Multivariate regression analyses demonstrated that higher attack rates were significantly associated with foodservice (b 0 14.70, p00.02) and winter outbreaks (b09.81, p0B0.01). A combination of strains was most common among food and waterborne outbreaks. Waterborne outbreaks were also significantly associated with GI strains (odds ratio [OR] 00.10, 95% confidence interval [CI] 00.03Á0.41 where the odds of an outbreak being caused by a GII strain are less than the odds of the outbreak being caused by a GI strain), while healthcarerelated (OR 040.42, 95% CI 02.09Á783.15 where the odds of the outbreak being caused by a GII strain are greater than the odds of the outbreak being caused by a GI strain) and winter outbreaks (OR 05.56, 95% CI 01.97Á15.69) were associated with GII strains (Table 1) .