ATTACKS AND DEFENSES
EPIDEMIOLOGY & PUBLIC HEALTH: CASE
STUDY 8
Preventing the Spread of Disease: Outbreak Investigations
Epidemiology Topic: Epidemic Curves and Attack Rates
Through international air travel, severe acute respiratory syndrome (SARS) rapidly
spread from its origins in Guangdong Province, China, in November 2002 to many parts of the
world, including to Canada, in February 2003. This syndrome, which is characterized by fever,
headache, myalgia, cough and shortness of breath, can lead to pneumonia and sometimes to
acute respiratory distress syndrome and death. There is strong evidence that a novel corona virus,
spread primarily through respiratory droplets, causes SARS.
The index case of SARS in Canada was an elderly woman who returned to Toronto on
Feb. 23, 2003, after a visit to Hong Kong. She had been exposed to SARS during her stay at a
hotel in Kowloon, where a cluster of 13 cases of SARS was subsequently reported. She became
ill upon her return to Toronto, and the infection spread to family members, one of whom was
later admitted to the emergency department of a local hospital where a large nosocomial
outbreak occurred. A peer-reviewed report describes the results of the investigation of the initial
128 cases of SARS associated with this outbreak, including those who became infected in the
hospital, as well as the secondary generation of cases among their contacts.1

Assuming the public health investigation followed a standard approach to an
outbreak investigation, what was the order of activities and what were the
likely difficulties at each step?
o Establish dx (check hospital records)
o Establish epidemiologic case definition: sx’s + SARS PCR (+), etc
o Determine whether an epidemic is occurring. Epidemic threshold is 0 for
SARS in Canada
 Take appropriate precautions while we try to figure out
transmission
o Make sure you follow up w/ cases once the public hype dies down
Consider the following scenario as part of the outbreak: Case A presented to the
emergency room on March 7, 2003, with severe respiratory symptoms. Four nurses
worked for 5 hours to stabilize the patient before he was transferred to the ICU. Over the
1
Varia M., Wilson S, Sarwal S, McGeer A, Gournis E., et al. Investigation of a nosocomial outbreak of
severe acute respiratory syndrome (SARS) in Toronto, Canada. CMAJ. 2003;169(4):285-92.