This nice piece by @carlzimmer lays out very clearly how our understanding of how #SARSCoV2 spread around the world and established itself in certain places is evolving (with all the necessary caveats). https://www.nytimes.com/2020/05/27/health/coronavirus-spread-united-states.html

Scientists had long argued that the outbreak in Italy likely did not descend from earlier cluster in Bavaria. But it is clearer now that the two first Washington cases (both in Snohomish County) also weren‘t connected, as @trvrb conceded in a great thread https://twitter.com/trvrb/status/1265063921880150016?s=21 https://twitter.com/trvrb/status/1265063921880150016

The larger point here is that, as @AdamJKucharski and others have pointed out, this pattern of introductions that die out is exactly what you would expect of a virus with overdispersed (‚very patchy’) transmission, since most infected people do not infect anyone else.