Gut Microbiota Influences Fever Reactions After Vaccination

People who develop a fever after receiving a vaccination tend to host specific mobile bacteria and mild intestinal inflammation in their gut beforehand, according to a study published in the scientific journal Science.

Key Clinical Takeaways:

• Temporary fever following mRNA vaccination does not alter the overall composition of a person’s underlying gut microbiome.
• Individuals who experience post-vaccine fevers display higher baseline levels of bacteria that produce the flagellin protein.
• Dietary habits show a correlation with thermal reactions, as regular meat consumers experience post-vaccine fever more frequently than vegetarians or vegans.

Side effects like fever and fatigue remain difficult to predict ahead of time. An international research team led by Kelsey Huus from the Max Planck Institute for Biology in Tübingen examined 171 healthy young adults receiving a Covid-19 mRNA vaccine to understand why reactions vary so widely.

The researchers collected stool and blood samples before and after immunization to track microorganisms and serum markers over time, while participants recorded their body temperature three times daily. The temporary fever phase itself left the microbiome composition undisturbed, but distinct baseline differences separated those who developed a fever from those who did not.

Those who developed fever after vaccination had different bacteria in the gut, especially bacteria that produce the flagellin protein.

Flagellin builds the fine flagella that certain bacteria use for locomotion. The human body mounts a strong immune reaction to the protein, which could explain the temperature spike. Participants with fever reactions also showed elevated levels of the inflammatory marker calprotectin, a sign often found in individuals with disrupted gut flora.

To test these findings further, the team exposed lab-grown gut tissue to the participants’ stool samples. The gut flora originating from participants with stronger fever reactions triggered much clearer inflammatory responses in this simplified model.

Researchers also transferred participant stool samples into mice lacking their own gut flora before vaccinating the animals. The transferred bacteria alone drove distinct reaction levels. Animals receiving samples from participants with high fevers developed more inflammation than the others.

A dietary survey of the participants revealed that individuals following vegetarian or vegan diets experienced lower temperature increases after vaccination. Regular meat eaters developed fevers more frequently, particularly when they also had elevated blood lipid levels.

Disclaimer: The information provided in this article is for educational and scientific communication purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider regarding any medical condition, diagnosis, or treatment plan.