Quantifying dynamics of SARS-CoV-2 transmission suggests that epidemic control and avoidance is feasible through instantaneous digital contact tracing

Luca Ferretti; Chris Wymant; Michelle Kendall; Lele Zhao; Anel Nurtay; David G Bonsall; Christophe Fraser

In this study, we estimated key parameters of the SARS-CoV-2 epidemic, using an analytically solvable model of the exponential phase of spread and of the impact of interventions. Our estimate of R0 is lower than many previous published estimates, for example (12), (29), (30). These studies assumed SARS-like generation times; however, the emerging evidence for shorter generation times for COVID-19 implies a smaller R0. This means a smaller fraction of transmissions need to be blocked for sustained epidemic suppression (R < 1). However, it does not mean sustained epidemic suppression will be easier to achieve because each individual's transmissions occur in a shorter window of time after their infection, and a greater fraction of them occurs before the warning sign of symptoms. Specifically, our approaches suggest that between a third and a half of transmissions occur from pre-symptomatic individuals. This is in line with estimates of 48% of transmission being presymptomatic in Singapore and 62% in Tianjin, China (31), and 44% in transmission pairs from various countries (32). Our infectiousness model suggests that the total contribution to R0 from presymptomatics is 0.9 (0.2 -1.1), almost enough to sustain an epidemic on its own. For SARS, the corresponding estimate was almost zero (9), immediately telling us that different containment strategies will be needed for COVID-19.