  Digital contact tracing/notification was initially hailed as a promising
strategy to combat SARS-CoV-2, but in most jurisdictions it did not live up to
its promise. To avert a given transmission event, both parties must have
adopted the tech, it must detect the contact, the primary case must be promptly
diagnosed, notifications must be triggered, and the secondary case must change
their behavior to avoid the focal tertiary transmission event. If we
approximate these as independent events, achieving a 26% reduction in R(t)
would require 80% success rates at each of these six points of failure. Here we
review the six failure rates experienced by a variety of digital contact
tracing/notification schemes, including Singapore's TraceTogether, India's
Aarogya Setu, and leading implementations of the Google Apple Exposure
Notification system. This leads to a number of recommendations, e.g. that
tracing/notification apps be multi-functional and integrated with testing,
manual contact tracing, and the gathering of critical scientific data, and that
the narrative be framed in terms of user autonomy rather than user privacy.
