Research shows that your gut bacteria may determine whether you develop a fever after vaccination. A 2026 study of 171 adults found that people with certain types of gut bacteria and low-grade intestinal inflammation were significantly more likely to get fever after COVID-19 vaccines. According to Gram Research analysis, the good news is that diet can change your gut bacteria composition, suggesting that dietary changes before vaccination might reduce fever risk.
A groundbreaking study published in Science found that the bacteria living in your gut may determine whether you get a fever after receiving a vaccine. Researchers tracked 171 healthy adults who received COVID-19 vaccines and discovered that people with certain types of gut bacteria were more likely to develop fever. The good news: diet can change your gut bacteria composition. According to Gram Research analysis, this discovery could help doctors predict who might have vaccine side effects and potentially reduce vaccine hesitancy by allowing people to prepare for or prevent fevers before getting vaccinated.
Key Statistics
A 2026 study published in Science tracked 171 healthy adults and found that people with flagellated Lachnospiraceae bacteria and low-grade intestinal inflammation had significantly higher fever risk after COVID-19 vaccination.
Researchers demonstrated that bacteria from people who developed high fevers triggered stronger inflammatory responses in lab-grown human intestinal tissue compared to bacteria from people without fever.
In germ-free mice, dietary changes reduced the abundance of fever-causing bacteria and decreased vaccine-induced inflammatory reactions, showing that diet directly influences microbiome composition and vaccine side effects.
The study identified flagellin, a bacterial protein, as a key trigger for vaccine fever that activates specific immune receptors and amplifies inflammatory responses after vaccination.
The Quick Take
- What they studied: Whether the types of bacteria living in your digestive system affect your chances of getting a fever after a COVID-19 vaccine
- Who participated: 171 healthy adults who received COVID-19 mRNA vaccines. Researchers tracked their gut bacteria, body temperature, and immune response before and after vaccination
- Key finding: People with low-grade inflammation in their intestines and higher amounts of flagellated Lachnospiraceae bacteria (a specific type of gut bacteria) were significantly more likely to develop fever after vaccination
- What it means for you: Your gut bacteria composition may predict vaccine side effects. Since diet can change your gut bacteria, eating certain foods before vaccination might reduce fever risk. However, more research is needed before doctors can make specific dietary recommendations
The Research Details
Researchers followed 171 healthy adults over time as they received COVID-19 vaccines. Before and after vaccination, they collected stool samples to analyze gut bacteria, measured body temperature, and took blood samples to check immune response. This longitudinal approach, meaning they tracked the same people over time, allowed researchers to see which bacteria were present before fever developed.
The team then took bacteria from people who developed high fevers and grew them in lab-grown human intestinal tissue to see if those bacteria caused more inflammation. They also transferred these bacteria into germ-free mice (mice born without any bacteria) to test whether the bacteria alone could trigger vaccine reactions.
Finally, they examined whether changing diet could alter the bacteria and reduce fever risk in mice. This multi-step approach moved from observation in humans to controlled experiments in the lab, strengthening confidence in the findings.
Understanding what causes vaccine fever is important because fever is one of the most common reasons people hesitate to get vaccinated or avoid future doses. If doctors can predict who will get fever and potentially prevent it through dietary changes, more people might be willing to get vaccinated. This research also reveals a new way the immune system works: through gut bacteria rather than just immune cells alone.
This study was published in Science, one of the world’s most prestigious scientific journals, which means it underwent rigorous peer review. The researchers used multiple approaches to confirm findings: human observation, lab tissue experiments, and animal models. The longitudinal design tracking the same people over time is stronger than studies that only look at one point in time. However, the study was limited to healthy adults receiving one type of vaccine, so results may not apply to all populations or all vaccines
What the Results Show
The research revealed a clear connection between gut bacteria and vaccine fever. People who developed fever after vaccination had specific bacterial characteristics before getting the shot: they had low-grade inflammation already present in their intestines, higher amounts of flagellated Lachnospiraceae bacteria, and increased flagellin expression (a bacterial protein that triggers immune responses).
When researchers took bacteria from people who got high fevers and placed them in lab-grown human intestinal tissue, these bacteria triggered stronger inflammatory responses compared to bacteria from people who didn’t get fever. This proved the bacteria themselves, not something else about those people, were responsible.
In germ-free mice that received these bacteria, the mice developed stronger vaccine reactions when exposed to the vaccine. Importantly, when researchers changed the mice’s diet, they could reduce the amount of these fever-causing bacteria and decrease vaccine reactions. This demonstrated that diet directly influences which bacteria thrive in the gut and how strong a vaccine fever response will be.
The human data supported this: people who reported eating certain diets had different fever risks, and their metabolic markers (measurements of how their body processes food) correlated with fever likelihood.
The study identified flagellin, a protein on the surface of certain bacteria, as a key trigger for vaccine fever. This protein activates specific immune receptors that amplify inflammatory responses. The research also showed that the relationship between gut bacteria and fever was not random but followed predictable patterns based on bacterial composition and diet. Additionally, the findings suggest that microbiome differences could explain why some people experience vaccine side effects while others don’t, pointing to a biological basis for individual variation in vaccine responses.
