Safety and Immunogenicity of Different Formulations of a Clostridium Difficile Toxoid Vaccine Administered at Three Different Schedules in Adults Aged 40 to 75 Years at Risk of C. Difficile Infection

This study will further evaluate the ACAM-CDIFF™ vaccine in a population of middle-aged to
elderly individuals at risk of exposure to Clostridium difficile because of impending
hospitalization or residence in a care facility.

Primary Objectives:

- To describe the safety profile of subjects in each of the study groups.

- To describe the immune responses elicited by toxoid A and toxoid B of subjects in each
of the study groups.

Observational Objective:

- To describe the occurrence of first-time Clostridium difficile infection (CDI) episodes.