HEALTH
Does your gut health predict a vaccine fever?
Karl Hille
The Baltimore Sun
Oct. 19, 2026, 10:43 a.m. ET
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What’s living in your gut determines whether you have a fever reaction to a vaccination, new research shows
Fever is the most common reaction to vaccination and could lead some people to delay or forgo future shots, especially modern vaccines such as the mRNA vaccine developed to fight COVID-19
“The fear of vaccine-induced adverse reactions is a major underlying cause of vaccine hesitancy and thus poses an important risk to global public health,” the multinational group of researchers wrote in their paper, published in the journal Bacterial Immunity. “The risk of fever and other side effects after vaccination is individualized; here, we find that the gut microbiome sets inflammatory immune tone to prime vaccine-induced fevers.”
The researchers monitored the diet, gut microbes, body temperature and immune response through blood tests for 171 healthy adults receiving COVID-19 vaccinations. They found that those who developed a fever also tended to have low-grade inflammation in their intestine linked to specific Lachnospiraceae bacteria, as well as testing high for certain proteins produced by those bacteria before their vaccination.
In addition, microbiome samples from those who developed a fever agitated living intestinal tissues in a lab setting and caused similar fevers in mice
On the other hand, the researchers found that a healthier diet could result in an easier time after getting the jab, suggesting new studies to develop a “vaccine diet” to help those with strong reactions
Vegetarian diets tended to correlate with a gut microbiome that contributed less to fever, they found, while those who ate fewer plant foods, more fat or more processed foods fell into the high-fever group
In all cases, they found, the presence or absence of a fever did not affect whether the vaccine worked
“Diet, microbial ecology, and the metabolic state are not immutable, and people might one day prepare for vaccination as deliberately as the vaccine itself is formulated,” immunologist Bali Pulendran of Stanford Medicine wrote in a commentary on the research
