Digital tools are being developed to support contact tracing as part of the global effort to control the spread of Covid-19. These include smartphone apps, Bluetooth-based proximity detection, location tracking, and automatic exposure notification features.

Evidence on the effectiveness of alternative approaches to digital contact tracing is so far limited. We use an age-structured branching process model of the transmission of Covid-19 in different settings to estimate the potential of manual contact tracing and digital tracing systems to help control the epidemic. We investigate the effect of the uptake rate and proportion of contacts recorded by the digital system on key model outputs: the effective reproduction number, the mean outbreak size after 30 days, and the probability of elimination.

We show that effective manual contact tracing can reduce the effective reproduction number from 2.4 to around 1.5. The addition of a digital tracing system with a high uptake rate over 75% could further reduce the effective reproduction number to around 1.1. Fully automated digital tracing without manual contact tracing is predicted to be much less effective. We conclude that, for digital tracing systems to make a significant contribution to the control of Covid-19, they need be designed in close conjunction with public health agencies to support and complement manual contact tracing by trained professionals.

Executive summary

  • To maintain elimination of Covid-19, digital contact tracing systems should be designed to complement manual contact tracing, for example by enhancing coverage or speed of tracing, rather than as a separate or fully automated system.
  • To reduce the effective reproduction number to around 1 requires a combination of rapid testing and case isolation, a well-functioning manual contact tracing system, digital contact tracing with an uptake rate of at least 75% and recording 90% of close contacts, and highly effective quarantine of traced contacts.
  • Ensuring that individuals with Covid-19 symptoms get tested quickly and are able to isolate effectively is just as important as investment in contact tracing.
  • Digital systems based on QR codes with no proximity detection are likely to be less effective as a result of recording fewer contacts.
  • Bluetooth apps and card-based proximity detection systems perform comparably at a given level of coverage, but other factors such as usability, reliability and longevity need to be considered.
  • In the event of a large ongoing outbreak, scalability and false positive rates are more important, but significant population-wide control measures are also likely be required to prevent a major epidemic.
  • Tracing and quarantining second-order contacts of a confirmed case provides a relatively small additional benefit. This could be useful in the very early stages of an outbreak, but for a larger outbreak ensuring fast and effective quarantine of first-order contacts should be a higher priority.