Covid-19 could already be circulating in the US, according to similar genetic sequences in viruses from two people who contracted the disease several weeks apart. One comes from a recent case in Snohomish County, Washington, in which the infected person had no contact with another case or outbreak location. Despite this, the sequence of the virus in their sample closely matches that of the first infection detected in the US, on 21 January, in a man in the same county who had visited Wuhan.

Both sequences, which were posted to …