This study is about a vaccine for Clostridioides difficile (C. difficile), a bacteria that can cause diarrhea in older adults. People 65 years or older will get either the C. difficile vaccine or a placebo (a shot with no medicine) in their arm muscle. The study checks how well the vaccine works, if it's safe, and how people feel after getting the shot.

The study will last up to about three and a half years. Participants will visit the clinic 3 times and have 3 phone visits. After that, they will have yearly visits until the study ends. If someone has 3 or more loose stools in a day, they should contact the study team.

• Participants may be involved for up to 3.5 years.
• There are 3 in-person and 3 phone visits, plus yearly visits.
• Participants must report 3+ loose stools in 24 hours.

People who have had C. difficile before, certain surgeries, or frequent loose stools cannot join. Also excluded are those with serious allergies, previous C. difficile vaccine or treatment, or major health issues.