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Two scientists studying the spread of the coronavirus say the number of infected residents in Massachusetts could be at least five times the number the state is reporting and possibly much higher, with one estimate ranging up to 6,500.

The official tally reported by the state rose to 218 on Tuesday, but that number does not account for large numbers of infected people who have not been tested, particularly those who are not yet symptomatic or have only mild symptoms. Those unconfirmed cases are considered the most dangerous in spreading the virus because people who don’t feel severely ill often continue to go about life, interacting with others and infecting them.

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Baker announces $5m ‘down payment’ to local boards of health to address coronavirus
Massachusetts Governor Charlie Baker said Tuesday the state would distribute a down payment of $5 million in emergency funds to local boards of health. (Photo by MATTHEW J. LEE/GLOBE STAFF)

“What you are looking at is just the tip of the iceberg, and the question is: how much of the iceberg is submerged?” said Jeffrey Shaman, a Columbia University environmental sciences professor who coauthored a new study on the scale of undetected cases of COVID-19.

Shaman’s study, published Monday in the journal Science, used epidemiological research and cellphone data to conclude that early on, only one in seven cases was detected in Wuhan, the Chinese city where the pandemic originated, and that “stealth transmissions” from seemingly healthy people were responsible for the explosive speed of the outbreak.

Based on his estimate of a US detection rate of somewhere between one in five and one in 10, Shaman said the total number of cases in Massachusetts could range from as few as 1,090 to as many as 2,180.

Another scientist who spoke to the Globe, Samuel Scarpino, a Northeastern University professor who specializes in infectious disease modeling, said he used an algorithm developed by an epidemiologist at Notre Dame University that calculates the United States has only detected between 1 of every 10 cases and 1 in 30 cases, due to its low rate of testing compared to other countries. He said Massachusetts likely has 2,180 to 6,540 or more actual cases.

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Quickly ramping up testing is critical, Shaman and Scarpino said, particularly testing that won’t foster more infections, such as drive-through facilities. Two such sites have opened in Hyannis and Middleton, and Governor Charlie Baker said the state’s testing capabilities are increasing.

The federal government’s testing effort has been sharply criticized for delays and shortages of test kits and strict guidelines limiting who could be tested. In Massachusetts, several people who had COVID-19 symptoms after a meeting of Biogen executives, which is thought to be the root of the outbreak in Boston, were turned away because they did not meet the criteria.

Undetected cases are particularly dangerous because they obscure the true level of risk facing the public, potentially affecting how seriously people take government warnings and affecting leaders’ decisions, they said.

“If you’re going to respond effectively, you have to take decisive action while the case counts are still low, otherwise it’ll be too late,” said Scarpino.

Scarpino said that in other countries the number of people infected doubled every few days and that under that scenario in the Boston area, hospitals could be full in just 10 days.

“Even if you only have a few hundred cases, in a matter of one to two weeks you have cases in the thousands,” Scarpino said. "With 10 percent of those cases needing intensive care, you very quickly run out of intensive care beds.”

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The Massachusetts Department of Public Health declined to say whether it has its own projections of the state’s undetected cases. In Ohio on Thursday, the governor’s top health official said she believed 1 percent of the state’s population, or 100,000 people, were carrying the virus. And in Washington, the governor said he believed at least 1,000 infected people had gone undetected.

The study Shaman cowrote found that the portion of cases evading detection dropped dramatically in Wuhan, to 35 percent, after Chinese officials boosted testing and imposed quarantines and travel restrictions.

The undetected cases were the source of infection for 79 percent of documented cases, the study found.

The findings “indicate that a radical increase in the identification and isolation of currently undocumented infections would be needed to fully control” the virus, the authors wrote.

In Italy, where doctors now must ration ventilators and other life-saving equipment among crowds of patients, it’s too late.

“Once you’re past the point where your hospitals are getting inundated," Shaman said, widespread testing "is not something that’s going to be feasible, and unfortunately, may be beside the point.”

As the government rolls out more testing supplies and infrastructure, the larger case numbers will hopefully paint a more accurate picture, Scarpino said. That’s good, he said, because more people who are contagious will be quarantined and health systems will be more prepared.

“That can look really scary because the numbers will jump," he said, “but that’s a good thing."

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The vast number of undetected cases should concern the public and policy makers alike, specialists said.

“Most of the country doesn’t have enough testing capacity,” said Elizabeth Halloran, a professor at the University of Washington who studied coronavirus. “Hopefully it will be ramped up in the next few weeks because it will be crucial to helping to slow the epidemic.”



Naomi Martin can be reached at naomi.martin@globe.com.