Gut Bacteria May Help Explain Post-Vaccine Fever
Researchers analyzed stool samples from over 170 people before and after COVID vaccine doses to study post-shot fever. Those who developed fevers tended to have more flagellin, a bacterial protein tied to movement, and more gut inflammation. Follow-up mouse experiments suggested that sensing flagellin can trigger a fever-like immune response.
Researchers studied stool samples from over 170 individuals before and after a second or third COVID vaccine dose. Fever after vaccination was linked to higher flagellin, a bacterial protein forming tail-like flagella used for movement, and to greater gut inflammation. Vegetarians in the study reported lower flagellin levels, though the reason remains unclear.
In mice, exposure to fever-associated human gut bacteria produced a hypothermic immune response, especially in animals able to detect flagellin. Mice lacking flagellin receptors reacted less. Human fever severity tracked with the mouse response, suggesting flagellin sensing may contribute to post-shot fever.
This finding may interest people who experience fever after vaccination, clinicians assessing side effects, and vaccine communicators. If future work confirms gut bacteria influence reactions, strategies that alter microbiome motility could potentially reduce discomfort and may help address hesitancy tied to side effects. However, benefits remain speculative, and dietary or microbial interventions should not be assumed safe or effective without trials.