Why do some people get fever after taking a  vaccine, while others don't?

Why do some people get fever after taking a  vaccine, while others don't?

New research makes us understand this in a clear way.

Some people develop a fever after receiving a vaccine and others don’t.

To investigate whether intestinal microbiome differences might explain postvaccination fever responses, researchers studied the gut microbial community and inflammation markers in people who received COVID-19 vaccines.

People with measurable fever tended to have higher levels of certain flagellated bacteria in their intestinal samples. Surveys of the participants, combined with mouse and in vitro studies, suggested that diets high in fat or processed food additives may promote the expansion of these motile bacteria and contribute to an overall inflammatory state that primes the body for fever.

The makeup of a person’s gut microbiome might affect whether they experience a minor fever after a vaccination, researchers collected stool samples from people before and after they received a dose of a COVID-19 vaccine. They found that people who experienced a fever after the jab had more flagellin — a protein that forms part of the bacterial flagellum — in their microbiome. The finding suggests that bacteria associated with post-vaccine fever are more motile. A person’s diet might influence the flagellin content of their microbiome, the research team suggests.

Researchers found that fever had very little effect on the microbiome; by contrast, fever-positive participants displayed chronic low-grade intestinal inflammation and a microbiome characterized by high expression of bacterial flagellins, both pre- and post-vaccination.

Microbiomes from fever-positive participants induced heightened inflammatory responses in human colonic organoids and exacerbated acute vaccine responses when transplanted into  mice, and this was dependent on sensing of bacterial flagellin through host innate immune receptors.

Moreover, microbial flagellin production was modifiable by diet: Dietary additives increased bacterial flagellin production in vitro, and a Western-style diet exacerbated vaccine reactions in mice. Consistent with this, fever was associated with dietary and metabolic markers in humans.

Although interventional human studies are needed to establish causality, this suggests that diet modification could be a viable strategy to influence microbiome-dependent vaccine responses.

These data suggest that chronic production of flagellin within the human gut microbiome sets the immune tone to drive inflammatory vaccine responses.

The ability to predict and control the risk of vaccine-induced adverse reactions is a crucial factor in vaccine hesitancy and global public health as some people are afraid of these inflammatory reactions and fever and refuse to take vaccines.

https://www.science.org/doi/10.1126/science.aea7733