Clinical Spectrum and Transmission Characteristics of Infection with Norwalk‐Like Virus: Findings from a Large Community Outbreak in Sweden

Abstract

A large foodborne outbreak caused by Norwalk-like virus (NLV) among children and staff at 30 day care centers provided an opportunity to study symptomatology and attack rates among patients in different age groups, as well as secondary transmission rates in centers and households. A retrospective cohort study of 775 subjects from 13 randomly chosen centers was performed. Diarrhea was more common in adults than in children (P=.001), whereas the reverse was noted with regard to vomiting (P=.003). The primary attack rate was 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. The secondary attack rate was 17%. Risk factors for spread into households were 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 incubation time for person-to-person transmission was approximated by a mean serial interval of 52 hours. This is the first reported outbreak of NLV infection in which secondary transmission into households by individuals has been studied.

Risk factors for symptomatic and asymptomatic norovirus infection in the community

“…Furthermore, when a household contact with IID symptoms was reported, the risk of norovirusassociated IID was greatest when this contact was a young child. This pattern of transmission, from young children to adults, was also observed in a large household transmission study following a pointsource norovirus outbreak [27]. However, after calculation of the PAFs, a substantial proportion of norovirus cases were not attributed to any of the recognized risk factors for norovirus transmission.…”

Waterborne gastroenteritis outbreak at a scouting camp caused by two norovirus genogroups: GI and GII

“…The sudden simultaneous onset of gastrointestinal symptoms, the high attack rate (85%) and clear secondary attack rate (20%) clearly correspond with a point-source outbreak, like a waterborne outbreak rather than person-to-person transmission. 17 The latter would hardly have been possible because of the immediate hospital admissions and camp cancellation. Using the latrines could have been an additional transmission route, but it is not likely to be the primary source considering the rationale for water mentioned above and the fact that all epidemiological sensitivity analyses revealed water to be the strongest independent risk factor.…”

Enhanced Hygiene Measures and Norovirus Transmission during an Outbreak

“…The method we used to estimate the time course of reproduction number depends crucially on a correct specifi cation of the generation time distribution. Here we obtained this distribution from a study of a norovirus outbreak in child daycare centers in Sweden (25) and estimated that the generation time distribution peaked at 2.6 days ( Figure 2). This estimation agrees with results from volunteer studies in which adults showed a peak in virus shedding at 1-3 days postchallenge (31).…”

“…To calculate the probability of transmission between any pair of cases, we needed information from the distribution of generation times (defi ned as the time between day of symptom onset in a secondary case and day of onset in its primary case) (19,24). To estimate the generation time distribution for norovirus infections, we used observations of generation times from several large norovirus outbreaks in child daycare centers in Sweden during 1999 (25). These generation times were well described by a gamma distribution ( distribution of generation times was used to assign a likelihood of transmission for any pair of cases, allowing estimation of the transmission probabilities.…”