Public health measures to control the spread of the severe acute respiratory syndrome during the outbreak in Toronto

Background: Toronto was the site of North America's largest outbreak of the severe acute respiratory syndrome (SARS). An understanding of the patterns of transmission and the effects on public health in relation to control measures that were taken will help health officials prepare for any future outbreaks. Methods: We analyzed SARS case, quarantine, and hotline records in relation to control measures. The two phases of the outbreak were compared. Results: Toronto Public Health investigated 2132 potential cases of SARS, identified 23,103 contacts of SARS patients as requiring quarantine, and logged 316,615 calls on its SARS hotline. In Toronto, 225 residents met the case definition of SARS, and all but 3 travel-related cases were linked to the index patient, from Hong Kong. SARS spread to 11 (58 percent) of Toronto's acute care hospitals. Unrecognized SARS among in-patients with underlying illness caused a resurgence, or a second phase, of the outbreak, which was finally controlled through active surveillance of hospitalized patients. In response to the control measures of Toronto Public Health, the number of persons who were exposed to SARS in nonhospital and nonhousehold settings dropped from 20 (13 percent) before the control measures were instituted (phase 1) to 0 afterward (phase 2). The number of patients who were exposed while in a hospital ward rose from 25 (17 percent) in phase 1 to 68 (88 percent) in phase 2, and the number exposed while in the intensive care unit dropped from 13 (9 percent) in phase 1 to 0 in phase 2. Community spread (the length of the chains of transmission outside of hospital settings) was significantly reduced in phase 2 of the outbreak (P<0.001). Conclusions: The transmission of SARS in Toronto was limited primarily to hospitals and to households that had had contact with patients. For every case of SARS, health authorities should expect to quarantine up to 100 contacts of the patients and to investigate 8 possible cases. During an outbreak, active in-hospital surveillance for SARS-like illnesses and heightened infection-control measures are essential.