Temporal-spatial analysis of severe acute respiratory syndrome among hospital inpatients.

BACKGROUND We report the temporal-spatial spread of severe acute respiratory syndrome (SARS) among inpatients in a hospital ward during a major nosocomial outbreak and discuss possible mechanisms for the outbreak. METHODS All inpatients who had stayed in the same ward as the initial index case patient for any duration before isolation were recruited into a cohort and followed up to document the occurrence of SARS. The normalized concentration of virus-laden aerosols at different locations of the ward was estimated by use of computational fluid dynamics modeling. The attack rates in the various subgroups stratified by bed location were calculated. Multivariate Cox proportional hazards regression was used to document important risk factors. RESULTS The overall attack rate of SARS was 41% (30 of 74 subjects). It was 65%, 52%, and 18% in the same bay, adjacent bay, and distant bays, respectively (P = .001). Computation fluid dynamics modeling indicated that the normalized concentration of virus-laden aerosols was highest in the same bay and lowest in the distant bays. Cox regression indicated that staying in the ward on 6 or 10 March entailed higher risk, as well as staying in the same or adjacent bays. The epidemic curve showed 2 peaks, and stratified analyses by bed location suggested >1 generation of spread. CONCLUSIONS The temporal-spatial spread of SARS in the ward was consistent with airborne transmission, as modeled by use of computational fluid dynamics. Infected health care workers likely acted as secondary sources in the latter phase of the outbreak.