Fig. 1: Forward-only and bidirectional contact tracing and digital exposure notification.
From: Bidirectional contact tracing could dramatically improve COVID-19 control
a Notifying people exposed to known cases (black) and isolating them (green) can prevent further transmission, but will miss asymptomatic and undiagnosed cases (gray) and descendants. b Bidirectional tracing also notifies and tests potential infectors, enabling isolation of additional cases. c Manual contact tracing requires individuals to share recent contacts with health authorities. d In digital exposure notification, smartphones broadcast rotating pseudorandom “chirps” and record those emitted by nearby devices48. e Individuals diagnosed with COVID-19 can “opt-in” by uploading broadcasted chirps to a diagnosis server48. All devices frequently check the server and alert the user if the calculated exposure exceeds a threshold set by the local health authority. In hybrid manual+digital systems, human tracers would seek to identify contacts without smartphones.
