Approximately 20% of cases were responsible for 80% of all SARS-CoV-2 transmission in Hong Kong, but these results should be interpreted in the context of constrained community transmission given moderate levels of physical distancing currently practiced in Hong Kong, including school closures, some adults working at home, cancellation of mass gatherings, as well as improved hygiene and universal mask wearing. Such degrees of overdispersion can be advantageous to disease control efforts if interventions can effectively target the core high-risk group responsible for the majority of transmission.

Previous modelling has suggested that reduced delays between symptom onset and isolation are important indicators for the control of SARS-CoV-2 outbreaks (20), however this did not appear to play a major role in Hong Kong: in our analysis, delay from symptom onset to isolation were not associated with the number of individual secondary cases, nor the final cluster size (Figure 4A and 4B).