Fig. 1

Structure of the ST/DT Model for epidemic risk assessment and method for calculating seeding number (SN) and seeding time (ST). a The structure of the ST/DT Model is illustrated by a coordinate plane with ST plotted in days on the x-axis and doubling time (DT) plotted in days on the y-axis. Dashed lines indicate mean ST and mean DT. The four quadrants created are ascribed risk levels—short ST and DT indicate high risk (red), long ST and short DT indicate moderately high risk (orange), short ST and long DT indicate moderately low risk (yellow), and long ST and DT indicate low risk (green). b On a hypothetical epidemic curve the date at which the epidemic appears to “take-off” was determined independently by two authors (blue). The number of cumulative cases on the day before is the SN and the date on which the SN is reached is the ST (pink)