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Why Do Some People Get Fevers After Vaccines? Scientists May Have Found the Answer (opens in a new tab)

scitechdaily.com · 2026-10-26

Short answerEvidenceSource

Short answer

Mixed

Mixed.

The claims we could check match the study, but some claims were not covered by the evidence reviewed.

  • 4 supported
  • 2 not covered

Checked against the study summary. The full text wasn't available, so some details couldn't be settled either way.

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Follow the evidence trail
1
2

NewsLink checks it

Mixed

Every claim we could check holds up. Four of six claims match the study. This overall rating is based only on the claims we could check. Two claims the study doesn't address.

  • 4 supported
  • 2 not covered
Open claim evidence
3
Then inspect each claim

Evidence layer

Claim by claim

Each claim gets a verdict. Expand it to see the evidence directly below.

6 claims in this story

Showing all 6 claimsChoose a verdict to focus the list.

Claim 1 of 6Not coveredParticipants eating plant-based diets tended to have weaker fever responses, and the team suggests diet could be one factor capable of modifying a pro-inflammatory microbiome.View evidenceHide evidence

Why this verdict

The abstract-level profile supports only that human fever risk was associated with self-reported diet and metabolic markers, and that diet modified microbiome flagellin phenotypes and murine vaccine reactions. It does not specify, at this depth, that plant-based diets were associated with weaker fever responses. The broader diet-modifiability idea is consistent with the profile, but the specific plant-based direction is not verifiable from the supplied abstract-level evidence.

1

Study evidence

Prevaccine low-grade intestinal inflammation and increased abundance of flagellated Lachnospiraceae, along with higher microbial flagellin expression, were associated with increased risk of fever after SARS-CoV-2 mRNA vaccination.

“we longitudinally profiled fecal microbiota, oral temperature, and serological markers in 171 healthy adults receiving Severe acute respiratory syndrome coronavirus 2 mRNA vaccines”
2

Study evidence

Dietary interventions in mice altered gut microbiome flagellin phenotypes.

“Moreover, microbiome flagellin phenotypes and murine vaccine reactions were modifiable by diet.”
Claim 2 of 6Not coveredAnimal experiments suggested that a high-fat Western-style diet enhanced inflammatory vaccine responses in mice.View evidenceHide evidence

Why this verdict

The profile says murine microbiome flagellin phenotypes and vaccine reactions were modifiable by diet, but the abstract-level evidence does not specify a high-fat Western-style diet or that it enhanced inflammatory vaccine responses. The claim may be consistent with fuller methods/results, but it is not verifiable at abstract depth.

1

Study evidence

Dietary interventions in mice altered gut microbiome flagellin phenotypes.

“Moreover, microbiome flagellin phenotypes and murine vaccine reactions were modifiable by diet.”
Claim 3 of 6SupportedThe microbes in your gut may help explain why you develop a fever after a vaccine while someone else feels fine.View evidenceHide evidence

Why this verdict

The paper profile supports the hedged idea that gut microbes may help explain variation in post-vaccination fever: the human cohort found prevaccine microbiome features associated with fever risk, and gnotobiotic mouse experiments reported donor-microbiome-driven vaccine reactions. The headline is broad, but its speculative wording is consistent with the abstract-level evidence.

1

Study evidence

Prevaccine low-grade intestinal inflammation and increased abundance of flagellated Lachnospiraceae, along with higher microbial flagellin expression, were associated with increased risk of fever after SARS-CoV-2 mRNA vaccination.

“we longitudinally profiled fecal microbiota, oral temperature, and serological markers in 171 healthy adults receiving Severe acute respiratory syndrome coronavirus 2 mRNA vaccines”
2

Study evidence

Microbiomes from fever-high human donors transferred into gnotobiotic mice drove flagellin-dependent vaccine reactions.

“and drove flagellin-dependent vaccine reactions in gnotobiotic mice.”
Claim 4 of 6SupportedAn international research team found that people who developed stronger fevers after vaccination already showed signs of heightened inflammation and distinctive gut microbial activity before receiving their shots.View evidenceHide evidence

Why this verdict

Supported by the human cohort unit: fever risk correlated with prevaccine low-grade intestinal inflammation, increased abundance of flagellated Lachnospiraceae, and increased flagellin expression. This is framed as a finding about pre-vaccination associations rather than proof of human causation.

