Phase I dose finding studies of an adjuvanted Clostridium difficile toxoid vaccine

Fifty healthy adult (18-55 years) and 48 elderly (≥ 65 years) volunteers were randomized to receive a candidate Clostridium difficile toxoid vaccine (2 μg, 10 μg, or 50 μg) or placebo on Days 0, 28, and 56. No volunteer receiving placebo seroconverted. For toxin A, seroconversion by Day 56 (post-dose 2) was observed in 100% of volunteers aged 18-55 years in all dose groups and in 50%, 89%, and 100% of elderly participants in the 2 μg, 10 μg, and 50 μg dose groups, respectively. For both age groups, seroconversion for toxin B was lower than toxin A. There were no safety concerns.