Losing the Trace?

Concerns have been raised about the 'fragility' of the NHS contact tracing app that is currently being trialled on the Isle of Wight (https://www.theguardian.com/world/2020/may/06/critical-mass-of-android-users-needed-for-success-of-nhs-coronavirus-contact-tracing-app). In essence, the app is 'centralised', meaning that it sends data about interactions back to the Health Service to support modelling. Such apps cannot use the 'decentralised' tools developed by Apple and Google for contact tracing (there are privacy issues). This means that a critical mass of Android users will be required to ensure that any iPhone user remains covered by the app's tracing ability. Otherwise, the iPhone simply stops broadcasting the signal needed for the app to operate (it can be switched back on but people would have to remember to do this). Apps developed in Australia and Singapore had similar  issues.