A closely watched model shows that COVID-19 increases could be coming to Mass. this fall. Here are three scenarios

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The alarming resurgence of the pandemic since early July in Massachusetts will continue into the fall, according to a closely watched forecast model and several experts.

While some said they didn’t expect the situation to become as dire as it did during last winter’s surge, the experts also emphasized that the trajectory of the highly infectious Delta variant is unclear, and human behavior — including whether people get vaccinated and wear masks — will make a crucial difference.

“I think we are likely to see an increasing number of cases, though I don’t think it will be as dramatic as last fall and winter,” said Dr. Daniel Kuritzkes, chief of the Division of Infectious Diseases at Brigham and Women’s Hospital.

“We’re not going to see an explosive surge,” thanks to the state’s high vaccination rate, he said. “It will be smaller because it will be primarily among the smaller group of unvaccinated people.” Massachusetts is a national leader in vaccinations, with nearly 75 percent of the population having received at least one shot, according to federal data.

Matthew P. Fox, a professor in the epidemiology and global health departments at Boston University, said, “I anticipate cases will continue to rise as we head into the fall. The Delta variant is highly contagious, and as we move into school reopenings, I think you will see a sharp rise.”

A widely cited model from the University of Washington’s Institute for Health Metrics and Evaluation offers three scenarios: one that is “most likely”; another that assumes 95 percent mask-wearing; and a third that assumes zero masking by the vaccinated, a faster-spreading virus, and a slower government response.

There are three scenarios for “estimated infections” (a number that the institute says is the total number infected daily, including people who are sick but not tested) and deaths.

The numbers for the “most likely” scenarios — the middle scenario — climb in the fall but don’t reach levels as high as last winter’s surge. They taper off later in the fall until the projection ends Dec. 1.

“I am concerned. Delta is causing a lot of increases in cases and in our hospitals,” Ali Mokdad, a professor at the institute told the Globe in a recent story on the grim fall outlook. “We have underestimated this virus and celebrated prematurely.”

An ensemble forecast from the Centers for Disease Control and Prevention, which combines multiple forecasts from respected researchers and only looks four weeks ahead, is less alarming, suggesting that weekly case numbers will stabilize in the short term, while weekly death numbers will remain in the single digits.

The ensemble projections for cases and deaths are shown below, with upper and lower confidence limits. (Click on “All projections” to see the widely divergent results from the different models that were combined to make the ensemble projection.)

While the short-range CDC forecast looks promising, experts said there are plenty of reasons to believe increases are ahead in the coming months, including:

- People going indoors due to cooler weather;

- The number of people still reluctant to get vaccinated;

- The fact that no vaccine has been authorized yet for the hundreds of thousands of children under the age of 11;

- The extreme infectiousness of the Delta variant; and

- The waning of protection offered by the vaccines, particularly among older people.

The possible impact of the reopening of schools and colleges was a recurring theme among the experts.

“I think we just have to anticipate that there’ll be an increase in the number of cases,” said Dr. Philip Landrigan, a pediatrician who is director of Boston College’s Global Public Health Program. Landrigan said one necessary step is requiring universal masking inside schools, a step the Baker administration took Wednesday, effective until at least Oct. 1.

As case numbers rise, the vaccines, which are highly effective against severe disease, are expected to reduce the percentage of people who eventually become hospitalized with COVID-19 and die.

But hospitalizations and deaths will still happen. Hospitalizations are already up more than sevenfold since early July, though the daily average of deaths, which tends to lag hospitalizations, is still low. Fox said he expected hospitalizations and deaths to go higher in the fall.

“The hope is that the sharp rise will be followed by a fairly sharp decline. So while I see early fall being a time of increase, I’m hopeful it doesn’t last long,” he said.

While Kuritzkes predicted the coming increases wouldn’t be as bad as in the winter surge, he sounded a note of caution, saying, “I don’t think anybody really knows how bad it’s going to get. There’s so many variables. I don’t think anyone can put a number on it.”

The sentiment was echoed by other experts.

Dr. Howard Koh, a professor at the Harvard T.H. Chan School of Public Health and a former high-ranking state and federal health official, said in an e-mail, “The public health forecast for Massachusetts remains so very guarded, despite our state’s nationally leading vaccination rates. Looking to the future, we can’t assume anything. This virus and its variants continue to humble the state, the nation and the world.”

Samuel Scarpino, an epidemiologist who is managing director of pathogen surveillance at the Rockefeller Foundation and an affiliate faculty member at Northeastern University, said models at the beginning of the summer predicted that Massachusetts would be sailing now into a time of zero deaths, but the “Delta variant has completely changed the game.”

“For everything we know about this virus, we can still fill up books with things we don’t have the answers for yet. That causes a lot of uncertainty in the forecasts,” he said. Borrowing a phrase from hurricane warnings, he said, “The cone of uncertainty as we get into the fall is large.”

One thing is certain: What people do next will have a major impact on what happens next, the experts said.

“We still have it within our power to alter the shape of the pandemic by taking appropriate precautions and making sure that everybody gets vaccinated,” Kuritzkes said.

He said the surge could be blunted by factors including more people wearing masks, by the federal government’s plan to offer boosters beginning in September, and by more people stepping up to get their shots, either because they’ve had a change of heart or been required to by their employer after the recent full approval of the Pfizer vaccine.

Scarpino said the University of Washington model might dramatically highlight the difference that masking can make, but there are many variables also at play.

“We have a lot of agency over what happens in the fall,” he said. “If we focus on vaccinations, if we have thoughtful, coordinated plans ready to vaccinate 5- to 11-year-olds once they’re eligible, if we have indoor masking, if we have surveillance testing, we can bring this under control and keep it under control.”

“We are in a better position in Massachusetts than many, many other states, and we need to build on that,” he said.

“We know there’s another alternative out there, which is more hospitalizations and more deaths. And that’s one that we can avoid . . . but it requires taking action now,” he said.