1

Study evidence

Prevaccine low-grade intestinal inflammation and increased abundance of flagellated Lachnospiraceae, along with higher microbial flagellin expression, were associated with increased risk of fever after SARS-CoV-2 mRNA vaccination.

“we longitudinally profiled fecal microbiota, oral temperature, and serological markers in 171 healthy adults receiving Severe acute respiratory syndrome coronavirus 2 mRNA vaccines”
Claim 5 of 6SupportedWhen researchers transferred participants' gut microbes into mice, the animals also developed stronger inflammatory responses to vaccination.View evidenceHide evidence

Why this verdict

The abstract-level profile reports causal in vivo animal evidence: microbiomes from fever-high human donors drove flagellin-dependent vaccine reactions in gnotobiotic mice. The exact inflammatory readouts are not detailed at abstract depth, but the core transfer-to-mice and stronger vaccine-reaction claim is supported.

1

Study evidence

Microbiomes from fever-high human donors transferred into gnotobiotic mice drove flagellin-dependent vaccine reactions.

“and drove flagellin-dependent vaccine reactions in gnotobiotic mice.”
Claim 6 of 6SupportedThe findings point to a bacterial protein called flagellin as a possible contributor.View evidenceHide evidence

Why this verdict

The paper profile repeatedly implicates flagellin: prevaccine flagellin expression was associated with human fever risk, mouse vaccine reactions were described as flagellin-dependent, and diet modified microbiome flagellin phenotypes in mice. The story’s hedged wording, 'possible contributor,' matches the evidence.

1

Study evidence

Prevaccine low-grade intestinal inflammation and increased abundance of flagellated Lachnospiraceae, along with higher microbial flagellin expression, were associated with increased risk of fever after SARS-CoV-2 mRNA vaccination.

“we longitudinally profiled fecal microbiota, oral temperature, and serological markers in 171 healthy adults receiving Severe acute respiratory syndrome coronavirus 2 mRNA vaccines”
2

Study evidence

Microbiomes from fever-high human donors transferred into gnotobiotic mice drove flagellin-dependent vaccine reactions.

“and drove flagellin-dependent vaccine reactions in gnotobiotic mice.”
Then look for missing context

Context layer

What the story left out

Important study details the story did not include.

  • The study population and vaccine context were limited to healthy adults receiving SARS-CoV-2 mRNA vaccines, so generalizability to other populations or vaccine platforms is uncertain.

    The story presents the issue broadly as fever after 'a vaccine' or 'vaccination' and the supplied caveats do not mention the SARS-CoV-2 mRNA-vaccine context or limits on generalizability.

    From Prospective longitudinal cohort

  • Human intestinal organoid experiments found that microbiomes from fever-high donors triggered stronger inflammatory responses ex vivo.

    The story mentions human associations and mouse transfer experiments, but it does not mention the organoid evidence, which is a distinct mechanistic component of the paper profile.

    From in vitro organoid stimulation

6 things the story did carry across
  • Prospective human cohort evidence in 171 healthy adults receiving SARS-CoV-2 mRNA vaccines linked prevaccine gut microbiome features, flagellin expression, and low-grade intestinal inflammation with post-vaccination fever risk.
  • The human cohort evidence is observational and cannot by itself prove that microbiome features, inflammation, or diet caused fever responses.
  • Gnotobiotic mouse experiments provided causal animal evidence that fever-high donor microbiomes drove flagellin-dependent vaccine reactions.
  • Diet-related evidence included observational human diet/metabolic associations and murine diet experiments showing modifiability of microbiome flagellin phenotypes and vaccine reactions.
  • Dietary data in humans were self-reported, and the abstract-level profile does not establish that changing human diet changes vaccine fever responses.
  • Translation from gnotobiotic mouse models to human vaccine-fever outcomes remains uncertain.
Then read the study layer

Study layer

Study at a glance

Scan the study first. Expand only the parts you want to inspect.

