The study is a phase I, single center, double-blind, placebo-controlled dose finding clinical study to evaluate the safety, tolerability and immunogenicity of the live attenuated, oral vaccine ACE527. The study is designed to evaluate 2 doses of the ACE527 in 2 cohorts. Each cohort will consist of approximately 18 subjects (healthy subjects), approximately 12 of them receiving ACE527 and approximately 6 receiving placebo.