Gut bacteria may explain why some people get fever after vaccination
A multi-institution study found that certain gut bacteria with flagella may prepare the body for fever after vaccination, helping explain why inflammatory responses vary between individuals. The microbiome interacts with the immune system before vaccination, shaping a person's immune state. The study recruited healthy young adults to investigate this link.
The µHEAT project followed healthy young adults who received a SARS-CoV-2 vaccine during 2021–2022. Participants supplied stool and blood samples and tracked their temperature before and after immunization, allowing researchers to compare microbiome features with immune responses.
Those who later developed higher fevers had greater activity in bacterial motility genes, especially genes for flagella. Because flagellins resemble molecules from pathogens such as Salmonella, the immune system may treat them as danger signals. This suggests pre-existing gut microbial traits can shape vaccine reactions.
Understanding why fever varies could help clinicians counsel patients and may reduce anxiety among people worried about side effects. If microbiome markers predict reactions, future vaccines or schedules might be tailored, though such applications remain speculative. This research may affect vaccine-hesitant individuals, healthy adults, and public health communication, but it does not show that gut bacteria cause fever or that altering them would prevent it.