Study links gut microbiome to fever response after COVID booster shot

In a new study, people who had worse fevers after a COVID shot tended to have similar gut characteristics. The researchers followed more than 1,200 adults who received a booster in 2023. They collected stool samples before the shot and asked participants to record any fever and how high it reached. The data showed that those who reported temperatures above 38.5 °C had a distinct mix of gut bacteria compared with those who stayed below that level. The finding suggests the gut microbiome may influence how the body reacts to the vaccine.

The gut microbiome is a known regulator of the immune system. Scientists have long suspected that the bacteria living in our intestines can affect how we respond to infections and vaccines. Prior work on flu and HPV vaccines found that people with a richer bacterial diversity tended to have stronger antibody responses. This new COVID study adds a side‑effect angle, showing that a less diverse or differently composed microbiome may trigger a stronger inflammatory response, like fever. Understanding this link is important because fever is a common reason people hesitate to get vaccinated or to keep coming back for boosters.

In the study, participants were grouped by the severity of their post‑shot fever. The researchers used 16S ribosomal RNA sequencing to profile the bacterial species in each stool sample. They found that high‑fever individuals had higher levels of Bacteroides and lower levels of beneficial short‑chain‑fatty‑acid producers such as Faecalibacterium. The overall microbial diversity was lower in the high‑fever group. Statistical models controlled for age, sex, weight, and prior COVID infection, and still showed a strong association between gut composition and fever severity. The authors cautioned that the study is observational and does not prove causation.

If the link is confirmed, it could lead to new ways to predict who might have strong side‑effects from vaccines. Health authorities could recommend probiotic or dietary adjustments before vaccination to reduce discomfort. Researchers are already planning interventional trials that give participants pre‑biotics or specific bacterial strains before a booster. In the long run, tailoring vaccine schedules to an individual’s gut microbiome could improve vaccine uptake and reduce the anxiety that fever sometimes causes.