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Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts

Our take —

This study demonstrates that the effectiveness of contact tracing may be limited, even in settings with very few cases. The model assumed that all cases and their contacts were successfully isolated and that there was minimal pre- and asymptomatic transmission, which we now know not to be the case. Thus, results should be interpreted as optimistic with respect to the potential effectiveness of contact tracing. Contact tracing in combination with other interventions may lower transmission rates, but only if it can be done quickly and cases and their contacts are effectively isolated.

Study design

Modeling/Simulation

Study population and setting

This modeling study simulated COVID-19 transmission from infected persons to their contacts and the potential impact of contact tracing in emerging epidemics (i.e., 40 cases or less in the population at the start of the outbreak) under a wide variety of scenarios, including varying levels of asymptomatic and pre-symptomatic transmission, reproductive numbers (range from 1.5-3.5), and contact tracing success levels. While the proportion of infected cases who were diagnosed varied across different scenarios, all individuals who tested positive were assumed to isolate and not transmit to any additional persons (i.e., 100% effectiveness of case isolation). Additionally, all traced contacts were assumed to immediately self-isolate and not infect any additional persons. Controlled outbreaks were defined as outbreaks with a final size of less than 5,000 cases.

Summary of Main Findings

Contact tracing was more likely to control a COVID-19 outbreak if: i)the number of cases to begin with was small (<20), ii)there were few pre- and asymptomatic cases as a proportion of all cases, and iii)there were shorter delays between symptom onset and case isolation. In moderate and high-transmission scenarios (Reproductive number, R0, equal to or greater than 2.5), the proportion of all contacts that need to be traced to control an outbreak exceeded 80%. Furthermore, even if we optimistically assume between five and 20 initial cases, the results show that large numbers of contacts (25-100 per week) would need to be identified and traced by public health departments in most outbreak scenarios.

Study Strengths

This study simulated the potential effectiveness of contact tracing of COVID-19 symptomatic cases under a wide variety of epidemic scenarios, demonstrating that contact tracing will be difficult even in settings with small numbers of cases. The authors clearly lay out all of the steps in the contact tracing process, and use the model to illustrate how gaps in each step might impact effectiveness.

Limitations

The study makes several assumptions that are unrealistic, including no onward transmission once cases are isolated. This model only applies to settings with very few cases (<40) and was published prior to learning that there is substantial pre-symptomatic transmission; thus, the results most likely overestimate the potential effectiveness of contact tracing to control COVID-19 spread.

Value added

This modeling study was among the first to show that contact tracing of COVID-19 symptomatic cases will have limited effectiveness if not done rapidly and in conjunction with other interventions.