Fig. 4. Manageable influx events that recur periodically can overwhelm the tracing capacity.

For the default capacity scenario, we explore whether periodic influx events can overwhelm the tracing capacity: A ‘manageable" influx that would not overwhelm the tracing capacity on its own (3331 externally acquired infections, 92% of which occur in 7 days) repeats every 1.5 months (a–c) or every 3 months (d–f). In the first case, the system is already unstable after the second event because case numbers remained high after the first influx (b). In the second case, the system remains stable after both the first and second event (e), but it becomes unstable after the third (f).