Certain Gut Bacteria May Intensify Inflammatory Responses to Vaccines

Certain Gut Bacteria May Intensify Inflammatory Responses to Vaccines

New research suggests that the bacteria living inside the gut may influence how the body reacts after vaccination. Scientists have found that certain gut microbes could contribute to vaccine-related fever and inflammation, potentially explaining why some people experience stronger side effects than others.

The study, published in the journal Science, examined the relationship between the human gut microbiome, immune activity and reactions following an mRNA vaccine against SARS-CoV-2. Researchers say the findings could eventually lead to new ways of reducing unwanted vaccine-related inflammation without weakening the protection vaccines provide.

Why Do Some People Get Fever After Vaccination?

As part of this process, temporary symptoms such as tiredness, muscle aches, chills or fever can occur.

However, people do not all experience these reactions in the same way. Some develop noticeable fever and flu-like symptoms, while others experience few or no side effects.

Researchers wanted to understand why these differences occur. Because the gut microbiome plays an important role in regulating immune activity, scientists investigated whether differences in gut bacteria could be involved.

Study Included 171 Healthy Adults

Researchers led by Kelsey Huus studied 171 healthy adults who received an mRNA vaccine against SARS-CoV-2.

The team analysed participants' gut microbiomes before vaccination and monitored body temperature and immune markers following vaccination. This allowed researchers to compare the composition and activity of gut bacteria with the intensity of vaccine-related reactions.

The results pointed towards a connection between particular bacterial activity in the gut and the development of fever after vaccination.

Lachnospiraceae Bacteria May Play a Role

People with greater levels of flagellin expression in their gut tended to have stronger inflammatory responses following vaccination. Those who already had higher levels of intestinal inflammation before receiving the vaccine were also more likely to develop a fever.

This does not mean that Lachnospiraceae are inherently harmful. Gut bacteria can have different effects depending on their species, abundance and surrounding environment.

The Gut and Immune System Are Closely Connected

The findings add to growing evidence that the gut microbiome can influence immune function beyond the digestive system.

Billions of microorganisms live in the human digestive tract, forming a complex ecosystem that interacts continuously with the body's immune system.

Changes in this ecosystem may alter inflammatory signalling. In the new research, scientists found evidence that gut microbial activity could help prime the body for a stronger inflammatory response when it encounters the immune stimulus created by vaccination.

Diet May Influence the Gut Response

One of the most interesting aspects of the research was the potential relationship between diet and vaccine reactions.

Researchers found that dietary patterns associated with greater consumption of whole plant foods tended to correlate with microbiome characteristics linked to less fever.

In contrast, lower consumption of whole plant foods and diets higher in fat or processed foods were associated with the inflammatory microbiome pattern linked to fever.

The researchers also found that dietary manipulation could change aspects of the microbiome and its relationship with vaccine responses.

However, this does not mean that people should change their diet immediately before vaccination in an attempt to prevent side effects.

Important Finding: Vaccine Protection Was Not Reduced

Perhaps the most reassuring finding was that the inflammatory response associated with gut bacteria did not appear to reduce the antibody response to vaccination.

This distinction is important because fever and inflammation following vaccination are not the same thing as the body's protective antibody response.

Could Gut Bacteria Be Targeted in the Future?

The findings raise the possibility that scientists could eventually develop strategies to reduce unpleasant vaccine reactions by modifying the gut microbiome.

Diet, microbial ecology and metabolic activity are potentially changeable factors. Researchers therefore suggest that the microbiome could one day become part of personalised approaches to vaccination.

For example, scientists may investigate whether particular dietary patterns, microbial interventions or other approaches could reduce excessive inflammatory responses while maintaining vaccine protection.

What the Research Means for Vaccination

The study provides another example of how individual biology can influence responses to medical treatments.

Two people can receive the same vaccine yet experience very different levels of fatigue, fever or other temporary symptoms. Genetics, previous immune exposure, age, health and other biological factors may all contribute.

The gut microbiome could be another piece of this complicated puzzle.

More Research Is Needed

The study focused on healthy adults receiving an mRNA SARS-CoV-2 vaccine, so the findings should not automatically be applied to every vaccine or every population.

Further research will be needed to determine whether similar microbiome-related effects occur with other types of vaccines and in people with different health conditions.

Scientists will also need to identify which specific bacteria, microbial products and dietary factors are most important.
Conclusion

The new research provides intriguing evidence that the gut microbiome may influence vaccine-related fever and inflammation.

Diet also appeared to be connected with the microbiome patterns associated with these reactions, although the findings are not yet strong enough to support specific dietary changes around vaccination.

As scientists continue exploring the relationshipbetween the gut and immune system, the microbiome could eventually become an important part of personalised strategies for making vaccination more comfortable while preserving its protective benefits.