Quantifying the relative effects of environmental and direct transmission of norovirus

Background: Norovirus is a common cause of outbreaks of acute gastroenteritis in health- and child-care settings, with serial outbreaks also frequently observed aboard cruise ships. The relative contributions of environmental and direct person-to-person transmission of norovirus has hitherto not been quantified. Objective: We employ a novel mathematical model of norovirus transmission, and fit the model to daily incidence data from a major norovirus outbreak on a cruise ship, and examine the relative efficacy of potential control strategies aimed at reducing environmental and/or direct transmission. Results: The reproduction number for environmental and direct transmission combined is Rtot = 7.2 [6.1,9.5], and of environmental transmission alone is Renviron = 1.6 [0.9,2.6]. Direct transmission is overwhelmingly due to passenger-to-passenger contacts, but crew can act as a reservoir of infection from cruise-to-cruise. Implications: This is the first quantification of the relative roles o...