Herd immunity threshold may be out of reach in U.S.

Experts say still good reasons to get vaccinated

BOSTON — It is a blank that experts cannot accurately fill, though they’ve been trying for months: normalcy will return when __% of the population has immunity to COVID-19.

“When the virus initially appeared, it would take 60-70% to get to the place where the virus couldn’t spread,” said Dr. Robin Colgrove, a virologist at Mt. Auburn Hospital in Cambridge. “Hard but achievable.”

But then it got harder.

“With the spread of more resistant variants and a pretty significant hard-edged group of Americans who just will not get vaccinated, it looks like the number that it would take to be vaccinated, the fraction, is more in the 75-to-85% range,” Dr. Colgrove said. “And that really doesn’t look like that’s going to be achievable over the country as a whole.”

Still, individually, vaccination is important, according to Colgrove.

“Getting vaccinated makes it extremely unlikely that you’ll die of COVID-19,” the doctor said.

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Many refer to the point of majority immunity as ‘herd immunity.’ But that’s a misnomer, said Dr. Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Penn State University.

“Herd immunity – that concept – that’s a journey. It’s not a destination,” Ferrari said. “And what it describes is the incremental and additional reduction in risk to everybody of being in a population where many people are immune. So that’s a journey we started the first time somebody became immune to COVID.”

Ferrari said the actual thing under discussion now, as vaccination programs continue, is the herd immunity threshold.

“That’s a point where there’s enough accumulated immunity in the population that an epidemic is unlikely to start,” he said. “So, we’re really talking about apples and oranges here.”

And Ferrari suggested talking about numbers and percentages vaccinated as a way to gauge victory over the virus could be unhelpful.

“If you set a target, then everybody focuses on that target,” he said. “And if that target is far away that can be really motivating. We’ve got a long way to go. Let’s all get going. But as you get closer and closer and closer to that target, people start to ease their foot off the accelerator.”

And there’s always the possibility that number could be wrong.

“We might get it generally right,” Ferrari said. “And it’s on the order of somewhere between 65 and 85% of the population immune that we might need. But that’s a pretty big window. And so not everybody in the population is going to respond to that window in the same way. Some people are going to ease off the gas when we approach 65, others are more risk-averse and they’ll stay engaged until 85.”

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One reason to stay engaged until 85: picture constant COVID.

“The virus doesn’t just go away,” Colgrove said. “That it’s likely to be with us for a while. It will also percolate in the communities that have low vaccination rates and low levels of immunity.”

And that will not be beneficial to any of us.

“The longer the virus continues to spread, it continues to have a chance to develop newer and more contagious and more vaccine-resistant variants,” Colgrove said.

And that’s the point when there might be no ‘new normal’ anymore. Because it will have become – normal.

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