On Aug. 25, 2020, data from the Broad Institute of MIT and Harvard, Massachusetts General Hospital, the Massachusetts Department of Public Health, and the Boston Healthcare for the Homeless Program described how the SARS-CoV-2 virus entered the Boston area, and how certain events shaped the trajectory of the epidemic in the region. These data were described in a manuscript on medRxiv.

Two features of the data are particularly striking. The first is the sheer number of times the virus entered Massachusetts over a period of some three months, from the beginning of February 2020 to early May. We estimate 80 separate introductions, mostly from elsewhere in the US and from Europe, but including sources on four continents. The exact number and sources of the introductions are uncertain (we lack good genetic data about exactly what viral variants were present in many parts of the world), but the introductions were clearly numerous. The rate of new introductions declined during March as travel decreased and various control measures took effect.

The second striking feature is how variable the effect of a single infection can be on the trajectory of an epidemic. The importance of ‘superspreading events’ in COVID-19, in which a single person infects a disproportionately large number of others, has been widely reported. This study, which details two superspreading events in Massachusetts, underscores this, but shows how variable the effect can be. 

One of these superspreading events took place within a skilled nursing facility. All of the residents and most of the staff were tested as a precaution prior to a planned relocation. Ultimately 85 percent of the residents and 37 percent of the staff tested positive. Genomic analysis of these infections showed that even though COVID-19 was not suspected, the virus had entered this community on three separate occasions — but only one of those introductions was responsible for more than 90 percent of the infections. The limited genetic diversity among these cases indicated a very rapid spread in the facility. 

The other superspreading event was associated with an international business conference in February. In that case, there is genetic evidence that a single person brought the virus (which had probably recently arrived from Europe) to the event.