Fig. 2. Distribution of phylogenetically inferred times of importation events for the 57 sites included in our analysis, also referred to as seeding time.

Ancestral states are reconstructed onto the phylogeny as exogenous or within the region of interest. The midpoint between an ancestral node which is exogenous to the region of interest and a node which is within the region of interest is the time of importation. Local epidemics can be seeded by many importation events. Central box-plot lines represent the central epidemic seeding time (CEST). Note that the CEST does not represent the earliest viral introduction but rather the mean time of viral introduction weighted by the number of samples descended from each viral introduction. Boxes represent the interquartile range of the distribution and whiskers indicate the 25th and 75th percentiles of the distributions, derived from n = 2500 data points (100 time-resolved phylogenies for each location, with ancestral states reconstructed 25 times on each). Ranges reflect the distribution of importation dates, rather than uncertainty around an estimate of seeding times. Sites from the same country or US state are grouped together and sites are ordered based on the first CEST for that country or US state. Width serves only to compress multiple sites from the same country or US state.