Clostridoides Difficile Associated Disease

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.

Re-exposure to systemic antibiotics (i.e., antibiotics absorbed into the bloodstream) is common after a Clostridioides difficile infection (CDI) and is the strongest risk factor for a recurrent episode. Oral vancomycin to prevent a recurrence during antibiotic re-exposure may reduce this risk but the data supporting this practice are limited. The aim of this trial is: 1\) Does oral vancomycin prophylaxis prevent CDI recurrences in patients with recent CDI (within 120 days) and who are re-exposed to systemic antibiotics? The trial will compare oral vancomycin to placebo. Participants will: * Take the study drug (either vancomycin or placebo) twice daily for the duration of systemic antibiotics plus once daily for 7 days after completion of systemic antibiotics. * Attend an in-person or over the phone follow-up at day 56 * Respond to weekly electronic questionnaires