Reading the Numbers on Approved Microbiome Medicines for Recurrent C. diff

If a patient keeps relapsing with Clostridioides difficile infection (CDI), two microbiome medicines are now approved to lower the odds of the next episode: Rebyota, a rectal product cleared in November 2022, and Vowst, an oral capsule cleared in April 2023, according to the FDA product pages for each. Both are prevention tools given after antibiotics have quieted the current infection, not treatments for active disease. If you are reading their trial data, as a clinician or as the patient, the skill that matters is knowing which number to trust, because the headline success rates are easy to misread.

Key points#

• Rebyota and Vowst are the first standardized, regulated products built from what used to be delivered as raw donor stool.
• Both are approved to prevent recurrence in adults after a course of antibiotics for recurrent CDI, not to treat a first or active infection.
• In their pivotal trials, everyone had already been treated with antibiotics, so the honest measure of benefit is the gap between the product and placebo, not the raw percentage who stayed well.
• Rebyota beat placebo by roughly 13 percentage points; Vowst by roughly 28. Comparing 70.6 percent against 88 percent across the two trials is not a fair contest.

Why the gut keeps losing to one bug#

Recurrent CDI is less a story about a stubborn germ and more a story about a wrecked neighborhood. Each round of antibiotics that suppresses C. difficile also thins out the hundreds of other species that normally share the colon. That loss of diversity removes the competition and the metabolic signals that ordinarily keep C. difficile in check, so the spores that survive treatment can bloom again the moment antibiotics stop.

Fecal microbiota transplantation (FMT) grew from a simple counter-idea: instead of hitting the same organism harder, rebuild the community around it. Screened stool from a healthy donor is transferred into the patient, and a normal microbial ecosystem re-establishes itself. Case series and small randomized trials reported resolution of recurrent CDI in the range of 80 to 90 percent, and the review in the cited PMC article traces how that early experience built the biological case for restoring the microbiome rather than sterilizing it.

The catch was never the concept. It was consistency. Donor stool varies from batch to batch, screening protocols differed between centers, and the FDA has documented transmission of drug-resistant organisms through investigational FMT, including serious outcomes. The agency treated stool used this way as a biologic and permitted its use for recurrent CDI not responding to standard care under enforcement discretion. That was a holding pattern, not an endorsement.

Turning a procedure into a product#

The two approved medicines are what happens when that bedside practice becomes a manufactured drug, with defined donor screening, controlled processing, characterized composition, and release testing on every lot. They took different engineering routes to the same goal.

Rebyota#

Rebyota (fecal microbiota, live-jslm) is a single-dose suspension given rectally, indicated to prevent recurrence of CDI in people 18 and older after antibiotic treatment for recurrent CDI, per the FDA. It is a broad consortium: a full microbial community prepared from screened human stool, closest in spirit to the original transplant. Its pivotal evidence came from the PUNCH CD3 program, a randomized, double-blind, placebo-controlled trial. In the primary analysis, modeled treatment success (no recurrence at eight weeks) was about 70.6 percent with Rebyota versus 57.5 percent with placebo, as summarized in the cited review.

Vowst#

Vowst (fecal microbiota spores, live-brpk) is an oral capsule, the first fecal microbiota product swallowed rather than instilled, indicated for the same prevention setting in adults after antibacterial treatment, per the FDA. Instead of a whole community, it is a purified fraction of bacterial spores (Firmicutes) chosen because they survive processing and the acid of the stomach. Its pivotal trial, ECOSPOR III, appeared in the New England Journal of Medicine. Recurrence at eight weeks was 12 percent with the product versus 40 percent with placebo, a relative risk near 0.32. Turned around, roughly 88 percent stayed recurrence-free on the product versus about 60 percent on placebo, and the separation held out to six months.

The trap in the headline percentages#

Line up 70.6 percent against 88 percent and it looks like one product is far stronger. That comparison does not hold, for three reasons.

The placebo arms were not untreated. Every participant had already completed standard antibiotics (vancomycin or fidaxomicin) before randomization, and a real fraction of people stay well on antibiotics alone. That is exactly why the placebo groups did so well, 57.5 and 60 percent. The benefit a microbiome product actually adds is the distance above its own placebo, about 13 percentage points for Rebyota and about 28 for Vowst. That within-trial gap is the number to read; a single-arm percentage floating on its own tells you nothing about what the product added.

The endpoints were not the same. Rebyota's headline is a modeled treatment-success estimate and Vowst's is an observed recurrence rate, so you are not reading the same kind of number twice. The trials also differed in how many prior episodes were required and in how recurrence was confirmed. A toxin-based test and a PCR test disagree about who is truly infected versus merely carrying the organism, and that choice shifts the counts in both arms. When two placebo responses differ this much (57.5 versus 40 percent), the populations and definitions were clearly not identical.

The interventions differ physically. A rectal whole-community suspension and an oral purified-spore capsule ask different things of a patient, and preference, access, and adherence all live outside the efficacy figure.

The disciplined reading is simple: both products beat an already-treated placebo in adequate, well-controlled trials, which is what earned each approval. If a cross-trial ranking of the two is offered, that is marketing, not evidence.

What these approvals do not cover#

Be precise about the job these medicines were approved to do. They prevent the next recurrence once antibiotics have controlled the current episode. They are not treatments for a first infection, not treatments for active severe CDI, and not general gut-health supplements. Long-term follow-up is still accumulating, the trials enrolled adults, and durability beyond the reported windows remains under study. Investigational donor FMT still exists alongside the approved products under the FDA's enforcement-discretion posture for eligible patients, which is a regulatory status and not a claim that the two are interchangeable.