From the 539 infections that occurred in clusters, all 210 solely imported cases (38.9%) were excluded from subsequent analysis due to uncertainties concerning transmission within each cluster whilst overseas. Within the remaining 53 clusters initiated by a local or imported infection, 245 (245/329; 74.5%) could be linked resulting in 171 unique infector-infectee transmission pairs with 94 unique infectors.

Given a superspreading threshold of 6-8 secondary cases, 2-4 SSEs were directly observed where the source was known. Likelihood analysis based on final cluster sizes increased the estimate of R to 0.75 (95% CI: 0.6 – 0.96) and decreased estimates of k to 0.37 (95% CI: 0.16 – 1.16). Based on these estimates we determined a coefficient of variation of 2.5 and inferred that approximately 20% of SARS-CoV-2 infections are responsible for 80% of all transmission events in Hong Kong.