A retrospective analysis of the start of the COVID-19 epidemic in Canada, January 15– March 12, 2020

We describe the epidemiology of COVID-19 at the start of Canada's epidemic from January 1 to March 12, 2020, before governments at all levels implemented aggressive communitybased public health measures.During this time, 153 laboratory-confirmed cases were reported in Canada.Due to delays inherent in the diagnosis and reporting process, these cases represented a small subset of the 1,360 confirmed cases subsequently reported, whose symptom onset occurred on or before March 12.More than half (57.8%) of these 1,360 cases had a history of international travel or were linked to a case that had travelled, most commonly from countries where few cases had been reported at that time.Community transmission, marked by cases that could not be traced back to another case, was first noted on February 20 and increased steadily thereafter.This descriptive analysis indicates that COVID-19 was spreading internationally and in Canada more broadly than was initially detected by surveillance systems from January to mid-March 2020.To limit the impact of future waves, an expanded surveillance system is now being implemented with multiple data streams to provide a more complete picture of the epidemic, including early signals of cases and clusters.Improved access to laboratory testing and expanded contact tracing are critical elements to detect and isolate cases early, including those with mild symptoms.