Mathematical model for the control of nosocomial norovirus.

A gastroenteritis outbreak in a long-term care facility was analysed by means of a SEIR (Susceptible, Exposed/Latent phase, Infected/Infectious, and Recovered) compartment model of infection dynamics in a closed population [96 beds; attack rate=41%; R0 (basic reproductive number)=3.74; generation time approximately 1 day; duration of disease approximately 2 days; theoretical infinite (1000 days) duration of hospital stay]. The patient-turnover variation was simulated to determine the effect of the length of hospital stay on the endemic level of gastroenteritis perpetuating the epidemic phase in an open population. With all the other parameters held constant, the prevalence of infected patients in the endemic phase (50 days after the beginning of the outbreak) increased markedly from five to 18 cases as the hospital stay increased from one-tenth of a day (one-day care) to one or two days; the prevalence decreased exponentially with the length of hospital stay, being fewer than five cases for hospital stays >50 days. In conclusion, the endemic prevalence of norovirus gastroenteritis is critically dependent on the patient turnover within hospital wards. For the usual range of hospital stay (0.1-20 days), the prevalence level is sufficiently elevated to maintain the perpetuation of gastroenteritis within the population of institutionalised patients. In long-term care facilities (hospital stay >20 days), the patient turnover is sufficiently low for one to expect a spontaneous extinction of epidemic outbreak without endemic perpetuation. When an epidemic outbreak occurs in an acute-care setting, reinforcement of infection control measures, including closure of the ward, is required to break the transmission chain.