Investigation of the second wave (phase 2) of severe acute respiratory syndrome (SARS) in Toronto, Canada. What happened?

BACKGROUND: Severe acute respiratory syndrome (SARS) was introduced to Toronto on 23 February, 2003. The outbreak was believed to be over in early May after two incubation periods had passed with no newly identified cases. However, on 20 May, 2003 a cluster of respiratory illnesses occurred in a rehabilitation facility in Toronto. These cases were later identified as SARS but not until a community hospital in which one of these cases was previously hospitalized (Hospital X) had already experienced nosocomial transmission. This report describes the outbreak investigation of nosocomial transmission of SARS at Hospital X.
METHODS: An investigation of 90 probable and suspect cases of SARS associated with a hospital outbreak was performed. The investigation included death record reviews, chart reviews, case finding and contact tracing. Nursing cohorts who worked on the specific ward in which nosocomial transmission occurred had work-shift and patient-assignment records reviewed to determine source of infection.
RESULTS: The greatest number of cases occurred within hospital employees (42.5%) with an average age of 51 years and 69% being female. The mean incubation period for one time exposure patients was 6.3 days (range 4 to 10 days). Twelve (13.8%) of the cases died. Five of seven nurses who cared for a specific SARS patient during this period acquired SARS. Twelve of 17 (70.6%) nursing staff who developed SARS worked with another nursing staff who was symptomatic for SARS. Staff members who worked the evening shift or the morning shift and therefore likely attended a nursing appreciation breakfast were five times more likely to acquire SARS than those who did not attend.
INTERPRETATION: What was believed to be the end of the Toronto SARS outbreak led the Provincial Operations Centre (POC) to issue a directive allowing a more relaxed use of infection-control precautions during the beginning of Phase 2 of the outbreak. These relaxations of precautions were temporally associated with the nosocomial transmission of SARS to hospital staff, other patients and visitors at Hospital X. As a result of this outbreak significant changes have been made with respect to infection-control practices within Canada.