  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 = 11.0 [9.4,15.6], and of environmental transmission alone is
Renviron = 0.85 [0.18,2.04]. Direct transmission is overwhelmingly due to 0
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 of environmental and direct transmission of norovirus. While
environmental transmission has the potential to maintain a sustained series of
outbreaks aboard a cruise ship in the absence of strict sanitation practices,
direct transmission dominates. Quarantine of ill passengers and cleaning are
likely to have little impact on final outbreak size, but intensive promotion of
good hand washing practices can prevent outbreaks.