Pieces of work

4

Evidence read

study summary

Lead result

human in vivo

1Lead resulthuman in vivoLongitudinal human cohort evidence that prevaccine gut microbiome features and low-grade intestinal inflammation are associated with post–mRNA-vaccine fever risk, and that diet/metabolic markers relate to fever risk.Prospective longitudinal cohortExpand

In plain English

Prospective longitudinal cohort study of 171 healthy adults receiving SARS-CoV-2 mRNA vaccines reporting that prevaccine gut microbiome features (higher abundance of flagellated Lachnospiraceae and increased flagellin expression) and low-grade intestinal inflammation are associated with increased risk of post-vaccination fever, with additional associations between fever risk and self-reported diet and metabolic markers.

Key findings

  • Prevaccine low-grade intestinal inflammation and increased abundance of flagellated Lachnospiraceae, along with higher microbial flagellin expression, were associated with increased risk of fever after SARS-CoV-2 mRNA vaccination.
  • Human fever risk was associated with self-reported diet and with metabolic markers measured in the cohort.
“we longitudinally profiled fecal microbiota, oral temperature, and serological markers in 171 healthy adults receiving Severe acute respiratory syndrome coronavirus 2 mRNA vaccines”
What this piece can’t prove
  • Observational design of the human cohort limits causal inference between microbiome features/inflammation and fever outcome.
  • Abstract does not provide quantitative effect sizes, confidence intervals, or detailed statistical adjustment models.
  • Dietary data were self-reported; potential for recall or reporting bias.
  • Study population described as healthy adults receiving SARS-CoV-2 mRNA vaccines (n = 171); applicability to other demographics, comorbidities, age groups, or vaccine platforms is unclear.
2in vitroMechanistic ex vivo evidence that microbiomes from fever-high human donors provoke stronger inflammatory responses in human intestinal organoids.in vitro organoid stimulationExpand

In plain English

Human intestinal organoids were exposed to microbiome preparations from human donors stratified by fever-risk phenotype; organoids exposed to microbiomes from fever-high donors exhibited stronger inflammatory responses compared with microbiomes from lower-fever-risk donors (as reported in the abstract).

Key findings

  • Microbiomes from fever-high human donors triggered stronger inflammation in human intestinal organoids compared with microbiomes from other donors (reported in the abstract).
“Microbiomes from fever-high donors triggered stronger inflammation in human intestinal organoids”
What this piece can’t prove
  • Organoid models represent epithelial and some local cell interactions but do not recapitulate systemic immune components; extrapolation to whole-organism fever responses requires additional evidence.

2 further details could not be confirmed from the summary.

3in vivo animalCausal in vivo animal evidence that fever-high donor microbiomes drive flagellin-dependent vaccine reactions in gnotobiotic mice.Gnotobiotic mouse colonization and vaccination challengeExpand

In plain English

In gnotobiotic mice colonized with microbiomes from human donors who had high fever after vaccination, vaccination elicited stronger vaccine reactions that the authors report were dependent on bacterial flagellin; the murine phenotype was also reported to be modifiable by diet (abstract-level).

Key findings

  • Microbiomes from fever-high human donors transferred into gnotobiotic mice drove flagellin-dependent vaccine reactions.
  • Murine vaccine reactions driven by fever-high donor microbiomes were reported to be modifiable by diet.
“and drove flagellin-dependent vaccine reactions in gnotobiotic mice.”
What this piece can’t prove
  • All information available here is drawn from the paper abstract; the abstract lacks methodological and quantitative detail (sample sizes, controls, statistical analyses).

3 further details could not be confirmed from the summary.

4in vivo animalModifiability evidence that diet can alter microbiome flagellin phenotypes and murine vaccine reactions, consistent with human diet associations.animal dietary interventionExpand

In plain English

The paper reports that dietary manipulation in murine models altered gut microbiome flagellin phenotypes and changed the magnitude of vaccine-induced reactions in mice; these murine diet effects are presented as modifiable and are described as consistent with observed human diet and metabolic marker associations in the cohort.

Key findings

  • Dietary interventions in mice altered gut microbiome flagellin phenotypes.
  • Dietary interventions in mice modified the magnitude/severity of vaccine-induced reactions in the murine model.
“Moreover, microbiome flagellin phenotypes and murine vaccine reactions were modifiable by diet.”
What this piece can’t prove

3 further details could not be confirmed from the summary.

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Method layer

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