Scientists warn May 18 ‘still feels too early’ to begin reopening Mass.

A woman pushed a cleaning cart walked past a store as retail shops remain closed in Assembly Square.Jessica Rinaldi/Globe Staff

Governor Charlie Baker’s plan to reopen the economy in phases starting as soon as Monday may come as a relief to struggling business owners and nearly 1 million people left unemployed by the coronavirus crisis.

But many scientists caution that it may be too soon to broadly reopen Massachusetts, which has been among the hardest-hit states in the country, with 5,141 deaths so far. If the state moves too quickly, they warn, it risks a bleak calculus: a catastrophic second wave of infections, hospitalizations, and deaths — while further harming the economy.

They warned the state may not yet have the needed infrastructure to adequately test and trace the contacts of those infected to control those new outbreaks that will inevitably occur as people mingle more, especially endangering the sick and elderly.

“It still feels too early,” said Erin Bromage, a biology professor who studies infectious diseases at the University of Massachusetts Dartmouth. “The fact that we’re still seeing 1,000 to 1,500 new cases every day and we’re in lockdown means quite a lot of community transmission is still happening, and the biology says you increase contacts, you increase spread, and you’re off again.”

By next week, the state may meet one metric for reopening proposed by the White House: declining virus cases for 14 days, the time it can take for symptoms to show up. The state’s cases have largely decreased since May 1.

The scientists praised Baker’s establishment of a statewide contact-tracing program, aimed at isolating those who interacted with anyone infected. But they said the state still needs to be testing more people every day.

Dr. Ashish Jha, director of the Harvard Global Health Institute, said the state needs to be testing possibly several times more people than currently to adequately detect and stop new outbreaks.

“If you go too fast, you get more outbreaks, you get increases in cases, increases in deaths, and you set the economy back," Jha said. “It isn’t about when a governor says things are reopened, it’s really about how much confidence people feel.”

Baker plans to unveil his full plan by May 18, when his stay-home advisory expires. On Monday, he released a three-page document that offered hints at his thinking. The plan lacked details on which businesses would open first, how long each phase would last, or what the triggers would be to move forward or back.

Baker has consulted with public health experts and business leaders. He said he would base his decisions on scientific data and revert to more restrictions if the virus takes off again uncontrolled.

“We have to ensure that when we take one step forward, we do not end up taking two steps back,” Baker said.

The first wave will likely focus on industries that rely on few face-to-face interactions with customers. Baker also suggested that he won’t open businesses based on whether they’re essential, but rather whether they can meet safety standards.

The governor has moved cautiously toward lifting restrictions amid numbers that have only started to improve recently. About 3,100 people remained hospitalized with COVID-19 as of Sunday, a near 22 percent drop from the peak nearly three weeks earlier.

Other states in New England and across the country have slowly reopened businesses. In Maine, hair salons and barbershops were among the first to welcome back customers. In Rhode Island, some stores opened last weekend.

Massachusetts, however, has tallied far more deaths and hospitalizations than its neighbors. Its 79,332 confirmed cases are the fourth most in the country, behind only New York, New Jersey, and Illinois.

Dr. David Hamer, an infectious disease specialist at Boston Medical Center, said he was “concerned” that Baker’s plan to start reopening was too early. He also questioned whether the state had sufficient testing or enough contact tracing.

“While there is evidence of decreasing numbers of new cases based on statewide testing and fewer hospitalizations, the decrease is very gradual,” he said. “We need to proceed cautiously, given the potential risk.”

But Dr. C. Robert Horsburgh Jr., a professor of epidemiology at Boston University, said he appreciated that the governor is wrestling with a quandary.

“I don’t think anyone really knows what’s best,” Horsburgh said. “From a public health plan, I think the governor’s plan is a bit risky, but if I were governor, I’d probably be doing the same thing.”

He said he could understand opening some stores, in which patrons could be limited and maintain their distance. But other businesses, such as movie theaters, would be risky.

“You have to start somewhere,” Horsburgh said.

A second wave of infections could be devastating, as the vast majority of Massachusetts residents remain susceptible to the virus.

In Chelsea, which was among the state’s hardest-hit cities, 30 percent of those randomly tested had coronavirus antibodies, suggesting they previously had the disease. That’s probably among the highest rates in Massachusetts.

The emphasis in Baker’s preliminary plan of prohibiting those with COVID-19 symptoms from coming to work belies the fact that a significant portion of those infected don’t experience symptoms at all or after they’ve been contagious for days, said Samuel Scarpino, an epidemiologist at Northeastern University.

He praised the governor’s order requiring people to wear masks in public but added that contact tracing is crucial to addressing that issue. Until things are more under control, he said, he likely wouldn’t feel comfortable meeting with friends or eating in restaurants.

“There’s a lot of COVID-19 out there right now,” Scarpino said. “I’m going to continue to physical distance until either the cases come way down or we’re much further along with a pharmaceutical treatment or a vaccine."

Businesses should be required to file plans to explain how they’re protecting their workers and customers, Scarpino said. In South Korea, one person may have caused the infections of 94 others at the call center where they all worked, a study showed.

Indoors, remaining 6 feet apart is not enough to guard against infection because droplets from someone’s sneeze or cough could end up across a large room, Bromage of UMass said. At a restaurant in China, one customer without symptoms unwittingly infected nine others at their table and others nearby because of an air conditioner’s flow, researchers believe.

In Florida and elsewhere, restaurants have been allowed to reopen with 25 percent of their fire capacity and some streets have been closed to traffic to allow for outdoor dining. That is something Massachusetts should consider, Bromage said.

Based on what scientists know about how the virus is transmitted through droplets, he said, it’s important for people to wear masks and to reduce the amount of time they’re indoors around others. They should also minimize the time they’re near others in general, he said, avoid anyone singing or shouting, and ensure indoor spaces are well ventilated.

For that reason, hair salons may actually be relatively low-risk, especially for cuts and other quick services, Bromage said.

Dr. Edward Nardell, a professor of medicine at Harvard Medical School who spent years working at the state Department of Public Health, noted that the last two months of social distancing appears to have succeeded in ensuring that the state’s hospitals weren’t overwhelmed.

But he worries that opening too quickly could swing the state back to the exponential growth of the virus.

“If we go back to business as usual," he said, “we’ll go back to transmission as usual.”