On Jun. 4, 2020, data from the Broad Institute of MIT and Harvard, Massachusetts General Hospital, and the Massachusetts Department of Public Health provided insights into the entry of SARS-CoV-2 virus in the Boston area and epidemiological events that shaped the trajectory of the epidemic in the region.

The first confirmed case of COVID-19 in Massachusetts, a traveler returning from Wuhan, China, was reported in early February. In the weeks that followed, no new cases were detected. Instead, the local narrative about the virus’s early spread in the greater Boston area — among the worst hit regions in the country — focused on an international professional conference in Boston in late February.

The researchers released the first 331 complete genomes from this ongoing effort. These genomes represent a dense sample from MGH (a large tertiary referral hospital), during the first month of the outbreak, and help tell the story of the introduction and early spread of SARS-CoV-2 in the Boston area and beyond.

The data suggest that the virus from the first diagnosed case in the state (sequenced by the CDC) was indeed contained effectively, leading to no subsequent cases. But in the weeks that followed, the virus entered the region many more times — we estimate at least 30 unique introductions — in many cases leading to onward community transmission.

No single event or importation alone was responsible for the ongoing spread of COVID-19 in the Boston area; rather, there were multiple entries within a few weeks, from both domestic and international sources.