The unwelcome houseguest: secondary household transmission of norovirus.

Norovirus is the leading cause of acute gastroenteritis in the USA. Although secondary household transmission of norovirus is frequently reported in outbreaks, little is known about specific risk factors for susceptibility and infectiousness in the household. Three norovirus outbreaks were investigated and data were collected on individuals exposed in the primary outbreak setting and their household members. Potential individual- and household-level risk factors for susceptibility and infectiousness were assessed using univariate and multivariate generalised linear mixed models. In the univariate models, the secondary attack rate (SAR) was significantly higher when living in a household with two or more primary cases (incidence rate ratio (IRR) = 2·1; 95% confidence interval (CI) 1·37-3·29), more than one primary case with vomiting (IRR = 1·9; CI 1·11-3·37), and at least one primary case with diarrhoea (IRR = 3·0; CI 1·46-6·01). After controlling for other risk factors in the multivariate models, the SAR was significantly higher among those living in a household with two or more primary cases (adjusted IRR = 2·0; CI 1·17-3·47) and at least one primary case with diarrhoea (adjusted IRR = 2·8; CI 1·35-5·93). These findings underscore the importance of maintaining proper hygiene and isolating ill household members to prevent norovirus transmission in the household.