Previous research knew that fever was a common vaccine side effect, but the cause remained unclear. Some studies suggested it was purely the vaccine itself triggering immune cells. This research adds a new layer: the gut microbiome acts as a primer that determines how strongly your immune system will react. This aligns with growing evidence that gut bacteria influence many aspects of immune function, but it’s the first study to directly link microbiome composition to vaccine fever specifically. The finding that diet can modify this response is novel and opens new possibilities for preventing vaccine side effects.
The study only included 171 healthy adults, so results may not apply to children, elderly people, or people with existing health conditions. Only one type of vaccine (COVID-19 mRNA) was tested, so these findings may not apply to other vaccines. The research was conducted in a specific time period and population, which may have different dietary patterns and microbiomes than other groups. While the mouse experiments prove bacteria can cause fever, human responses are more complex and involve many other factors. Finally, the study shows correlation between bacteria and fever but doesn’t fully explain all the biological mechanisms involved
The Bottom Line
Based on this research, people concerned about vaccine fever might consider dietary changes before vaccination that promote healthy gut bacteria, such as eating more fiber and fermented foods. However, this is a preliminary recommendation because specific dietary protocols haven’t been tested in humans yet. The strongest recommendation is for future research to develop personalized approaches based on individual microbiome testing. People should not avoid vaccination based on fever risk, as fever is typically mild and temporary while vaccine protection is substantial. Confidence level: moderate for the bacterial connection, low for specific dietary interventions
Anyone hesitant about vaccines due to fever concerns should pay attention to this research. Healthcare providers may eventually use this information to predict which patients might experience fever and provide preventive strategies. People with existing digestive issues or inflammatory conditions may be particularly interested in how their microbiome affects vaccine responses. Parents considering vaccines for children should note this research applies to adults and more work is needed for younger populations. People should not use this as a reason to delay or avoid vaccination
If dietary changes are pursued, modifications to gut bacteria typically take 2 to 4 weeks to show measurable effects. Fever from vaccination typically appears within 24 to 48 hours of the shot and resolves within 1 to 2 days. Any preventive dietary strategy would need to be started several weeks before vaccination to allow gut bacteria composition to shift. Long-term benefits of a microbiome-friendly diet extend beyond vaccine responses to overall immune health and digestive wellness
Frequently Asked Questions
Can changing my diet before vaccination prevent fever?
Possibly, but it’s not yet proven in humans. A 2026 study showed that diet changes reduced fever-causing bacteria in mice, but human studies are needed. Eating more fiber and fermented foods supports healthy gut bacteria, which may help, but specific recommendations await further research.
What gut bacteria cause vaccine fever?
Flagellated Lachnospiraceae bacteria, particularly those expressing high levels of flagellin protein, were associated with higher fever risk in the study. These bacteria trigger immune responses through specific receptors, amplifying the inflammatory reaction to vaccination.
Should I avoid vaccination if I’m worried about fever?
No. Vaccine fever is typically mild, temporary, and resolves within 1 to 2 days. Vaccine protection against serious illness is substantial and far outweighs the risk of brief fever. Instead, discuss microbiome-friendly dietary strategies with your doctor before vaccination.
How long does it take for diet to change gut bacteria?
Measurable changes in gut bacteria composition typically occur within 2 to 4 weeks of dietary changes. To potentially reduce vaccine fever, start dietary modifications 4 weeks before your scheduled vaccination to allow sufficient time for bacterial composition to shift.
Does this research apply to all vaccines or just COVID-19?
This study specifically tested COVID-19 mRNA vaccines in healthy adults. The findings may apply to other vaccines, but that hasn’t been proven yet. More research is needed to determine if microbiome effects on fever are universal across different vaccine types and populations.
Want to Apply This Research?
- Track daily diet quality (servings of fiber, fermented foods, processed foods) and correlate with any fever or immune symptoms after vaccination. Record specific foods eaten in the 4 weeks before vaccination and compare to fever outcomes
- Users can log dietary changes aimed at supporting healthy gut bacteria: increasing fiber intake, adding fermented foods like yogurt or sauerkraut, reducing ultra-processed foods, and staying hydrated. Set reminders to start these dietary changes 4 weeks before scheduled vaccination
- Create a pre-vaccination microbiome health score based on diet quality over the previous month. After vaccination, log temperature readings and symptom severity. Over time, users can see if their dietary improvements correlate with reduced fever or side effects. Share patterns with healthcare providers before future vaccinations
This research is preliminary and has not yet been translated into clinical practice. Do not delay or avoid vaccination based on this information. Fever is a normal immune response and typically mild. Anyone with concerns about vaccine side effects should consult their healthcare provider before vaccination. This article summarizes research findings and does not constitute medical advice. Individual responses to vaccines vary, and this study was conducted in healthy adults only. People with existing health conditions, taking medications, or with compromised immune systems should discuss vaccination with their doctor.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.