Clinical spectrum and transmission characteristics of infection with Norwalk-like virus: Findings from a large community outbreak in Sweden

A large foodborne outbreak caused by Norwalk-like virus (NLV) among childre n and staff at 30 day care centers provided an opportunity to study symptom atology and attack rates among patients in different age groups, as well as secondary transmission rates in centers and households. A retrospective co hort study of 775 subjects from 13 randomly chosen centers was performed. D iarrhea was more common in adults than in children (P=.001), whereas the re verse was noted with regard to vomiting (P=.003). The primary attack rate w as 27% (142 of 524 subjects): 54% of adults versus 19% of children (P<.001) . The mean incubation time for foodborne cases of infection was 34 hours. T he secondary attack rate was 17%. Risk factors for spread into households w ere the primary case being a child (relative risk [RR], 3.8; 95% confidence interval [CI], 1.9-7.6) and vomiting (RR, 2.4; 95% CI, 1.0-5.5). The incub ation time for person-to-person transmission was approximated by a mean ser ial interval of 52 hours. This is the first reported outbreak of NLV infect ion in which secondary transmission into households by individuals has been studied.