The state Department of Public Health released new details last week about the sources of COVID-19 clusters in Massachusetts, using data from the state's contact tracing efforts. But in some ways, the data raise more questions than answers.
While Gov. Charlie Baker and other public officials have repeatedly said the biggest risk of infection was coming from social gatherings, particularly among young people, the data show that the vast majority of COVID-19 clusters were traced back to transmission within households.
A household "cluster" is defined as two or more people living at the same address who were infected within four weeks of each other. The state's weekly COVID-19 report, released Thursday, stated that there have been 3,854 such household clusters in the state, leading to 7,428 confirmed cases.
By comparison, contact tracing found 33 clusters that could be traced back to social gatherings, leading to 79 confirmed cases. Those clusters led to testing of another 96 "close contacts," who were found not to be infected.
Long-term care facilities were the second most common setting for clusters, with 141 clusters leading to 811 cases. Fifty-five child care clusters led to 78 coronavirus infections.
Epidemiologists and infectious disease experts say the rate of spread within households is unsurprising.
"It's an issue of proximity and duration," said Dr. Erica Shenoy, associate chief of the Infection Control Unit at Massachusetts General Hospital. "We spend a lot of time in our home together. And if there are more of us in close quarters, it provides more opportunity for the infection to spread."
That is a factor in the higher rates seen in many of the state's lower income communities, said Dr. Paul Biddinger, chief of the Division of Emergency Preparedness at MGH, and director of the MGH Center for Disaster Medicine. "I think [it] underscores some of the disparities that we see in transmission, where communities with high housing density, high number of people in any given household, especially multi-generational, remains a significant risk," Biddinger said.
People let their guard down at home, said Tory Mazzola, a spokesperson for the state COVID-19 Command Center, in a statement. "It's critical that residents are aware of this and -- especially those living in multi-generational homes or with family members who have underlying conditions -- take precautions even in their home, such as wearing a mask, washing hands and not sharing utensils, as a few examples."
But household clusters don't happen in isolation. Someone still has to get infected somewhere else and bring the virus into a home. The new data doesn't shed much light on where those infections are happening. The cases the state was able to trace to clusters account for 54 percent of the more than 150,000 confirmed cases the state has seen since the beginning of the pandemic.
"The case investigations and the tracing that we're doing is showing us that there is not one reason. There's not these super-spreader events that you see in other places," Boston Chief of Health and Human Services Marty Martinez said in a press call Friday.
"There's a lot of small little scenarios that are happening that is really increasing the spread, especially in different neighborhoods and different communities," he said.
And that, Martinez said, underscores the need for people to wear masks and keep their distance when out in public.
"Massachusetts, I think, in general, has done a pretty good job at social distancing and mask wearing and things like that," Dr. Michael Mina of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health said in a press call Friday. "Of course, we've seen even in Massachusetts that people are getting tired. People are going back to work. People are going back to school. And so community transmission is increasing again. And that will cause exponential increase through the households."
And people infected in those households, he added, then go back out into the community and infect others.
More than a third of the state's communities are now in the so-called "red zone," meaning over the last two weeks, they've reported more than eight cases per day on average, per 100,00 residents. That's almost double the number of cities and towns in the red zone two weeks ago. The state as a whole is in the red zone for the third week in a row. Hospitalizations and deaths have been climbing steadily since early September, though not as sharply as the daily case counts. But that could change if the rising number of cases translates into more severely sick people.
With the data from the state's contact tracing efforts showing that household transmission was the single biggest risk for clusters, the upcoming holidays could result in an additional spike in cases, as visiting family members and friends essentially increase the size of households overnight.
"We are all looking for connections, we all miss our families. We all want to have some joy at the holiday time and I very, very much appreciate that," said Biddinger. "But I think hopefully people will try to find ways to have joy and to connect with their families in a way that doesn't significantly increase transmission. Because if we disregard the virus and just gather together without precautions, what happens in December and January could be very concerning."
GBH News' Mark Herz contributed to this report.
Correction: This story has been updated to correct the percentage of total infections the state has been able to trace to clusters.