Abstract Clostridium difficile is the most common cause of healthcare-associated infectious diarrhea among adults in industrialized countries. In addition to diarrhea, C. difficile infection (CDI) may also result in serious complications such as shock, toxic megacolon, colectomy, and death. The Centers for Disease Control and Prevention (CDC) has estimated C. difficile results in 250,000 hospital infections, 14,000 deaths, and $1 billion in excess costs annually. Recurrent CDI is the most challenging clinical dilemma facing clinicians who treat this disease. An estimated 30% of patients who respond to initial treatment with either vancomycin or metronidazole develop recurrent CDI, usually within 1-4 weeks of completing treatment.

The primary objective of this study is to determine whether 1) standard fidaxomicin treatment and 2) standard vancomycin treatment followed by taper and pulse vancomycin treatment are superior to standard vancomycin treatment alone for sustained clinical response at day 59 for all treatments, for participants with either their first or second recurrence of CDI. Veterans presenting with a first or second CDI recurrence will be screened, consented and randomly assigned in a double-blind manner equally to one of three treatment groups: 1) a 10 day course of oral vancomycin (VAN-TX), 2) a 10 day course of fidaxomicin (FID-TX) or 3) a 31 day course of vancomycin which includes a taper and pulse following daily treatment (VAN-TP/P). Symptom resolution is defined as an improvement or resolution of diarrhea ( 3 unformed bowel movements over 24 hours) for 48 consecutive hours compared to the participant's baseline. Recurrence is defined as having diarrhea (\>3 loose or semi-formed stools over 24 hours for 48 consecutive hours). A sample size of 459 randomized study participants is required to obtain 91% global power to detect a 16% absolute difference (expected proportion of 31% in the VAN-TX group) in sustained clinical response (D- COM) proportion for at least one comparison (VAN-TP/P vs. VAN-TX, FID-TX vs. VAN-TX) at the family wised error rate (FWER) 0.05 level. The marginal probability (disjunctive power) of detecting 16% absolute difference for each comparison is 81%. The expected withdrawal rate prior to day 59 (prior outcome assessment) is estimated to be 10%. If both FID-TX and VAN-TP/P are found to be superior to VAN-TX, then the non-inferiority of VAN-TP/P to FID-TX will be assessed.

With the assumption that sites recruit 4 participants (site average) per year for sites primarily recruiting from the main hospital and nearby CBOCS, and 6 participants (site average) per year for sites that could partner with independent VAMCs (Independent VAMCs LSI Application will be reviewed and approved by Central IRB) that are close in distance to allow a shared site coordinator (WOC appointed) at an increased funding level, the study is expected to complete enrollment of 459 participants within 6 years with 90 days of follow-up. This includes 2 years of pilot phase plus transitioning period from pilot phase to full study, and 4 years of full study. There were 6 sites in the pilot phase and will have 24 units (26 sites) in full phase (including 5 pilot sites and 21 additional sites). Sites that are significantly below the recruitment target for an extensive period may be considered for termination. The recruitment timeline and the number of sites will be re-evaluated based on the actual recruitment rate, the number of sites still recruiting, whether replacement or additional sites will be added, the study time period on administrative recruitment hold due to COVID-19 pandemic, and available funding resources.