A vaccine, a syringe, a single action performed identically for two people, and yet two bodies that react as if they have nothing in common. On one side, a slightly sore arm and nothing more. On the other, a fever of 39°C, aches, and a day spent under the covers. This scenario, many couples have lived through without questioning it, attributing it to chance or to a poorly tolerated dose. But a study published in Science and presented by Inserm on October 9, 2026 challenges this seemingly simple explanation, by revealing what truly lies behind these reaction gaps.
- A study from Inserm, published in Science on October 9, 2026, shows that the gut microbiota influences the fever intensity after vaccination.
- The inflammatory state of the body before injection, shaped by the microbiota, could explain the differences in reactions among individuals to the same vaccine.
- Post-vaccination fever is not a marker of vaccine efficacy, and no probiotic treatment has been validated to reduce these reactions in humans.
- Why two people react so differently to the same vaccine
- What the Inserm study published in Science reveals
- The gut microbiota, a key yet still poorly understood factor in personalized medicine
Why two people react so differently to the same vaccine
When faced with an injection that is strictly identical in product and quantity, the body never responds in exactly the same way from one person to another. Why, for instance, can two young adults experience such different post-vaccination effects? It is precisely this question that led researchers to explore an unsuspected actor in vaccine medicine lurking in our guts: the gut microbiota.
What the Inserm study published in Science reveals
The study in question draws a link between certain characteristics of the gut microbiota and the intensity of fever felt after vaccination. The researchers based their work on observations from young adults vaccinated against Covid-19. These observations were then complemented by experiments in mice, allowing a closer look at the mechanisms at play. A particularly interesting lead emerges: the inflammatory state of the body even before the injection seems to contribute to the differences in reaction observed among individuals. In other words, it is not so much the vaccine that would vary in its effects, but rather the terrain on which it lands—a terrain shaped in part by the microbiota living in our intestines for years, long before anyone even considers vaccination.
The gut microbiota, a key yet still poorly understood factor in personalized medicine
These findings open a window into what tomorrow’s medicine could become: personalized medicine that accounts for the uniqueness of each microbiota to better anticipate individual reactions. But it is essential to remain cautious in the face of these early results. Fever after an injection should never be taken as an indicator of the vaccine’s effectiveness: a person who feels nothing is not less protected than another brought down by a fever of 39 degrees. Similarly, these discoveries do not validate the idea that a probiotic treatment could prevent or lessen post-vaccination reactions in humans. We are still a long way from that, and it would be premature to draw concrete dietary recommendations. The gut microbiota remains a key but still poorly understood factor.
This gives us something to ponder the next time a difference in reaction appears within a couple or a family after the same vaccination. Rather than luck or a matter of dose, perhaps an entire invisible ecosystem, housed for years in the intestines, quietly pulls the strings.
👉 A small gesture for Space Group? Follow us on Google, add us to your favorites, and find us on Google News so you don’t miss any discoveries.
