It likely took just one of the 175 people gathered in February at a Biogen conference at the Boston Marriott Long Wharf hotel to ignite a COVID-19 wildfire. Within a week, attendees began falling ill. More than 99 would ultimately test positive. By then, many of them had hopped aboard planes to head home or even attend other conferences. And the spread only exploded from there.
Between 205,000 and 300,000 COVID-19 cases across the nation and around the world can likely be traced back to the two-day Boston conference, according to Jacob Lemieux, the lead author of a new study published Thursday in the journal Science. The study estimates some 71,540 of the cases with a genetic link to the Biogen conference were discovered in Florida, more than 1,000 miles from the waterfront lobby and banquet rooms in Boston that served as the perfect incubator for an eager virus to multiply.
In all, the study estimates the Biogen conference is responsible for roughly 1.6 percent of all cases in the United States since the start of the pandemic.
But the same study found that not all mass gatherings spell nationwide disaster. Early this spring, a chance facility-wide screening uncovered widespread infection at a nursing home in Wilmington. Eighty-two out of 97 residents and 36 staff members at the home would test positive for the virus despite visitor restrictions and universal masking. Two dozen residents would die within two weeks of testing. Yet the devastation of that outbreak was mostly limited to within the walls of the facility. Few outside cases have been linked to the home, the study found.
Although both the Biogen outbreak and the nursing home cases occurred in the early chapter of the pandemic, when little was known about transmission and prevention, researchers believe the two events serve as divergent examples of what happens when COVID-19 is left to spread unchecked.
The study noted that outbreaks that hit vulnerable nursing home populations are often deadly — but those that hit younger populations are dangerous, too, because young people are more likely to interact with others.
“The implications may be greater, when measured as a cost to society, for super-spreading events that involve younger, healthier and more mobile populations because of the increased risk of subsequent transmission,” the study concludes.
To reach those findings, researchers analyzed the genetic sequences of the virus that caused COVID-19 in thousands of cases, both in Massachusetts and nationwide. Mutations in the genetic code that naturally occur as the virus makes copies of itself act like passport stamps that show where the pathogen has been. In combination with epidemiology, it can generate a level of detail in contact tracing largely unachievable by a web of interviews.
Researchers identified more than 80 distinct SARS-CoV-2 genomes of viruses that infected people in the Boston area in the first five months of the year, but one virus with a unique genetic signature had an outsize impact. That specific signature bore a passport stamp tracing back to the Long Wharf hotel on the last week in February.
In an early conversation with the Globe before the study was peer-reviewed, the authors estimated the Biogen conference may have been the root of as many as 20,000 cases in the Boston area, including a large cluster within a city homeless shelter. But they have since revised that number upward to include nationwide numbers after acquiring more data that reflects the transient nature of the attendees.
The Biogen conference was held Feb. 26-27. At its end, many attendees traveled overseas. Others boarded planes to Indiana and Tennessee. By November, the genetic signature nearly synonymous with the Biogen conference could be found in COVID-19 cases as far away as Australia, Slovakia, and Sweden, as well as 29 different states.
“The COVID-19 pandemic had a very direct and personal impact on the Biogen community — as it has on many communities across the country and world,” Biogen said in a statement Thursday. “Tremendous progress has been made since the start of the pandemic to gain a better understanding of this novel virus and its transmission, develop vaccines, and investigate potential treatment options. As a company rooted in science, we understand the value of the data that came from the first wave of the pandemic in the Boston area and we hope that information gleaned from these data will help continue to drive a better understanding of the transmission of this virus and efforts to address it.”
A month or so after the Biogen outbreak, within the H-shaped building of AdviniaCare at Wilmington, a single virus signature appears to have swept through most of the residents and staff from late March to early April. Like other outbreaks reported in nursing facilities, the mortality rate was tragically high. Among the deaths was 73-year-old Janice Cox Pratillo, whose daughter watched her take her last breath while peering through a parking lot window, as described in the Globe’s “A home to die in” Spotlight report.
Both super-spreading events show two different sides of the virus, according to researchers. The first is its cunning ability to jump from host to host with breakneck spread and with little detection. The second is its cruel tendency to prey upon society’s most vulnerable populations. Researchers believe both narratives are important to keep in mind as society enters what could be the final chapter of the pandemic.
Long-term-care residents, like those at AdviniaCare, will likely be vaccinated this winter, according to the timeline introduced by the Baker administration Wednesday. But those who might typically attend international conferences and the happy hours that accompany them may not receive their first dose for several months, enabling the virus to continue its spread.
“If there is a public health message here, it is that the conditions that enable these types of massive super-spreading events to occur are still with us,” said Lemieux, an infectious disease physician at Massachusetts General Hospital and one of two scientists at the Broad Institute of MIT and Harvard who spoke about the study they authored with 50 other researchers.
“They’re still possible if we let our guard down. They’re still possible if infected but otherwise healthy people mingle and travel without restriction.”