Fig. 2: Empirical transmission cluster size distribution and cluster category frequency during COVID−19 pandemic in Hong Kong from 23 January 2020 and 15 December 2021.

a Boxplots of cluster size distribution overall and stratified by transmission setting, single index case was counted as size of 1 in each setting. Box showed the inter-quartile range (IQR), bold line within box showed the median, whisker showed upper quartile plus 1.5 times IQR, dots showed the empirical values. The y-axis shows the cluster size, the numbers under the labels of x-axis indicate number of clusters identified in each transmission setting. Number of clusters involved at least 2 cases in each setting sum together equals to 2228 as in all clusters, while number of single index cases in each setting sum together equals to 2298, but number of single index cases in all clusters is 1582, due to single index cases could belong to more than one setting. b Histogram of empirical cluster size distribution in each setting. Colors in green, grey, orange, blue, teal, purple, pink and brown represent all clusters, households, office work, restaurants, manual labour work, retail & leisure, nosocomial, close-social indoor and care homes respectively.