COVID-19 contact-tracing smartphone application usage-The New Zealand COVID Tracer experience.

Contact tracing has been a central pillar of the nonpharmaceutical health system response to the COVID-19 pandemic. Countries around the world have devoted unprecedented levels of resources to build up their testing and tracing capabilities, including the development and deployment of smartphone-based applications. Yet despite these nontrivial investments, the body of academic literature evaluating the effects of the smartphone-based applications remains scant and many apps have not delivered the promised benefits (Bano et al. 2021). We contribute to this body of empirical evidence by analysing data on uptake and usage of New Zealand's QR code-based application New Zealand COVID Tracer (NZCT). Our paper uses descriptive statistics and regression analysis to focus on the likelihood of an individual registering to use the application, the likelihood that a registered user will use the application to scan QR codes, and the extent of that usage, measured as the number of codes scanned daily. In addition, we take advantage of the "natural experiment" offered by the decision in August 2021 following the establishment of endemic community transmission of COVID-19 to make use of the application mandatory from 7 September 2021 to assess the effect of this policy on usage. We find that despite increasing numbers of individuals downloading the application over time, the number of active devices and scanning activity was very low, even when community outbreaks occurred (i.e. actual infection risk was higher). Mandating use (separate and distinct from increased infection risk) led to an increase in the number of scans in total via an increase in the number of active devices only.