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Here’s the rosy scenario: By Labor Day, if all goes well, nearly every adult in the United States who wants a COVID-19 vaccine will have received one. Deaths and hospitalizations from the coronavirus will have fallen sharply; transmission will have plummeted as the population reaches herd immunity. Everything from schools to restaurants will have fully reopened, allowing for something approximating pre-pandemic life to resume.

There is also, regrettably, a far bleaker scenario for the coming months: The virus remains implacable, mutating in ways that either end-run the protection of vaccines or keep it contagious enough to sicken a large portion of the population, including younger people at lower risk from previous strains and the millions who refuse to be vaccinated. As drug makers race to tailor vaccines to the variants, the country might remain trapped in a purgatory of stressed hospitals, shuttered businesses, and remote learning.

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Dr. Edward Nardell, a professor at Harvard Medical School who studies infectious diseases, prefers to think we’re heading toward the rosy outcome. He is already planning to attend a cabaret festival in Provincetown in June, and hopes to resume organizing concerts this fall at his condominium complex in Brookline.

“My natural tendency is to be optimistic and believe things by the fall will be dramatically better,” said Nardell, who on Thursday received his second dose of vaccine. “But what makes me concerned is that there could be a tsunami offshore in the form of variants of the virus that could dramatically increase transmission. We’re in a strange period of apprehension, and nobody has a crystal ball about what will happen.”

As the pace of vaccinations accelerates, there has been a spate of encouraging virus news. Cases in the United States have fallen substantially — nearly 60 percent from the peak last month. Hospitalizations have declined by 36 percent and deaths have fallen 23 percent.

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But those declines must be considered in context. They follow a catastrophic surge of the virus, which has left about 475,000 Americans dead, nearly 100,000 of them in the past month. The seven-day average of new infections this week remained at more than 100,000 a day, a level that would have been unthinkable last spring. On Feb. 10, at least 3,255 people died as a result of COVID-19 — more than all of those who died as a result of the attacks on Sept. 11, 2001 — while 77,000 people were hospitalized.

The rampant spread of the virus over the past year has had this side effect: a significant number of Americans with immunity to the virus, at least to the initial strain that emerged from China. In January, the Centers for Disease Control and Prevention estimated that more than 83 million Americans had been infected, which includes unconfirmed cases. With 35 million Americans having received at least one dose of the vaccine, that would mean at least 36 percent of the population has some measure of immunity.

That number is likely to increase significantly in the coming months as vaccinations become more universally available, public health officials say.

“By the time we get to April, that will be what I would call, for better wording, ‘open season,’ namely, virtually everybody and anybody in any category could start to get vaccinated,” Dr. Anthony Fauci, the nation’s top infectious disease expert, told NBC’s “Today” show on Thursday.

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But herd immunity — which occurs when so many people have been immunized, either by illness or vaccination, that transmission of the virus effectively stops — is likely to require at least 80 percent of the population to be vaccinated or protected as a result of a previous infection, Fauci has said.

It’s unclear how long that will take, or indeed whether it’s possible. Achieving herd immunity depends on how many people choose to be vaccinated and how well the vaccines stand up to potentially more virulent variants. It also will depend on a host of unanswered questions, such as how long natural immunity lasts, whether natural immunity extends to the variants, and whether those who are immunized can, nonetheless, transmit the virus to others.

“I do think that herd immunity is still possible, but it is not guaranteed that we can achieve it without some possible future modifications to our vaccines,” said Dr. Paul Biddinger, medical director for emergency preparedness at Mass General Brigham and chairman of the state’s COVID-19 vaccine advisory group. “It may be that people will need booster shots of modified vaccines in the future to help us reach our goals.”

He mainly attributed the recent decline in new cases to more people wearing masks and the time that has passed since the holidays, when millions of Americans traveled and gathered with relatives against the advice of health officials.

But that progress could be reversed, especially if, as seems likely, the more contagious variant that emerged in the United Kingdom becomes the dominant strain in the United States, as it has in other countries. On Wednesday, health authorities said the variant had been detected in Maine for the first time, less than a month after it was discovered in Massachusetts.

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And as cases decline, the move to increase capacity limits in restaurants, gyms, and other businesses, as is already happening in Massachusetts, could also spur the spread of the virus.

“We may still see large surges of infections in the coming months,” said Erin Bromage, a biology professor who studies infectious diseases at the University of Massachusetts Dartmouth. “We are in for a rough ride.”

When asked whether it was wise for Governor Charlie Baker to relax restrictions in Massachusetts, he noted that similar actions in the United Kingdom and elsewhere had led to a resurgence of the virus.

“I am torn with this decision,” he said. “I really don’t want us to repeat what happened in London. Their November lockdown worked wonderfully, but as soon as they reopened, B1.17 [the UK variant] infection took off, and they saw a wave of infections and deaths that was much larger than their earlier surges.”

With the virus spreading more among younger people now — few of whom have been vaccinated — Bromage said he expects infections will remain high but fewer people will die or require treatment at a hospital.

Dr. Christopher Gill, an associate professor of global health and infectious disease specialist at the Boston University School of Public Health, said those who are fully vaccinated should be able to start to live a more worry-free life, going to restaurants and theaters, accompanied by others who have been vaccinated.

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“It should be safe to do these things,” he said. “Is it possible that some transmission could still occur, say, due to the South African variant? Of course. But the question really is how much tolerance do we have for varying degrees of risk?”

He said the risks of going to a bar with someone who is vaccinated are likely to be similar to the risks of getting the flu in more ordinary times. What’s unclear, however, is the risks of bringing the virus home or to a job, where others aren’t vaccinated.

For that reason, health officials are advising the public to continue wearing masks for months to come, even well after they’ve been vaccinated.

“We need to get to a much lower level of viral prevalence in the community, and a much higher level of vaccination of the population,” before people should stop wearing masks, Biddinger said. “It could be by the fall or winter, but I think it’s too soon to estimate with confidence.”

Although there remain many reasons to be concerned about potential setbacks, Nardell thinks there are grounds for cautious optimism.

The vaccines are proving to be as remarkably effective as they were in clinical trials, with no one who has been fully immunized known to have died or become gravely ill. The federal government is ramping up vaccinations, with President Biden announcing this week that the United States will have enough supply to immunize 300 million Americans by the end of July.

And spring is coming, when warmer weather should reduce the spread of the virus.

Nardell, who was infected with COVID-19 shortly after the pandemic began, looks forward to a day soon when he’ll feel comfortable having friends over for dinner.

“There’s light at the end of the tunnel,” he said.

Gal Tziperman Lotan of the Globe Staff contributed to this report.


David Abel can be reached at david.abel@globe.com. Follow him on Twitter @davabel.