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Imported COVID-19 cases could overwhelm contact tracing system

Given the burden of tracing a large number of contacts to find new cases of COVID-19, there is a potential for the UK contact tracing system to be overwhelmed if imports of infection occur at a rapid rate, say experts from the University of Warwick, University of Oxford, Lancaster University and University of Liverpool.

In the Journal of Epidemiology and Community Health, the team presents the findings of detailed survey on social encounters carried out among over 5800 respondents and coupled with predictive models of contact tracing and control to investigate the likely efficacy of contact tracing and the distribution of secondary cases that may go untraced.

Based on recent estimates for COVID-19 transmission, the study predicts that under effective contact tracing, less than one in six cases will generate any subsequent untraced infections, but with an average of 36 individuals traced per case.

Changes to the definition of a close contact can reduce this burden, but with increased risk of untraced cases. Tracing using a contact definition requiring more than four hours of contact is unlikely to control spread.

The authors concluded that current contact tracing strategy within the UK is likely to identify a sufficient proportion of infected individuals and could prevent spread of the virus. But the success of the system will depend on the rapid detection of cases and isolation of contacts. They say, given the burden of tracing a large number of contacts to find new cases, there is potential for the system to get overwhelmed if the number of imported cases increases rapidly.


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