Since we can genetically sequence stuff, we can determine its lineage and age from the number of mutations. This shows that between the first and second cases in Washington state, there were likely 4 jumps between people, which means almost a month ago:
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Knowing that transmission was initiated on Jan 15 allows us to estimate the total number of infections that exist in this cluster today. Our preliminary analysis puts this at 570 with an 90% uncertainty interval of between 80 and 1500 infections.
