Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics
Fig 4
Variability in the timing and outcome of epidemics due to stochastic variation in individual transmission.
Lines show number of latently infected individuals in the E class over time for 1 year with moderate social distancing that reduces contact rates by 40% (κ = 0.6). Grey lines show runs from a single stochastic simulation and the black line shows the deterministic outcome. The fraction of epidemics that establish is the fraction of simulations where the maximum number of people infected at any time exceeds the starting number infected. The four scenarios shown include different starting numbers of latently infected individuals on day 0, E0 (A, C: 5; B, D: 50), and with (A, B) or without (C, D) contact tracing (CT) which lowered R0 from 1.57 to 1.33. The delay from symptom onset to testing and tracing 1/τIs was 10d. The modeled population of 100,000 people had 15 tracers making 12 calls/day, and each case had an average of 10 contacts which is intermediate between pre-lockdown and lockdown conditions; this scenario is the same as the yellow line in Fig 1.