A Science study followed 171 healthy adults receiving mRNA COVID vaccines. Before vaccination, those who later developed higher fevers were more likely to show low-grade intestinal inflammation, flagellated Lachnospiraceae, and higher expression of flagellin—the protein that forms bacterial flagella and activates host inflammatory sensing.
The important step goes beyond association. Microbiomes transferred from high-fever donors produced stronger inflammation in human intestinal organoids and stronger vaccine reactions in germ-free mice; the mouse effect was partly dependent on sensing flagellin. Diet is less settled: human diet was self-reported, while food-additive and Western-diet manipulations were in vitro or in mice. This is evidence that the microbiome can help set immune tone, not a prescription to change dinner before a shot. Science · Max Planck Society