The concept of herd immunity is often pitched as the key to a return to normalcy after living for more than a year under coronavirus restrictions, but public health experts warn that achieving and maintaining herd immunity for the coronavirus is increasingly unlikely.

“The concept of herd immunity with COVID-19 remains elusive,” said Dr. Todd Ellerin, director of infectious diseases at South Shore Health. “What I mean by that is that with influenza we don’t talk about herd immunity. Why? Because we know it’s coming back next year.”

Herd immunity is a form of indirect protection from viruses when a large portion of the population is immune, stopping transmission. Most estimates place the threshold at around 70% of the population gaining immunity — either through vaccinations or natural exposure to the virus.

But even as vaccination efforts take off around the world, factors such as vaccine hesitancy, emerging new variants and the delayed delivery of vaccinations to children — who make up about 22% of the United States population alone — make achieving true herd immunity difficult, Boston University infectious diseases specialist Dr. Davidson Hamer said.

Ellerin said there are still a lot of “unanswered questions” about the new virus.

“Will we get to a point where we’ve eradicated the virus? I don’t believe that’s happening. It’s highly contagious, there is an animal reservoir and all these variants,” Ellerin said.

While true herd immunity may be out of reach in Massachusetts, the United States and across the globe, experts are unanimous in their agreement that vaccinations play a key role in charting a path to normality.

“People should absolutely get vaccinated,” Ellerin said.

As of Sunday, more than 2.5 million people in the state are fully vaccinated and another 1.6 million have received their first dose. That’s well on the way to the 4.1 million people Gov. Charlie Baker has pledged to vaccinate to attain herd immunity in Massachusetts, but public health experts warned it might not be enough to keep the virus at bay.

COVID-19 Command Center spokeswoman Katelyn Reilly said Massachusetts “has the lowest rate of vaccine hesitancy in the country.”

But Hamer says even if Massachusetts does achieve herd immunity within its borders, there is still a “big concern” about what happens when travel opens up to areas with lower vaccination rates.

“There is a constant threat of reintroduction of the virus in parts of world where there is less vaccine,” Hamer said.

The good news is that booster shots — should they be required — will be relatively quick and easy to manufacture, which Hamer and Ellerin said would likely block a massive resurgence of viral transmission.

“I truly believe for foreseeable future, including next fall and winter, the worst is behind us and if in the future a COVID-19 isolate comes that can evade immune system, we have the tech to develop vaccines much faster,” Ellerin said.

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