Background: Norovirus (NoV) is a leading cause of epidemic non-bacterial acute gastroenteritis in young children and adults globally. Snow Mountain Virus (SMV), the prototype of genogroup II and genotype II NoV, has been used in three human challenge studies to examine the infectivity, pathogenicity, and immune response to NoV. Methods: This is a secondary data analysis. Clinical and laboratory data from two previously completed SMV human challenge trials using two different inocula (Inoculum 1 and Inoculum 2) were analyzed to compare infectivity, illness (including modified Vesikari severity scores of gastroenteritis in those subjects with clinical symptoms), viral shedding, and serum IgG conversion. SMV Inoculum 2 is a second-generation inoculum prepared from a stool sample collected from a study subject who was infected with SMV Inoculum 1. Results: Of 15 subjects orally challenged with SMV Inoculum 1 between 2000 and 2002, nine were infected, and seven presented with acute gastroenteritis. Of 33 subjects orally challenged with SMV Inoculum 2 between 2016 and 2018, 25 were infected, and nine presented with acute gastroenteritis. There were no statistically significant differences in overall infection and illness rates between subjects challenged with Inoculum 1 vs. Inoculum 2. However, subjects infected with Inoculum 1 experienced more severe clinical symptoms of acute gastroenteritis and had higher severity scores (6.00 vs. 2.94, P = 0.003) compared with those infected with Inoculum 2. We also observed that infection with Inoculum 2 resulted in longer viral shedding compared with Inoculum 1. This analysis also indicated that secretor-positive subjects had more severe gastroenteritis than secretor-negative subjects. Among ill subjects, no association was observed between challenge dose and severity of acute gastroenteritis. Conclusions: Understanding the differences between these two SMV inocula is critical for NoV vaccine evaluation because illness and viral shedding are two important outcomes in NoV challenge studies to determine vaccine efficacy. Using a less pathogenic inoculum for a vaccine trial will require more participants to meet the target reduction in illness when evaluating the efficacy of candidate vaccines.