Will Mass. achieve herd immunity against the coronavirus pandemic? Maybe, experts say, but it could be difficult

Dipika Patel of Somerville got her first COVID vaccine from registered nurse Dianurys Mota at the Encore Boston casino. Massachusetts may have reached 70 percent immunity.John Tlumacki/Globe Staff

Massachusetts’ coronavirus vaccination campaign has been among the most successful in the country, with 57.3 percent of residents — 3.9 million people — having received at least a first shot of the vaccines as of earlier this week, according to federal data.

That progress has people wondering if the state can vaccinate enough people to reach herd immunity. Here’s what some experts think about that prospect.

What is herd immunity?

Herd immunity occurs when a large proportion of a population is immune to a virus because of previous infection or vaccination. As a result, the virus can’t readily spread, because its chances of encountering a susceptible person are low.

What percentage of a population would have to be immune to achieve herd immunity?

“There’s no magic number,” said Dr. David Dowdy, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. And the number, he added in an e-mail, “depends partially on how many people have been infected, the degree of distancing that people are practicing, and the transmissibility of the virus” — a factor that can change with the season and the variant of the virus in circulation.

Still, many experts have recently settled on 80 percent as the likely target.

“We won’t know what the exact number is until we reach it and see what happens epidemiologically with cases, hospitalizations, and deaths,” said Dr. Nahid Bhadelia, an associate professor at the Boston University School of Medicine who specializes in infectious diseases.

Could Massachusetts achieve herd immunity by itself?

“We have a better shot than most other places in the world right now,” Bhadelia said in an e-mail.

But Dr. David Hamer, a physician at Boston Medical Center and a Boston University epidemiologist, said, “It’s going to be a challenge.” The federal government has not yet authorized the vaccines for people under 16, some people remain reluctant to get the vaccines, and some people have medical or religious reasons for declining.

“I think it’s more feasible in Massachusetts than in a number of other states, where there’s much greater reticence,” Hamer said.

Dr. Philip J. Landrigan, director of the Program for Global Public Health and the Common Good at Boston College, doesn’t think it’s likely to happen.

“Given the substantial number of people who seem intent on declining COVID immunization and our open borders, we will probably never achieve full herd immunity or eradicate COVID in Massachusetts,” he said in an e-mail. “But if we continue on the current trajectory (and the virus does not mutate out of control), we will get it down to very low numbers.”

In a Twitter thread earlier this week, Dr. Ashish K. Jha, dean of the Brown University School of Public Health, noted that Massachusetts, Rhode Island, and South Dakota have already reached about 70 percent population immunity, when you count immunity through infections. “They might get to 80 percent before long,” he wrote.

“We won’t be done even if we get to 80%,” Jha added. “We’ll need to monitor variants, vaccinate the world, continue testing, etc. . . . But this is all manageable. We’ll settle into a new equilibrium as we do with many viruses.”

Dr. Helen W. Boucher, chief of the Division of Geographic Medicine and Infectious Diseases at Tufts Medical Center, said that doctors and others are already planning to cope with COVID-19 for the long haul — preparing to possibly give booster shots in the fall, thinking ahead to caring for patients with a fever and a cough, ensuring tests and protective equipment are in adequate supply.

“We’ll have to be prepared to deal with a world like that,” Boucher said in an interview. “Those discussions are happening at many levels in this country.”

How depressed should we be if we can’t reach herd immunity?

If herd immunity is never achieved, said BC’s Landrigan, “COVID will continue to smolder in the population — endemic transmission — and we will have to hope that no new, resistant strain of the virus emerges.”

Still, the benefits of vaccination are clear.

Even if Massachusetts doesn’t attain herd immunity, Jha wrote on Twitter, as the state gets into summer and fall, “infection numbers will be low, vaccinated folks will be mainly safe, and with better treatment infections may become less problematic. And life will return to a recognizable normal. And that’ll be good.”

Bhadelia, from BU, cautioned, “We need to drive the number down through vaccination this summer because it’s possible cases may go up again during winter due to possible seasonality of this virus.”

At the same time, experts noted, even if herd immunity were technically reached, it is not necessarily the same as completely wiping out the disease.

“We’re not likely to completely eradicate the virus anytime soon,” said William Hanage, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health.

Can Massachusetts protect itself on its own?

“We don’t live in a microcosm,” said Boucher, the Tufts physician. “You can’t say we have herd immunity when people come in and out.”

Even if Massachusetts does well in stopping COVID-19, the virus could make its way back into the state if it is still circulating around the country and the world, the experts said.

But, Hanage said, “If population immunity can be kept high enough, then it won’t cause large outbreaks.

Bhadelia noted that if the virus keeps getting imported into the state, chances increase that it will encounter people who cannot mount a good immune response, and that new variants will reduce vaccine effectiveness.

“At the end of the day, we’re all connected,” said Dowdy, from Johns Hopkins. “As long as the virus is transmitting in one jurisdiction, reintroduction and outbreaks will be a possibility in all jurisdictions.”