A study of 171 adults links pre-existing gut inflammation and flagellin-producing bacteria with higher temperatures after mRNA vaccination, while experiments suggest diet could influence the response.
Researchers have reported that the gut microbiome may help determine how strongly people develop fever after vaccination, adding a new layer to understanding why side effects vary from one person to the next. In a paper in Science published on 8 October 2026, the team said people who went on to experience higher post-vaccination temperatures already showed signs of greater intestinal inflammation and stronger flagellin activity before they were jabbed. (pasteur.fr)
The study followed 171 healthy adults receiving mRNA vaccines against COVID-19 and combined stool profiling with temperature monitoring and immune marker analysis. According to the researchers’ summary, the findings point to a connection between a particular gut microbial profile and a more pronounced inflammatory response after vaccination. The group also reported that microbes associated with fever tended to include more flagellated bacteria, organisms that carry flagellin, a protein known to stimulate the immune system. (covidnewscast.com)
The work went beyond association. In experiments with human intestinal organoids, the scientists found that microbiomes from donors with higher fever responses provoked stronger inflammatory reactions. They also used gnotobiotic mice, which are raised in sterile conditions and then colonised with selected microbes, to show that the vaccine response could be driven by a flagellin-dependent mechanism. The researchers further reported that diet could alter those microbial and inflammatory patterns, suggesting the effect may not be fixed. (covidnewscast.com)
The findings come from an international collaboration that included researchers at the Max Planck Institute for Biology in Tübingen, the Institut Pasteur, the University of Manitoba, the University of California, San Diego, Georgia State University and the Institute for Tropical Medicine, with support from the Max Planck Society and the European Research Council. The authors said the work could eventually help scientists develop ways to reduce vaccine side effects without blunting protection, although that would require further study. (leylab.com)
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