammatory side effects while preserving the protective immune response vaccines provide.
Vaccine side effects vary widely
While vaccines remain essential for public health, individual reactions differ significantly. Fever and flu-like symptoms represent common adverse effects, particularly following mRNA and lipid nanoparticle-based inoculations. These physical discomforts can deter future uptake and fuel hesitancy or mistrust toward immunization programs.
Not every recipient experiences fever, yet those who do often react similarly across multiple doses. This pattern highlights the role of individual risk factors in determining reaction severity, though the underlying causes have remained unclear until now.
Gut bacteria activate inflammation
Kelsey Huus and colleagues hypothesized that the gut microbiome, known to modulate immune activity, influences these reactions. To investigate, the team analyzed fecal microbiomes, body temperature, and blood-based immune markers in 171 healthy adults before and after SARS-CoV-2 mRNA vaccination.
The data revealed that certain bacteria, especially Lachnospiraceae, produce flagellin. This protein activates inflammatory immune pathways, intensifying vaccine-related responses. Participants with higher pre-vaccination intestinal inflammation or elevated gut flagellin expression were more prone to developing fever.
Dietary patterns influence response
The authors observed that industrialized diets may promote this inflammatory activity. Vegetarian eating habits correlated with microbiome features associated with reduced fever incidence. Conversely, low intake of whole plant foods and high consumption of fats or processed items linked to the inflammatory, fever-associated microbiome profile.
Modifying diet altered both microbiome composition and subsequent vaccine responses. Crucially, Huus et al. demonstrated that flagellin, inflammation, and dietary changes did not impact antibody production. This suggests microbiome-driven inflammation increases side effects without compromising immunity.
Future preparation strategies
"Diet, microbial ecology, and the metabolic state are not immutable, and people might one day prepare for vaccination as well as they prepare for the vaccine itself," wrote Bali Pulran in an accompanying commentary. The study was authored by K. E. Huus et al. and titled "The human gut microbiome primes fever after vaccination."
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