Single IV infusion of MK-3415A (10 mg/kg of monoclonal antibody to C. difficile Toxin A and 10 mg/kg of monoclonal antibody to C. difficile Toxin B)

Single IV infusion of normal saline (0.9% sodium chloride)

SOC for CDI will be prescribed for 10 to 14 days and can begin on the day of study drug infusion; but the first dose must have been administered prior to or within a few hours following study drug infusion. SOC is defined as the receipt of oral metronidazole, oral vancomycin, IV metronidazole concurrent with oral vancomycin, oral fidaxomicin, or oral fidaxomicin concurrent with IV metronidazole.

An adverse event (AE) is defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the medicinal product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a preexisting condition which is temporally associated with the use of the medicinal product, is also an adverse event.

An adverse event (AE) is defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the medicinal product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a preexisting condition which is temporally associated with the use of the medicinal product, is also an adverse event.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. The 027 ribotype is a more virulent, epidemic strain responsible for several outbreaks of disease associated with an increased risk of severity and mortality.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. An epidemic strain includes ribotypes 027, 014, 002, 001, 106 or 020.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. Participants with clinically severe CDI have a Zar Score greater than or equal to 2 points based on the presence of 1 or more of the following: 1) age >60 years old (1 point); 2) body temperature >38.3°C (>100°F) (1 point); 3) albumin level ˂2.5 mg/dl (1 point); 4) peripheral white blood cell count >15,000 cells/mm^3 within 48 hours (1 point); 5) endoscopic evidence of pseudomembranous colitis (2 points); and 6) treatment in Intensive Care Unit (2 points).

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. Compromised immunity is an active hematological malignancy (including leukemia, lymphoma, multiple myeloma), an active malignancy requiring recent cytotoxic chemotherapy, receipt of a prior hematopoietic stem cell transplant, receipt of a prior solid organ transplant, asplenia, or neutropenia/pancytopenia due to other conditions.

Normal saline IV infusion (0.9% sodium chloride) + SOC for CDI

Total of all reporting groups

Single IV infusion of 10 mg/kg MK-6072 + SOC for CDI

Normal saline IV infusion (0.9% sodium chloride) + SOC for CDI

Global cure is defined as the clinical cure of the initial CDI episode with no CDI recurrence through Week 12. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

The FAS population consisting of all randomized participants with participants excluded for the failure to receive infusion of study medication; for lack of a positive local stool test for toxigenic C. difficile; or for failure to receive protocol defined standard of care therapy within a 1 day window of the infusion.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

Treated participants who achieved a clinical cure of the initial CDI episode.

An adverse event (AE) is defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the medicinal product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a preexisting condition which is temporally associated with the use of the medicinal product, is also an adverse event.

All Participants as Treated (APaT), based on the treatment actually received. One participant randomized to the MK- 3415A + SOC arm who was treated with MK-3415, but was not treated with MK-6072, was not analyzed.

An adverse event (AE) is defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the medicinal product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a preexisting condition which is temporally associated with the use of the medicinal product, is also an adverse event. A drug-related adverse event is determined by the investigator to be related to the drug.

APaT based on the treatment actually received. One participant randomized to the MK-3415A + SOC arm who was treated with MK-3415, but was not treated with MK-6072, was not analyzed.

A serious adverse event (SAE) is any AE occurring at any dose or during any use of the medicinal product that results in death; or is life threatening; or results in a persistent or significant disability/incapacity; or results in or prolongs an existing inpatient hospitalization; or is a congenital anomaly/birth defect; or other important medical events. A serious drug-related adverse event is determined by the investigator to be related to the drug.

APaT based on the treatment actually received. One participant randomized to the MK-3415A + SOC arm who was treated with MK-3415, but was not treated with MK-6072, was not analyzed.

An adverse event (AE) is defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the medicinal product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a preexisting condition which is temporally associated with the use of the medicinal product, is also an adverse event.

APaT based on the treatment actually received. One participant randomized to the MK-3415A + SOC arm who was treated with MK-3415, but was not treated with MK-6072, was not analyzed.

An adverse event (AE) is defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the medicinal product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a preexisting condition which is temporally associated with the use of the medicinal product, is also an adverse event.

APaT based on the treatment actually received. One participant randomized to the MK-3415A + SOC arm who was treated with MK-3415, but was not treated with MK-6072, was not analyzed.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

Treated participants with a history of CDI in the past 6 months.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. The 027 ribotype is a more virulent, epidemic strain responsible for several outbreaks of disease associated with an increased risk of severity and mortality.

Treated participants with the 027 ribotype

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. An epidemic strain includes ribotypes 027, 014, 002, 001, 106 or 020.

Treated participants with an epidemic strain

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. Participants with clinically severe CDI have a Zar Score greater than or equal to 2 points based on the presence of 1 or more of the following: 1) age >60 years old (1 point); 2) body temperature >38.3°C (>100°F) (1 point); 3) albumin level ˂2.5 mg/dl (1 point); 4) peripheral white blood cell count >15,000 cells/mm^3 within 48 hours (1 point); 5) endoscopic evidence of pseudomembranous colitis (2 points); and 6) treatment in Intensive Care Unit (2 points).

Treated participants with clinically severe CDI

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen.

Treated participants 65 years and older.

CDI recurrence is defined as the development of a new episode of diarrhea (3 or more loose stools in 24 or fewer hours) and a positive lab stool test (local or central) for toxigenic C. difficile. Clinical cure is defined as no diarrhea [2 or fewer loose stools per 24 hours] for 2 consecutive days following completion of SOC therapy for the initial CDI episode in participants who received =< 14 day regimen. Compromised immunity is an active hematological malignancy (including leukemia, lymphoma, multiple myeloma), an active malignancy requiring recent cytotoxic chemotherapy, receipt of a prior hematopoietic stem cell transplant, receipt of a prior solid organ transplant, asplenia, or neutropenia/pancytopenia due to other conditions.

Treated participants with compromised immunity

Single IV infusion of 10 mg/kg MK-6072 + SOC for CDI

Normal saline IV infusion (0.9% sodium chloride) + SOC for CDI

Single IV infusion of 10 mg/kg MK-3415 + SOC for CDI

9 Month Extension (9-ME) for participants treated with a single IV infusion of 10 mg/kg MK-3415A + SOC

9-ME for participants treated with a single IV infusion of 10 mg/kg MK-6072 + SOC

9-ME for participants treated with Placebo