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

Abstract

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.

Figure 1

Layout of the ward where the index case patient with severe acute respiratory syndrome (SARS) was hospitalized, showing the location of beds, air supply diffusers, and exhaust grilles. Supply and exhaust flow rates were measured in liters per second). The index case patient stayed in bed 11 during the period 4–12 March. Same bay, beds 9–16, 9x, and 16x; adjacent bay, beds 17–24, 17x, and 24x; distant bays, all other beds.

Figure 2

Date of fever onset for all patients with severe acute respiratory syndrome in the ward, stratified by bed location.

Table 1

Attack rates of severe acute respiratory syndrome (SARS) for inpatients according to the dates of presence in the hospital ward during the outbreak in March 2003.

Table 2

Risk factors for severe acute respiratory syndrome (SARS) among inpatients in the hospital ward during the outbreak in March 2003 identified by Cox regression.

Figure 3

Severe acute respiratory syndrome (SARS)-free survival curves for inpatients in the 3 bed locations in the ward.

Figure 4

Dates of fever onset for nurses (A), all health care and supporting workers (B), visitors (C), and all patients combined (D) involved in the outbreak of severe acute respiratory syndrome.

Figure 5

Steady-state normalized concentration contours of virus-laden aerosols in the ward at a height of 1.1 m