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Effectiveness of isolation, testing, contact tracing, and physical distancing on reducing transmission of SARS-CoV-2 in different settings: a mathematical modelling study

Our take —

Using setting- and disease-specific social contact data from the UK, authors demonstrated that combined test and trace strategies could reduce SARS-CoV-2 transmission more than self-isolation or mass testing alone (60-65% reduction compared to a 2-30% reduction). App-based contact tracing was estimated to be less effective in reducing transmission than manual contact tracing, but these comparative losses may be offset by the redirection of critical human resources to other activities.

Study design

Modeling/Simulation

Study population and setting

Using social contact data from 40,162 UK participants, authors explored the impact of different control measures for SARS-CoV-2 on the reduction in transmission including: self-isolation of cases showing symptoms, household quarantine, manual tracing of all contacts, manual tracing of contacts that have met before (acquaintances), app-based tracing, mass testing regardless of symptoms, limiting the number of daily contacts made outside home, work and school, and having a fraction of the adult population work from home. In addition, authors also estimated the number of primary cases and contacts newly quarantined per day under the different strategies for different levels of COVID-19 incidence.

Summary of Main Findings

Results showed that the Re (the effective reproductive number, the reproductive number after the implementation of interventions and community transmission has occurred) was most effectively reduced through the implementation of multiple control measures at once. For example, compared to self-isolation within the home, which reduced Re by an average of 29%, a combination of self-isolation, household quarantine, manual contact tracing of acquaintances, app-based tracing, and limiting daily contacts to four per person reduced Re by an average of 66%. The addition of app-based tracing resulted in overall relatively smaller reductions to Re compared to other combinations because both the primary case and contact would need to have and use the app. Mass random testing of 5% of the population every week resulted in the smallest reduction in transmission (2%), because through this method only a small number of infections would be detected, and those detected are likely to have already spread the infection to others. Assuming there were 10,000 new symptomatic cases per day, anywhere from 22,000 to 390,000 contacts would be newly quarantined each day under the different contact tracing strategies.

Study Strengths

Authors used setting-specific social contact data from over 40,000 individuals to estimate how singular and combinations of different control measures impact SARS-CoV-2 transmission. In addition, authors evaluated a wide range of different control strategies and included strategies that have been developed to deal with the magnitude of the pandemic, including app-based tracing methods.

Limitations

Authors did not account for different types of interactions between contacts, including the fact that contacts of primary cases might know one another. This would reduce the number of contacts that would need to be traced. Authors also assumed that contacts made outside the home were different each day. Repeated contacts outside the home would also reduce the number of contacts that need to be traced. Finally, authors only considered four types of settings, work, home, school, so findings may not be applicable to other specific settings, including mass gatherings.

Value added

Using data from over 40,000 individuals in the UK, authors compare different combinations of control measures to reduce transmission including contact tracing, self-isolation, and physical distancing. While previous analyses suggest isolation or tracing alone are not sufficient to control outbreaks, this is one of the first studies using setting- and disease-specific social contact data to assess the potential impact of combined control strategies on SARS-CoV-2 transmission.

This review was posted on: 10 September 2020