Contact tracing and physical distancing projected to significantly reduce SARS-CoV-2 infections in Canada

1. A computerized model predicted that 64.6% of Canadians will be infected with SARS-CoV-2 without any interventions.

2. With enhanced case detection and contact tracing, infection rate was predicted to be 0.4%, and 0.2% if sustained physical distancing was additionally achieved. 

3. Keeping infection rate under 0.2% was the only scenario that kept all ICU’s under capacity, prevented all deaths, and eliminated the epidemic. 

Evidence Rating Level: 2 (Good)

Six months have passed since the World Health Organization declared COVID-19 a pandemic. In these past months, Canada’s public health response has involved the following four measures: Detecting and isolating symptomatic cases, tracing contacts and quarantining exposed individuals, physical distancing measures, and closure of community spaces. However, because this level of response is unprecedented in recent times, it is unclear to what extent each of these measures is effective. The current study used a computer simulation to predict which combination of measures would be best for minimizing transmission. An agent-based model was used: Agents can represent a heterogeneous group of people, with each agent possessing unique characteristics. This allows for an element of randomness to influence the events in the simulation. The model extended from May 2020 to January 2022, comparing 4 possible combinations of interventions: Case detection and contact tracing at enhanced levels or baseline levels (from data collected between February 2020 and May 2020), paired with either maintenance of physical distancing or no physical distancing. Each scenario was then combined with community closures to determine their significance. The model predicted that with no interventions, the total attack rate (percentage of Canadians infected) would be 64.6%, with a 95% credible interval (CrI) of 63.9%-65.0%. With no physical distancing combined with baseline levels of case detection and contact tracing, the total attack rate would be 56.1% (95% CrI 0.05-57.1%), but with enhanced levels, it would be 0.4% (95% CrI 0.03%-23.5%). The deaths per 100,000 were 1113 (95% CrI 0-1208) and 739 (95% CrI 0-830) respectively. The best scenario was adding physical distancing to enhanced levels, dropping the attack rate to 0.2% (95% CrI 0.03%-1.7%) and deaths per 100,000 to 4 (95% CrI 0-296). As well, this was the only scenario where hospital and ICU bed capacity were consistently not exceeded, and the epidemic was eliminated in all realizations of the simulation. In terms of community closures, school closures were not seen to be as effective compared to closing workplaces and mixed-age venues, which have higher infection rates. Overall, this study has implications for guiding Canada’s continued response to COVID-19, with greater levels of detection and tracing needed, as well as maintained physical distancing, in order to minimize transmission and eventually eliminate the epidemic.

Click to read the study in CMAJ

Image: PD

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