Gut microbiota patterns tracked with weaker COVID-19 vaccine antibody responses in infliximab-treated IBD
Infliximab-treated patients with inflammatory bowel disease who mounted weaker antibody responses to SARS-CoV-2 vaccination also carried a less diverse gut microbiota and a distinct fecal metabolomic signature. The signal matters because these patients were already immunosuppressed, and anti-TNF therapy is known to blunt humoral responses; the added association with microbial composition points to a second biological layer that may help explain why vaccine responses vary so widely in this population.
The study focused on people enrolled in the CLARITY-IBD cohort who were receiving infliximab and had provided fecal and serum samples around vaccination. Antibody responses were measured after two doses of either ChAdOx1 nCoV-19 or BNT162b2, then compared with the geometric mean response across the wider CLARITY-IBD population. Investigators paired immunology with 16S rRNA gene sequencing, nuclear magnetic resonance spectroscopy, and bile acid profiling by ultra-high-performance liquid chromatography mass spectrometry, creating a linked readout of host immunity, bacterial ecology, and metabolite output.
Forty-three patients were included. Thirty had Crohn’s disease, 12 had ulcerative colitis, and 1 had IBD-unclassified disease. Twenty-six were also receiving thiopurine therapy, and eight had evidence of prior SARS-CoV-2 infection. Seventeen patients, or 39.5%, landed below the geometric mean antibody response. Even in a cohort already shaped by anti-TNF therapy, the spread in response was substantial.
Lower microbial diversity separated poorer responders from better responders
A reduced gut microbial diversity accompanied the weaker antibody response group. The association reached statistical significance, with p = 0.037. In practical biological terms, lower diversity often reflects a gut ecosystem under stress: fewer competing organisms, reduced metabolic redundancy, and a narrower repertoire of microbial products available to influence mucosal and systemic immunity. In a patient population already exposed to immune suppression and chronic intestinal inflammation, that ecological narrowing may carry direct immunologic consequences.
Specific taxa also moved in opposite directions. Bilophila abundance was associated with a better serological response, while Streptococcus tracked with poorer response. The data do not prove causality, but they suggest that the composition of the intestinal community may mirror, or perhaps help shape, the host’s capacity to generate vaccine antibodies under anti-TNF treatment. In a disease state where gut barrier function, antigen handling, and immune trafficking are already altered, microbial signals may be more than passive bystanders.
Fecal metabolites lined up with antibody strength
The metabolomic profile separated responders from poorer responders as well. Fecal samples showed a distinct pattern between the above-average and below-average groups, with an orthogonal partial least squares discriminant analysis profile that was modest but significant: R2X 0.25, R2Y 0.26, Q2 0.15, and CV-ANOVA p = 0.038. Those values indicate separation between groups without implying a fully deterministic biomarker model. The finding is biologically meaningful because metabolites are the end products of microbial activity and can influence immune cell signaling, epithelial integrity, and bile acid pathways.
Several compounds were associated with better serological response. Trimethylamine, isobutyrate, and omega-muricholic acid all tracked in that direction. Several others went the opposite way: succinate, phenylalanine, taurolithocholate, and taurodeoxycholate were associated with poorer response. The pattern suggests that both fermentation products and bile acid derivatives may be tied to how strongly the immune system reacts after vaccination in patients receiving infliximab.
One metabolite deserves special attention. Trimethylamine is part of a broader microbial-host metabolic axis and has been implicated in immune modulation through downstream pathways that extend beyond the gut lumen. Its association with better antibody response raises the possibility that microbial metabolic output could help offset some of the attenuating effects of anti-TNF therapy on vaccine immunity. The study does not establish a therapeutic target, but it does identify a plausible mechanistic bridge between the microbiome and the humoral response.
Why anti-TNF therapy may expose a microbial signature
Anti-TNF treatment dampens inflammatory signaling, and that can improve bowel disease control while also muting aspects of vaccine responsiveness. The immune system does not operate in isolation from the intestinal microbiota; microbial metabolites influence dendritic cell behavior, T-cell priming, B-cell maturation, and mucosal immune tone. In patients with IBD, the baseline microbiome is often altered by disease activity, diet, medications, and prior antibiotic exposure, so the intestinal ecosystem may either cushion or amplify the effect of immunosuppression on vaccine response.
This study places those ideas into a real-world immunosuppressed cohort rather than a theoretical model. Patients on infliximab do not simply differ by drug exposure alone. They also differ by microbial diversity, metabolite output, and possibly by how much immune reserve remains available to respond to vaccination. The combined picture is more useful than any single marker.
What the cohort can and cannot answer
The sample was small, and the cohort included only 43 patients, so these findings should be read as hypothesis-generating rather than practice-changing. Serological response was classified relative to the geometric mean of the wider CLARITY-IBD cohort, which is a sensible comparative framework but still leaves room for confounding by prior infection, concomitant thiopurine use, disease phenotype, and vaccine platform. Eight participants had prior SARS-CoV-2 infection, and 26 were taking thiopurines, both of which could influence immune readouts.
Even so, the study provides a clinically relevant clue: weaker antibody responses after vaccination in infliximab-treated IBD were linked to a distinct gut microbial and metabolomic state. That linkage raises the possibility that microbiome-aware strategies, or metabolite-informed risk stratification, could eventually help identify patients most likely to under-respond to SARS-CoV-2 vaccination. For now, the evidence is associative, but the biology is coherent and the signal is difficult to ignore.