This study represents the best estimate so far of COVID-19 transmissibility. Viral shedding was measured in 94 patients with confirmed COVID-19. Modelling estimates that 44% of transmission to secondary cases occurred in the 2 to 3 days prior to symptom onset, with most of the rest of transmission occurring up to 8 days after symptoms onset. The definition of contacts must be extended to 3 days prior to symptom onset for contact tracing and isolation (containment) to work.