There are 6.9 million people in Mass. Here’s why the state goal is to vaccinate 4.1 million of them

Nurse Jessica Pineault vaccinated Maria Saraceni at the Old North Church in the North End last week.Jessica Rinaldi/Globe Staff

The state’s coronavirus vaccination campaign, which has drawn an increasing amount of flak in recent days, has a goal of vaccinating 4.1 million Massachusetts adults. Experts say it would be a good step toward protecting the population, but the shots shouldn’t stop there.

Governor Charlie Baker said in his State of the State speech last month, “Vaccinating four million adults in Massachusetts as the doses are allocated by the federal government is not going to be easy. But be assured that we will make every effort to get this done as quickly and efficiently as possible.” The state’s COVID-19 Response Command Center said in a statement last week that the target number is 4.1 million.

The state’s population is about 6.9 million, including about 5.5 million people who are 18 and over, according to the US Census. A spokeswoman for the state didn’t immediately respond to a request for more details on the 4.1 million target.

But epidemiologists say you don’t need to give every single person a shot. That’s because after a certain level of vaccinations, herd immunity can kick in, a situation in which a large portion of the population is immune and the virus is less likely to spread.

Dr. Paul Biddinger, medical director for emergency preparedness at Mass General Brigham and chairman of the state’s COVID-19 vaccine advisory group, said the governor’s goal seemed to be a “reasonable, herd immunity-approaching strategy for the adult eligible population.”

He said estimates have varied on what percentage of the population needs to be vaccinated to provide herd immunity from the coronavirus, but at least 70 percent is reasonable. And he calculated that 70 percent of the adult population is roughly 3.8 million people, which the state would exceed among adults if it met Baker’s goal.

“If you ask anyone involved in vaccinations on the medical side, our goal would always be to vaccinate everyone that is medically eligible,” he said. But “setting shorter-term goals trying to get to levels that reach herd immunity among the eligible population makes sense.”

He said that he also expected that when vaccines are authorized for younger people, they would be administered and the percentage of the total population vaccinated would rise. “We would want to see that number come up to reflect at least 70 percent of everyone who’s medically eligible,” he said.

“It’s going to be challenging to get to 70 percent, but I think it’s absolutely the right goal,” he said.

The vaccines are currently only authorized for people 16 and up, in the case of the Pfizer vaccine, and 18 and up, in the case of the Moderna vaccine.

Samuel Scarpino, a Northeastern University epidemiologist, said he also expected that, at some point in the coming months, the focus of the vaccination effort in Massachusetts would turn to younger people.

“That would be my assumption — that this first goal is what we want to do for people over 18,” and once younger people become eligible, “we would very quickly move to vaccinate them,” Scarpino said.

Dr. Barry Bloom, a professor and former dean of the Harvard T.H. Chan School of Public Health, said he would like to see 75 percent of the entire population vaccinated.

He also warned that the percentage of people who need to receive vaccinations could rise if coronavirus variants take hold in Massachusetts. “If resistant variants arise or more rapidly transmitting variants, we would need a higher level of immunized people and/or booster shots that provide specific protection against the vaccine resistant variants, which are here but have not spread widely yet,” Bloom, who is also a member of the state’s vaccine advisory group, said in an e-mail.

He noted that the vaccines are being tested in younger people, including those as young as 6, but it’s not necessarily a simple process. “It is possible trials will have to be done to adjust the dose of vaccines and that will take time and add to costs,” he said.

As of Tuesday, 304,657 people, or about 4 percent of the total population of the state had been fully vaccinated, according to the Department of Public Health.

“Setting public health targets always involves a delicate balance involving both the aspirational and the achievable,” Dr. Howard Koh, a professor at the Harvard T.H. Chan School of Public Health and a former assistant secretary of health and human services in the Obama administration, said. “Doing so in the dynamic setting of COVID adds to the challenge, especially given the rise of variants, concerns about vaccine hesitancy, and the unknown duration of vaccine-induced immunity.”

“The stated target so far, initially announced in the context of 5.5 million eligible adults, may well need to be upgraded once children are able to be vaccinated, hopefully sometime later this year,” Koh, who also once served as public health commissioner in Massachusetts, said in an e-mail.

What will life be like if and when the state succeeds at getting the 4.1 million adults vaccinated?

Biddinger said that, as with many other infectious diseases, “You can’t drive prevalence down to zero. I think you can say, ‘There is very little prevalence of disease in the community and those who are at highest risk have received a very good chance of protection’ ... And those are the factors that will help us start getting back to a normal life.”

Travis Andersen of the Globe staff contributed to this report.