Different countries have adopted different approaches to
contact tracing. In some, GPS location tracking is used to establish when
people were collocated. In others, QR code solutions, that record when people
check in and out of buildings, can identify who was in the same building at the
same time, but not whether they were ever in close physical proximity. Both
these approaches may compromise people’s privacy as they depend on centrally
storing data that identifies the phone, location, and time. Their technical
effectiveness relies on the precision and accuracy of the data and the
reliability of the matching algorithm that identifies collocated people.
A third approach, facilitated by Apple and Google, is decentralised. Put simply, this relies on Bluetooth connections between devices: devices exchange IDs (and potentially distance, inferred from signal strength). If the owner of one of the devices subsequently reports a positive test result then notification is sent to all phones for which the IDs have been stored within the past 14 days. This avoids recording location and is better for respecting individuals’ privacy; it also limits the utility of data for other purposes such as public health research.
The practical effectiveness of any solution depends on how many people adopt it (60% of the population is a widely used figure) and on people being collocated with their phones; some countries achieve this through regulatory means (e.g., by controlling people’s right to travel); others rely on “public spiritedness”.
All approaches rely on the accuracy and timeliness of reporting test results, and on the quality of app programming. Another potential limitation of contact tracing apps is that while different regions adopt different solutions, each only applies locally, and may be undermined by international travel.
In summary, there are many obstacles to implementing an effective contact tracing app that respects individual rights as enshrined in the laws of many countries. A well designed and deployed app may reduce infection rates but no app will eliminate Covid-19 without other, complementary, interventions.