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Some people develop a fever after vaccination while others do not. A 2026 study of healthy young adults receiving SARS-CoV-2 vaccines found that pre-vaccination gut-microbe activity—particularly features linked to bacterial flagella—was associated with stronger fever responses. Experiments in mice support a possible role for those microbes in inflammation, but they do not prove that gut bacteria cause fever in people or that changing your diet can prevent it.
Why do some people feel feverish after a vaccine?
Fever is one possible response as the immune system reacts to vaccination, and its intensity varies from person to person. The µHEAT study investigated whether differences already present in the gut microbiome might help explain some of that variation. The Max Planck Society describes the study and its findings in its October 8, 2026 report.
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Researchers recruited healthy young adults receiving SARS-CoV-2 vaccines in 2021 and 2022. Participants provided stool and blood samples and recorded their temperatures during the week before and after vaccination. In the human observations, people who later had stronger fever responses already showed differences in microbiome activity and inflammation before vaccination.
What did the study find about gut bacteria and fever?
Human observations linked flagellin-related features with stronger fever
The study associated stronger fever responses with pre-vaccination microbial activity related to bacterial movement, including features connected with flagellin. Flagellin is a protein in bacterial flagella—the structures some bacteria use to move—and the immune system can recognize it as an inflammatory signal. The NCBI BioProject record reports 171 healthy adults and summarizes greater representation of the flagellated genus Waltera, along with more flagellin gene expression, among participants with high fever.
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Mouse experiments support a possible mechanism, not proof in people
To test whether the microbiome might contribute to inflammation, researchers transferred stool microbiomes from human participants into mice that lacked their own intestinal microbes, then vaccinated the mice. Mice given microbiomes from stronger human fever responders developed greater inflammation than those given microbiomes from milder responders. The effect was partly dependent on the mice’s ability to sense flagellin.
That experiment strengthens the case for a possible microbiome-and-flagellin mechanism in the mouse model. It cannot establish that the same microbes cause fever in human vaccine recipients: the human results were observational, while the transfer experiment was conducted in mice.
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Can changing what you eat stop vaccine fever?
No. The study does not show that a particular diet prevents fever after vaccination in people. Plant-based eating was associated with weaker fever responses in the reported human observations, even though antibody production was robust, but that association does not demonstrate that diet caused the difference. The NCBI study summary reports the diet observation; it is not evidence for a preventive diet.
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For now, these findings do not support using probiotics, supplements, a microbiome test, or a diet change to prevent vaccine fever. No such intervention was established by the study.
How strong is the evidence—and who does it apply to?
The findings are intriguing but narrow. µHEAT concerned healthy young adults receiving SARS-CoV-2 vaccines; it should not be treated as established evidence for children, older adults, people with chronic illness, other vaccines, or every kind of symptom after vaccination.
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The Paul-Ehrlich-Institut registry describes µHEAT as a non-interventional study in Germany and lists 256 planned participants. That is the registered target, not the analyzed sample size reported by NCBI, which is 171 healthy adults.
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Earlier work provides context, not independent confirmation of causation. A 2022 study of 138 recipients—37 CoronaVac and 101 BNT162b2—reported associations between gut microbiome composition, antibody response, and some adverse events in Gut. A 2024 systematic review covering 26 human studies found varied designs and outcomes and called for adequately powered studies to confirm associations and inform possible microbiota-targeted interventions (npj Vaccines).
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What should you do if you feel feverish after vaccination?
The microbiome findings do not change how to respond to a fever or other concerning symptoms. If you want to keep track of your temperature, record the reading and when you took it; the µHEAT researchers used temperature measurements to study changes around vaccination, not to test a treatment. Follow relevant medical guidance for your situation, and contact a healthcare professional if you are worried about your symptoms.
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