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

A PHASE 3, PLACEBO-CONTROLLED, DOUBLE-BLINDED, RANDOMIZED STUDY TO EVALUATE THE EFFICACY, SAFETY, AND TOLERABILITY OF A CLOSTRIDIOIDES DIFFICILE VACCINE IN ADULTS 65 YEARS OF AGE AND OLDER

Who can join

Ages 65 and older · All sexes · Healthy volunteers welcome

Who Can Join the Study (Inclusion Criteria):

• People with recent or future planned contact with healthcare systems, recent antibiotic exposure, or a diagnosis of certain chronic medical conditions.

Who Cannot Join the Study (Exclusion Criteria):

* Anyone who has had CDI before.
* Anyone who has had surgery to remove part of their small or large intestine.
* Anyone who often has diarrhea (meaning three or more loose stools in a day, more than once a month).
* Anyone who has already received a vaccine or special antibody treatment for C. difficile.
* Anyone who has had a serious allergic reaction to a vaccine or to any part of the study vaccines.
* Anyone who might not respond well to the vaccine because their immune system is weak (either from a disease or from treatment).
* Anyone with cancer that has spread or another serious health problem.

About the study

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.

What is being tested

• C.difficile vaccine (biological)
• Saline Placebo (other)

Sponsor: Pfizer · Participants: 32,500 · Started: Dec 10, 2025

Contact the study team

• Pfizer CT.gov Call Center · Phone: 1-800-718-1021

Official record on ClinicalTrials.gov — NCT07282665

Locations in the U.S.

From ClinicalTrials.gov, data retrieved Sep 29, 2026. Each study sets its own eligibility; the study team decides who can join.