As the number of new coronavirus cases in Massachusetts climbed past 1,000 for the second day in a row Sunday, the state acknowledged it has not been able to determine the source of infection in about half of COVID-19 cases, an information gap that epidemiologists say could limit the ability to respond to outbreaks and control transmission of the disease.
“For a disease like COVID, where superspreading is so important [to] prevent, not identifying the sources of infection means we’re risking not identifying superspreading events fast enough,” said Dr. Sam Scarpino, an epidemiologist at Northeastern University, in an e-mail to the Globe Sunday.
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The state reported 24 new confirmed deaths Sunday, bringing the death toll to 9,640, according to the Department of Public Health. The agency also reported 1,097 new confirmed cases of COVID-19, increasing the total number of cases to 147,120.
Governor Charlie Baker, earlier this month, told reporters he expected an increase in cases this fall, after the state reduced the rate of infections during the summer. Now there are signs that the state has returned to numbers not seen since late May, increasing positivity rates, and an increase in the average number of hospitalized COVID-19 patients.
Some experts have warned about the widespread economic impact of a second lockdown, favoring targeted efforts intended to shut down pockets of infections and prevent spread of the disease.
“The key to controlling epidemics like this is knowing where the spread is occurring, and then in a very targeted way, go in and your take steps to reduce spread in those places,” said Dr. Philip J. Landrigan, who directs Boston College’s Program for Global Public Health and the Common Good. “You don’t shut down the whole society if you can avoid it.”
But that effort requires precise information, including where and when infections occur, and how far the disease has spread. If the state doesn’t have that information, “it’s going to get in the way” of any specific targeted effort against the virus, Landrigan said in a phone interview Sunday.
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Earlier this month, state Health and Human Services Secretary Marylou Sudders, told lawmakers in a legislative call that about half of the state’s cases were due to spread within households and in families, as well as in other clusters.
The remaining half of the state’s cases, Sudders said, were due to “community/unknown transmission.” According to the US Centers for Disease Control, community spread of an illness means the source of infection is not known.
Of the known cases, approximately 30 to 37 percent of cases in Massachusetts were from households and families, according to Sudders’s talking points for the legislative call. The remaining 15 percent to 20 percent of cases were due to cluster-driven transmission, like restaurants, schools, and private events.
On Sunday, a state COVID-19 Command Center spokesman told the Globe that the “50 percent other category are cases not attributed to a defined cluster.” Those cases, state officials said, are due to people letting their guard down despite calls to follow rules like social distancing and wearing masks.
Dr. David Hamer, a physician at Boston Medical Center and a Boston University epidemiologist, said it can be difficult to identify all sources of COVID-19 infections as it’s highly contagious and someone can become infected from a relatively short contact.
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“It makes it hard for contact tracers to try and piece together where [people] have been, and who they’ve been in contact with,” Hamer said. “It’s like a big puzzle, and they may not have all the pieces to develop a complete picture.”
But it will be more difficult for the state to respond, if officials haven’t identified sources in so many as half of its cases, he said in a phone interview.
“It’s going to make it hard to target specific kinds of events, or gatherings, or places, to increase the intensity of prevention measures there,” Hamer said. “It may require, therefore, a broader approach that could include, unfortunately, shutting down certain businesses, a shelter-at-home [measure] again . . . the state is in a tough position with this one."
Landrigan said he hopes the state will be able to unearth more information on those cases from contact tracing. “They can [then] take very specific targeted action that minimizes the economic impact,” he said.
Scarpino said in a phone interview that if the state can’t define sources beyond community transmission in many cases, it will limit options on responding to the pandemic.
“What happens is what you see now — the case numbers begin to climb, and we are forced to decide between a much broader-scale lockdown or the potential return of a surge,” Scarpino said. “Me, personally, I’d rather have the data to make an informed decision around targeted measures that will have less of an effect in terms of our day-to-day lives now, and have a much bigger effect on the course of the epidemic.”
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The state must be aggressive, he said, in stepping up contact tracing, particularly with the growing signs the virus is spreading.
“Certainly, with hospitalizations up . . . with case numbers per day in the thousands, that to me is not evidence of a contact tracing case investigation system that is working,” Scarpino said.
Over the weekend, local officials reported new outbreaks.
In Marblehead, school officials announced Sunday that the high school will cease in-person classes and return to remote learning until Nov. 6 after police found teenagers sharing drinks and socializing without masks at a house party Friday.
Most of the partygoers scattered when police arrived and were not identified, Superintendent John Buckey wrote in a letter posted on the school district website.
“In choosing to ignore the rules set down by the Governor and our community in the pandemic, however, we are not just endangering individuals,” he wrote. “[W]e are also potentially harming the community at large.”
Stephen D. Curry, Fitchburg’s director of public health, said Sunday that 28 confirmed coronavirus cases stemmed from services at Crossroads Community Church.
“We are contract tracing all of the cases, encouraging anyone associated with the church to be tested and working with all the businesses and entities that have been affected by the outbreak,” Curry said in an e-mail to the Globe.
In Mashpee, Superintendent Patty DeBoer told families and staff Saturday in a letter that two Mashpee Middle School students and a Quashnet School student tested positive for the virus.
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In the letter, which DeBoer posted to Twitter, the superintendent said anyone identified as a close contact would receive further communication and guidance from the school district.
“Please continue to monitor your child daily for any signs or symptoms of COVID-19 and alert the school nurse if any symptoms develop,” DeBoer said.
And Winthrop officials Saturday said they connected 22 COVID-19 cases to social events at the Winthrop Lodge of Elks and the Pleasant Park Yacht Club. They asked anyone who visited those locations from Oct. 15 to Oct. 18 to get tested and socially isolate, according to a statement.
Winthrop Public Health director Meredith Hurley said everyone in town should “get tested if they have not already so that we can have a better understanding of how many cases there currently are in the community.”
Globe correspondents Abigail Feldman and Jeremy C. Fox contributed to this report.
John Hilliard can be reached at john.hilliard@globe.com.