Herd immunity, or as some experts now call it, “population” or “community” immunity, is when most of the population is immune to a particular disease, whether through natural infection or vaccination. When a population reaches this point, the virus has nowhere to go, and the disease fades away. Then even people who don’t have individual immunity are protected.

As with any disease, how many people need to be immune to provide community protection depends on how infectious it is. For Covid-19, experts think the magic number could be anywhere between 70 to 90% of a population immune to the virus. The world is nowhere near that level.

“Given where we are today, as we look around the United States and when we look around the globe, it just seems like that is not going to happen in the foreseeable future,” said Lauren Ancel Meyers, the director of the Covid-19 Modeling Consortium at the University of Texas at Austin.

It’s a good goal, Meyers said, but she ticks off a host of factors in this particular pandemic that suggest the odds are not in its favor: vaccinating so many people would be nearly impossible; this particular virus spreads too rapidly; more contagious variants threaten to make vaccines less effective; there are entire countries and pockets of the US that have few fully vaccinated people; there are vaccine access and equity issues; children are not yet vaccinated; and about a quarter of the population is hesitant or unwilling to get vaccinated.

“We know how fast this virus spreads and how stealthily. We really would have to have a lot of people immunized before we would eradicate this virus,” Meyers said.

For perspective, though, Dr. Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said few infectious diseases ever become globally extinct.

“We’ve only ever really eradicated smallpox and if it weren’t for a couple of further interventions beyond vaccines, even that could still have been with us,” Hanage said. “Most people who know anything about infectious disease don’t think total eradication is possible.”

But all is not lost. The world doesn’t have to live in lockdown forever.

In Israel, for instance, once about 50-55% of the population was vaccinated, cases dropped dramatically.

“We can likely get enough immunity in the population where the virus is not a major threat everywhere,” Hanage said. Public health officials will have to watch for cases and variants in the fall, but he thinks people can return to some level of normal behavior, anyway. “We’ll get there eventually, and hopefully that will be through vaccination, rather than infection, because infection can kill people,” he said.

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In the US more than 40% of adults are fully vaccinated, according to data published Monday by the US Centers for Disease Control and Prevention. Numbers of new cases are down 15% since last week, but the US is still averaging 49,209 daily cases over the last seven days. 

If ever the case numbers got low enough, even without herd immunity, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases said last Sunday, the country will start moving toward “normality.”

“It’s not going to be like a light switch on and off – we go from where we are right now to completely normal. It’s going to be a gradual,” Fauci told CNN’s Jim Acosta.”Little by little you’ll be seeing that approach to normal.”

It’s unclear just how many people will need to be vaccinated to get closer to normal, Fauci said. “I can’t give you what that exact number is now, because we don’t know it,” he said.

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If cases are low enough, Covid-19 becomes manageable.

“We may not get to zero. We probably won’t,” Dr. Ashish Jha, dean of the Brown University School of Health, told CNN’s New Day on Monday. “But if we can get the infections at very low levels, most of us can get back to our lives in normal ways. I think we can probably live with that.”

The way to get to normal is to continue to test, monitor for variants, and get as many people vaccinated as quickly as possible. Vaccines protect the individual and protect against the spread of variants.

“The more people get vaccinated, the fewer variants. You don’t want to see this disease develop a selective advantage that makes them more contagious and then to mutate in a way that it would make vaccines ineffective or make this disease more deadly,” said Dr. Claudia Hoyen, a specialist in infectious disease and molecular epidemiology with University Hospitals Cleveland Medical Center.

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Hoyen said the country is in a much better place than it was three months ago because so many of the vulnerable have been vaccinated. “But again, we still need to be sure the end game is that we try to stop this if we can,” Hoyen said.

Vaccination rates have been slowing.

“Every 1% from here on out will represent incredible progress for the country,” Andy Slavitt, a top official on the White House’s coronavirus response team, told CNN last week.

That’s why states and the Biden administration have started shifting their focus to incentivizing and encouraging people to get the vaccine.

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“I think we’re going to need so much more of these kinds of efforts — taking the vaccine to people, building trust with the communities,” said Samuel Scarpino, an assistant professor who runs the Emergent Epidemics lab at Northeastern University.

In Israel, he said, vaccinators visited a bar that picked up people’s tabs if they got vaccinated. New Jersey just started “Shot and a Beer,” a campaign that offers free beer to those of age who show their completed CDC vaccination card at participating breweries.

“I was joking that you could offer to let the tourists cut the long lobster roll lines in Maine in the summer if they got vaccinated,” said Scarpino.

“I think if we take that kind of attitude and get enough vaccines to people, we actually could get to the right level to make this severe disease that has been completely debilitating from a societal and economic perspective, to something that is not eradicated, but that is something more akin to the common cold, which becomes more manageable.”

CNN’s Ryan Prior, Jeremy Diamond, Kaitlan Collins, and Laura Ly contributed to this report

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