Massachusetts ranks behind only three other states, New York, New Jersey, and Connecticut, where the virus has infected a larger percent of their state population, the researchers said.
“Over 80 percent of [Massachusetts] hasn’t been infected yet,” said Samuel Scarpino, an epidemiologist at Northeastern University. “We’re going to be susceptible to another wave of infections.”
The model estimated about 96,000 Massachusetts residents were infectious as of May 17 and could spread the virus to others. Only Illinois had a higher number of infectious residents, according to the researchers, who prepared estimates for every state.
The outbreak in Massachusetts is at a crucial crossroads, according to the model, and the daily number of deaths could increase by more than twofold if residents push too quickly to resume their pre-pandemic lifestyle.
“In Massachusetts, you are on a knife’s edge,” Seth Flaxman, a senior lecturer at Imperial College London, said in an e-mail Saturday.
The state is among two dozen nationwide with a reproduction number greater than one, which refers to the average number of secondary infections caused by an infected person. It is commonly referred to as “R.”
“If Massachusetts is able to get R below 1 and keep it below 1 then the epidemic will die out. But if R goes above 1 then the epidemic is not under control — it will grow and in 2 to 3 weeks we will see deaths starting to increase,” Flaxman wrote.
A reproduction number above one in 24 states shows “the epidemic is not under control in much of the US,” the researchers wrote.
Last Monday, Massachusetts began relaxing restrictions and closures of businesses and houses of worship to stop the spread of the virus. More restrictions are scheduled to be lifted Monday as beaches, some offices, and certain service providers like hair salons and pet groomers are allowed to reopen.
The Imperial College London model examines how high the daily death count could go in Massachusetts as residents begin to leave their homes more frequently and move around the region.
If mobility returns to 20 percent of what it was before the pandemic, COVID-19 could kill about 500 people daily by the end of June, the researchers said. If mobility increases to 40 percent of pre-pandemic levels, the virus could claim the lives of about 800 people each day, the model said.
Massachusetts experienced the highest number of deaths in COVID-19 patients to date on April 24 when 195 patients died, state figures show.
As of Saturday, the state has confirmed the deaths of 6,304 people who were infected with COVID-19.
Samir Bhatt, a senior lecturer at Imperial College London, said the estimates are “pessimistic” and assume “that the relationship between mobility and disease transmission remains constant.”
“We do not address the potential effect of additional behavioural changes or interventions, such as increased mask-wearing or testing and tracing strategies,” Bhatt wrote in an e-mail. “These scenarios are pessimistic and should be interpreted as such.”
Governor Charlie Baker’s administration didn’t respond directly Saturday to Globe questions about the model.
“The Baker-Polito Administration is pursuing a four-phased reopening plan, guided by public health data, and will continue to carefully monitor public health metrics and trends as we work to safely return to a new normal,” said Baker spokeswoman Sarah Finlaw.
Scarpino, the Northeastern epidemiologist, said the number of COVID-19 infections and deaths in Massachusetts is likely much higher than what the state has reported. Researchers studying outbreaks in China and Europe have estimated the number of infections to be 10 times what was reported in official counts, he said.
A small study conducted by Massachusetts General Hospital and the Boston Public Health Commission found 10 percent of Boston residents have the antibodies that indicate they had COVID-19 and fought it off.
How people respond to preventative measures like wearing masks, social distancing, and handwashing will determine the path the virus takes in Massachusetts, Scarpino said.
“To the extent that people are taking that seriously is going to have a major effect on whether this gets better or begins to get worse again,” he said.
Imperial College London’s COVID-19 modeling has influenced responses to the pandemic at the highest levels of government.
Their estimate in March that 510,000 would die in Britain and 2.2 million in the United States if the outbreak remained unchecked persuaded British Prime Minister Boris Johnson to adopt more extreme measures to combat the virus and was cited during a news conference by Dr. Deborah Birx, coordinator for the White House Coronavirus Task Force.
Nationally, the researchers estimate 4.1 percent of Americans or 13.5 million people have been infected with COVID-19 and 1.3 million were infectious and susceptible to spreading the virus on May 17.
On Saturday, the Centers for Disease Control and Prevention said there were nearly 1.6 million cases in the US and 96,002 deaths.
Laura Crimaldi can be reached at firstname.lastname@example.org. Follow her on Twitter @lauracrimaldi.