Table S3.

Parameter estimates for the baseline analysis where the daily number of introductions follows a Poisson distribution and for an alternative model where it follows a negative-binomial distribution with high overdispersion (k = 0.2)

 Variable Summary statistic Poisson (baseline) Negative binomial Serial interval Mean 6.8 (6.0, 7.8) 6.8 (6.0, 7.7) SD 4.1 (3.4, 5.0) 4.1 (3.4, 5.0) Reproduction numbers Within cluster Initial mean RC 0.45 (0.33, 0.58) 0.44 (0.32, 0.58) Initial SD σC 0.53 (0.35, 0.78) 0.52 (0.35, 0.78) Probability it starts >1 12% (6%, 18%) 12% (6%, 19%) Value after 10 cases (relative to starting value) 47% (34%, 63%) 46% (34%, 64%) Within region RR 0.24 (0.19, 0.29) 0.24 (0.19, 0.30) Between region RO 0.05 (0.02, 0.09) 0.06 (0.02, 0.10) Risk of introduction from reservoir Nb at start of study period (weekly) 0.5 (0.2, 0.8) 0.5 (0.2, 1.0) Nb at end of study period (weekly) 2.1 (1.0, 3.6) 1.6 (0.7, 3.3) Exponential growth rate α (daily) 0.003 (0.001, 0.005) 0.002 (0.000, 0.004) Source of infection, % Within cluster 60% (57%, 63%) 60% (57%, 63%) Within region 23% (20%, 27%) 24% (20%, 28%) Between region 5% (2%, 8%) 6% (3%, 9%) Reservoir 12% (9%, 15%) 10% (7%, 13%)