BOSTON (CBS) — Is reaching herd immunity possible? As another million people become eligible for a COVID-19 vaccine in Massachusetts, experts continue to debate whether the public can reach herd immunity and safely return to normal life.

The concept of herd immunity is simple. Whether through vaccination or infection, the virus is ultimately stamped out when it runs out of bodies to infect.

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A few weeks ago, Governor Charlie Baker told a legislative oversight committee that the Commonwealth needed to fully vaccinate roughly 4.1 million people to reach herd immunity but experts say it is far more complicated than that.

“To kind of hang our hat on a number or a time or date is really difficult and you can’t just have herd immunity in a state that has borders that are open,” said Dr. Shira Doron, hospital epidemiologist at Tufts Medical Center.

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Doron argued that knowing when we could reach herd immunity is nearly impossible. Firstly, children, a large portion of the population, are still not eligible for the vaccine. It remains unknown how much of the population may be immune because it’s unknown exactly how many people have been infected by the virus. New variants muddy the waters further because scientists do not know how infectious they are.

“More realistically, what we should expect and what we should plan is that we vaccinate as many people as possible as quickly as we can. We get to a point where we start to see cases drop and we learn to live with some low levels of the virus in our lives,” Doron said.

Doron said there is still reason to be hopeful. She believes mask mandates and social distancing may slowly fall away as everyone who wants a vaccine gets one. And the time to get one, she said, is as soon as possible.

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“Make an appointment right away. Take the very first vaccine that’s offered to you. It’s an emergency. And none of us are safe until everyone is safe,” Doron said.

Louisa Moller