{
  "id": 13087865,
  "title": "Feverish after a vaccine? The reason could be your gut microbiome",
  "url": "https://urgent.news/2026/10/09/feverish-after-a-vaccine-the-reason-could-be-your-gut-microbiome",
  "topic": "health",
  "section": "Health & Medicine",
  "published": "2026-10-09T10:00:00.000Z",
  "source": {
    "name": "Scientific American",
    "slug": "scientific-american",
    "url": "https://www.scientificamerican.com/article/feverish-after-a-vaccine-the-reason-could-be-your-gut-microbiome/"
  },
  "original_language": "en",
  "account": "A recent study published in Science suggests a possible link between gut microbiome and fever experienced after receiving a COVID vaccine. Researchers collected stool samples from over 170 individuals before and after their second or third dose of the vaccine, observing that those who had more severe fevers after the shot shared similar microbiome traits. Specifically, individuals with higher fever levels had a greater presence of a protein called flagellin in their systems. Flagellin is found in the tail-like structures of bacteria, indicating that the fever-associated microbes possessed greater mobility. The participants with high-fever symptoms also exhibited heightened gut inflammation compared to others. To explore the reason behind this connection, the researchers conducted a follow-up experiment using genetically engineered mice. They found that exposure to the fever-associated human bacteria resulted in a human-like fever response, particularly in mice with flagellin receptors. Conversely, mice lacking flagellin receptors showed a reduced response, suggesting a direct link between flagellin and an immune system reaction. The correlation between humans' fever levels and their mouse counterparts' temperature drop indicates that the severity of the fever in humans is proportional to the response in mice. However, the exact reasons why some people have higher levels of flagellin in their guts remain unclear, though self-reported vegetarians had lower flagellin levels in the study. Further research is needed to understand the factors influencing flagellin levels in the gut. Ley hopes that these findings could eventually help alleviate vaccine-related fever side effects and potentially increase vaccination rates. This research emphasizes the importance of understanding the gut microbiome's role in vaccine reactions to improve public health outcomes.",
  "summary": "In a new study, people who had worse fevers after a COVID shot tended to have similar gut characteristics",
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}