The largest local cluster was 106 cases and was traced back to multiple social exposures among a collection of bars across Hong Kong (Figure 3A). Of the 73 primary bar cases, 39 customers, 20 staff, and 14 musicians were infected, while the remaining 33 infections were secondary, tertiary or quaternary contacts. This single outbreak accounts for 10.2% (106/1,038) of all cases in Hong Kong regardless of the source, but 30.4% of all local cases acquired in Hong Kong (106/349).

Transmission within families was the most frequent event among all recognized transmission pairs (96/171; 56.1%) followed by transmission in social settings (55/171; 32.2%) and at work (20/171; 11.1%). However, there was very strong evidence that the number of individual secondary cases resulting from social exposures was significantly higher compared to work or family exposures (μ = 2.89 vs. 1.36; p < 0.001, negative binomial regression model).