Transmission model parameters
| Variable | Units | Default value | Range examined in sensitivity analyses (uniform distribution) | Reference and notes |
|---|---|---|---|---|
| Epidemiological | ||||
| Population size of the GTA | Number | 6 196 731 | NA | Projected estimate from 2016 census (7), (8) and a 1% annual change as per the United Nations Urbanization Prospects, (9) and using the census metropolitan area of Toronto. (10) |
| R0 | Number | 2.4 | 1.4–3.0 | Range of estimates from modelling studies of outbreaks within and outside China, and on the Diamond Princess cruise ship. (20)– (24) The lower bound was based on the lower bound estimate of R0 from the World Health Organization report of outbreaks in China. (25) Systematic review and meta-analysis of studies of R0 suggest that R0 estimates have stablized in the range of 2–3 in more recent studies. (26) Our default estimate of 2.4 was consistent with the assumption used in other modelling studies. (27) |
| Incubation period | Days | 5.2 | 3–9 | Pooled analysis of 181 confirmed cases with identifiable exposure and symptom onset estimated a median incubation of 5.2 days. (28) We further extracted point (mean or median) estimates of incubation period from a list identified of studies in China and Singapore to inform the range estimates. (20), (29)– (38) |
| Duration of latent infection | Days | 2 | 1–3 | Assumption based on the relatively short incubation period (5.2 d) and serial interval (4.4 d) of COVID-19; other models have used a latent period of 3 days. (39) |
| Duration of subclinical infectiousness | Days | 3 | 2–6 | Calculated as the difference between incubation period and latent period (Appendix 1, available at www.cmajopen.ca/content/8/3/E593/suppl/DC1). |
| Duration of symptomatic infectiousness | Days | 7 | 5–10 | Based on duration of upper respiratory tract viral shedding among individuals with symptoms. (40) |
| Serial interval | Days | NA | 3.1–7.5 | No default estimate was used, as serial interval was not used as an input parameter; only the range estimates were used for internal parameter validation (detailed in the Methods section). (20), (38), (41), (42) |
| Initial seeding | % of total population | 0.0032 | 0.0011–0.0048 | Assumption on range based on detecting 3 travel-related cases by Feb. 23, 2020, at a time when testing criteria was limited to travel to China or contact with a person already diagnosed with COVID-19, (43) and likelihood of detection of imported cases between 11% and 40%. (44) We assumed seeding includes imported cases from outside Canada, between provinces and local transmission that had remained undetected. |
| Clinical | ||||
| Proportion diagnosed with COVID-19 who required admission to hospital | % | 10 | 6–20 | As of Mar. 23, 10% of confirmed cases in Canada were admitted to hospital. (45) Data on 55 924 confirmed cases in China suggested that 19.9% of confirmed cases were severe, including 6.1% in critical conditions. (34) We therefore assumed that a range from 6% to 20% of detected cases would require admission to hospital in the GTA. Subsequently, Toronto Public Health reported 18 (6.4%) cases admitted to hospital out of 280 confirmed cases of COVID-19 as of Mar. 24. (46) |
| Proportion infected with SARS-CoV-2 who were diagnosed | % | NA | 41–69 | Proportion infected who were diagnosed was not directly used as an input parameter, but was used indirectly to calculate the proportion infected who required admission to hospital (detailed below). Analyses on data from China as well as on Japanese citizens returning on repatriation flights showed that 31%–59% of infected cases may not be detected because of asymptomatic infections or mild symptoms. (47)– (49) We therefore assumed a default estimate of 55% (midpoint of the range) for proportion of infected cases that were detected. |
| Proportion infected with SARS-CoV-2 who required admission to hospital | % | 5.5 | 2.4–14 | We calculated the proportion of infected individuals who require admission to hospital using the proportion of detected cases that require admission to hospital, and multiplied by the proportion of infected cases that may be detected. |
| Proportion admitted to hospital who require ICU care | % | 33 | 30–52 | As of Mar. 25, 33% of cases admitted to hospital in the Toronto Public Health Unit required ICU admission. (46) Similarly, as of Mar. 23, 40% of cases admitted to hospital in Canada required ICU care. (45) Based on data of 55 924 confirmed cases in China, cases with critical conditions, and thus those that may require ICU care, comprise 30% of confirmed cases with severe or critical conditions. (34) Of 1590 patients admitted to hospital across 575 hospitals in China, 254 had severe conditions, and 52% of these required ICU care or invasive ventilation. (30) We did not estimate the proportion of ICU patients among all patients admitted to hospital in China as many patients were admitted for isolation only rather than because of disease severity in the settings of China. |
| Duration of hospital stay | Days | 12 | 10–13 | Among 1032 patients admitted to hospital, who did not require ICU care across 552 hospitals in China, their median length of hospital stay at the end of study follow-up was 12 (IQR 10–13) days. (31) This estimate was consistent with the estimates on length of hospital stay among discharged patients with COVID-19 (regardless of ICU stay) in China and Europe. (29), (31), (32), (50)– (52) |
| Duration of ICU stay | Days | 8 | 5–13 | There are limited data on the length of ICU stay before transfer to the medicine ward for post-ICU recovery. Of 23 ICU patients in Wuhan, who were discharged to the medicine ward from the ICU, their median length of stay in ICU was 8 (IQR 5–13) days. (53) |
| Case-fatality proportion among those in ICU care | % | 38 | 17–62 | Of 1590 patients admitted to hospital across 575 hospitals in China, 131 patients required ICU care or invasive ventilation, and 50 (38%) of these patients died. (30) We also extracted estimates from several studies in China and in Europe regarding the crude mortality among ICU patients, which ranged from 17% to 62%. (31), (34), (51), (52), (54)– (57) |
| Case-fatality proportion among those diagnosed | % | NA | 0.8–4.24 | No default estimate was used, as case-fatality proportion among those diagnosed was not used as an input parameter; only the range estimates were used for internal parameter validation (detailed in the Methods section). Our estimates of the case-fatality proportion among those diagnosed were informed by a range of evidence as shown below, accounting for the uncertainty and heterogeneity in estimates by geographic location and age. As of Mar. 23, 2020, cases were reported in Canada with 23 deaths, indicating a crude case fatality of 1.1%. (45) Using crude age-specific case-fatality among all confirmed cases in China, (58) and adjusted for the age distribution of confirmed cases in Canada as of Mar. 23, (45) we obtained an overall crude case fatality of 2.5% in Canada. Estimates of case-fatality rate among confirmed cases after adjusting for time lag to death ranged from 0.8% in China excluding Hubei province, 3.48% in China overall and 4.24% in other countries and regions. (59) Analyses using data of cases on Diamond Princess cruise ship estimated an infection fatality rate of 0.5% and case fatality rate of 1.1% after adjusting for time lag to death, and standardizing the age to approximate the age distribution among confirmed cases in China. (60) |
Note: COVID-19 = coronavirus disease 2019, GTA = Greater Toronto Area, ICU = intensive care unit, IQR = interquartile range, NA = not applicable, SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2.