The role of case importation in explaining differences in early SARS-CoV-2 transmission dynamics in Canada - a mathematical modeling study of surveillance data

Godin, Arnaud; Xia, Yiqing; Buckeridge, David L.; Mishra, Sharmistha; Douwes-Schultz, Dirk; Shen, Yannan; Lavigne, Maxime; Drolet, Mélanie; Schmidt, Alexandra M.; Brisson, Marc; Maheu-Giroux, Mathieu

OBJECTIVE: North American COVID-19 epidemics exhibited distinct early trajectories. In Canada, Quebec had the highest COVID-19 burden and its earlier March school break, taking place two weeks before those of the rest of the country, could have shaped early transmission dynamics. METHODS: We combined a semi-mechanistic model of SARS-CoV-2 transmission with detailed surveillance data from Quebec and Ontario (accounting for 85% of Canadian cases). We then explored the impact of case importation and timing of control measures on cumulative hospitalization. RESULTS: A total of 1,544 and 1,150 cases among returning travelers were laboratory-confirmed in Quebec and Ontario, respectively (symptoms onset ≤2020-03-25). Hospitalizations could have been reduced by 55% (95%CrI: 51-59%) had no cases been imported after Quebec’s March break. However, had Quebec experienced Ontario’s number of introductions, hospitalizations would have only been reduced by 12% (95%CrI: 8-16%). Early public health measures mitigated epidemic spread as delaying them by one week could have resulted in twice as many hospitalizations (95%CrI: 1.7-2.1). CONCLUSION: Beyond introductions, factors such as public health preparedness, responses, and capacity could play a role in explaining interprovincial differences. In a context where regions are considering lifting travel restrictions, coordinated strategies and proactive measures are to be considered.