Figure 1

Flow diagram of the transmission model. The population of each city is classified into seven different compartments, namely susceptible (S), latent (L), infectious (I), hospitalized who either recover (H _R ) or die (H _D ), dead (D) and recovered (R) individuals. We assume that hospitalized as well as infectious individuals are able to transmit the infection, given the large percentage of the cases among health care workers [37-39]. The actual efficiency of hospital isolation procedures is modeled through a reduction of the transmission rate β by a factor r _β for hospitalized patients, with r _β = 20% as estimated for the early stage of the epidemic in Hong Kong [9]. The infectiousness of patients in the compartments H _R and H _D are assumed to be equal (although this assumption can easily be changed in the model). Susceptible individuals exposed to SARS enter the latent class. Latents represent infected who are not yet contagious and are assumed to be asymptomatic, as suggested by results based on epidemiologic, clinical and diagnostic data in Canada [40]. They become infectious after an average time ε ^-1 (mean latency period). The individual is classified as infectious during an average time equal to μ ^-1 from the onset of clinical symptoms to his admission to the hospital where he eventually dies or recovers. Patients admitted to the hospital are not allowed to travel. The average periods spent in the hospital from admission to death or recovery are equal to μ _D ^-1 and μ _R ^-1, respectively. The average death rate is denoted by d.