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

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 cant 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?

Theres 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 wont 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, Its 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 its more feasible in Massachusetts than in a number of other states, where theres much greater reticence, Hamer said.

Dr. Philip J. Landrigan, director of the Program for Global Public Health and the Common Good at Boston College, doesnt think its 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 wont be done even if we get to 80%, Jha added. Well need to monitor variants, vaccinate the world, continue testing, etc. . . . But this is all manageable. Well 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.

Well 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 cant reach herd immunity?

If herd immunity is never achieved, said BCs 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 doesnt 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 thatll be good.

Bhadelia, from BU, cautioned, We need to drive the number down through vaccination this summer because its 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.

Were 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 dont live in a microcosm, said Boucher, the Tufts physician. You cant 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 wont 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, were 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.

Martin Finucane can be reached at martin.finucane@globe.com. Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer.

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