The main objective of the study is to evaluate whether the extended duration fidaxomicin therapy is superior to the standard vancomycin therapy in sustained clinical cure of CDI at 30 days after end of treatment (Day 40 or Day 55).

Inclusion Criteria:

• CDI is confirmed by clinical symptoms (either > 3 unformed bowel movements or ≥ 200ml of unformed stool (for subjects having rectal collection devices)) in the 24 hours prior to randomization and CDI test confirmed positive for presence of C. difficile toxin A or B in stool within 48 hr prior to randomization.
• Subject agrees not to participate in another interventional study whilst participating in this study.

Exclusion Criteria:

• Subject is taking or requiring to be treated with prohibited medications
• Subject has received more than one day of dosing of any therapy for CDI within the last 48 hours
• Subject has experienced more than 2 previous episodes of CDI in the 3 months prior to study enrolment
• Subject is unable to swallow oral study medication.
• Subject has a current diagnosis of toxic megacolon.
• Subject is not willing to adhere to the provisions of treatment and observation specified in the protocol.
• Subject has been randomized into this study previously, has taken any investigational drug within 28 days or 5 half lives, whichever is longer, prior to enrollment, or is currently participating in another clinical study which may influence the assessment of efficacy and/or safety endpoints of this study, in the opinion of the Sponsor.
• Subject has previously participated in a CDI vaccine study
• Subject has hypersensitivity to fidaxomicin, vancomycin or any of its components.

Access to anonymized individual participant level data collected during the study, in addition to study-related supporting documentation, is planned for studies conducted with approved product indications and formulations, as well as products terminated during development. Studies conducted with product indications or formulations that remain active in development are assessed after study completion to determine if Individual Participant Data can be shared. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.

Access to participant level data is offered to researchers after publication of the primary manuscript (if applicable) and is available as long as Astellas has legal authority to provide the data.

Researchers must submit a proposal to conduct a scientifically relevant analysis of the study data. The research proposal is reviewed by an Independent Research Panel. If the proposal is approved, access to the study data is provided in a secure data sharing environment after receipt of a signed Data Sharing Agreement.