A Study to Learn About a Clostridioides Difficile Vaccine in People 65 Years of Age and Older

Summary

The purpose of the study is to learn about how effective, safe, and tolerable the Clostridioides difficile (C. difficile) vaccine is in decreasing the number of C. difficile infections (pCDI) in adults 65 years of age or older. The participants will receive either the C. difficile vaccine or placebo (saline). A placebo does not contain any active ingredients. The vaccines will be given as a shot into the upper arm muscle. This study looks at the number of diarrhea (loose stools) events related to a C. difficile infection, so the length of the study can change. If these events happen quickly, the study may finish sooner; if they happen slowly, it could take longer. Sometimes, the study might stop early if the vaccine clearly does not work. Participants will stay in the study until enough events have occurred to answer the main question. On average, each person is expected to take part for up to about three and a half years. During this time, participants will have 3 planned clinic visits and 3 planned phone visits. After these visits, more scheduled clinic visits will happen every year until the end of study. Besides the scheduled study visits, if at any time during the study a participant has 3 or more loose stools in 24 hours, they will be asked to save the next one (the fourth or later) and contact the study site. The study site will check to see if it could be a C. difficile infection. This check may happen at the clinic or through a phone or video call.

Eligibility

Study design

Outcome measures

1. Primary outcome

   The percentage of participants reporting local reactions

   Injection site pain, redness, and swelling as self-reported in electronic diaries

   Time frame For 7 days after each vaccination

2. Primary outcome

   The percentage of participants reporting systemic events

   Fever, vomiting, fatigue, headache, muscle pain, and joint pain as self-reported in electronic diaries

   Time frame For 7 days after each vaccination

3. Primary outcome

   Percentage of participants reporting adverse events

   As elicited by investigator site staff

   Time frame From each vaccination through 1 month after each vaccination

4. Primary outcome

   Percentage of participants reporting serious adverse events

   As elicited by investigator site staff

   Time frame From Vaccination 1 (Day 1) through 12 months after Vaccination 2 (last dose) (18 months)

5. Primary outcome

   The incidence of the first episode of medically attended and clinically meaningful primary Clostridioides difficile infection (CDI)

   First episode of medically attended and clinically meaningful primary CDI incidence per 1000 person- years of follow- up

   Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

6. Primary outcome

   The incidence of the first episode of medically attended primary CDI

   First episode of medically attended primary CDI incidence per 1000 person- years of follow- up

   Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

7. Primary outcome

   The incidence of the first episode of clinically meaningful primary CDI

   First episode of clinically meaningful primary CDI incidence per 1000 person- years of follow- up

   Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

8. Secondary outcome

   The incidence of antibiotic use in the treatment of a first episode of the primary CDI

   First episode of primary CDI with antibiotic use incidence per 1000 person- years of follow- up

   Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

9. Secondary outcome

   The incidence of severe or fulminant primary CDI

   First episode of severe or fulminant primary CDI incidence per 1000 person- years of follow- up

   Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

10. Secondary outcome

    The incidence of a first episode of primary CDI (any severity)

    First episode of primary CDI incidence per 1000 person- years of follow- up

    Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

11. Secondary outcome

    The incidence of all CDI (primary and recurrent)

    Occurence of all C.difficile infection cases

    Time frame from 14 days after Vaccination 2 to the end of the surveillance period (up to approximately 3.5 years)

Dates

Actual means the event happened. Estimated means the sponsor expects it. The two mean different things.

Locations

180 sites are recruiting

135 further sites are listed in the registry record.

Study documents

No documents are linked in this registry record.

Changes over time

1. September 9, 2026

   Site added

   2 sites added (185 total)

   183 → 185

2. August 14, 2026

   Enrolment target changed

   Enrollment target revised up from 32000 to 32500

   32000 → 32500

3. August 14, 2026

   Site added

   9 sites added (183 total)

   174 → 183