A Phase 1b, multicenter, randomized, double-blind, placebo-controlled study to determine the safety and immunogenicity of an adenoviral-vector based oral norovirus vaccine expressing GI.1 VP1 administered orally to healthy older adult volunteers 55-80 years of age. The study is designed to assess the safety, tolerability, immunogenicity, and efficacy of 3 dose levels of vaccine with a 2-dose vaccination schedule (4 weeks apart) in healthy older adults (55 to 80 years old)