What Is Herd Immunity? (And What Happens When We Get There?)

If you haven't had your shot at a shot yet, you're probably getting much closer to that point—on May 1, states will open the COVID vaccines to the general adult population, and already, more than 143 million doses have been put into arms across the United States.

That's a huge step toward getting us to herd immunity, which experts expect us to achieve when 75 to 85 percent of the population has been immunized or had the illness and recovered. (We're currently at more than 15 percent of the population vaccinated—and according to NPR's vaccine tracker, we're on track to reach herd immunity sometime between Halloween and Thanksgiving.)

But what happens when we reach herd immunity? And what benefits can we expect when we have a majority of our population vaccinated?

What Is Herd Immunity?

Herd immunity happens when enough of the population cannot be infected by a particular virus, either due to vaccination or having had the virus and recovered. Infections may still happen here and there among the small percentage of people who aren't immune, but because the overwhelming majority of the population is taken out of the equation, it's harder for the virus to find a new host to spread into.

"Herd immunity will prevent these large surges like we're having now," says Jason Kessler, M.D., section chief of infectious disease at Morristown Medical Center in Morristown, N.J. "Herd immunity doesn't prevent anyone, anywhere from getting it—you will still see localized infections in particular communities."

The magic number for herd immunity is different for different diseases. "An infection that's very transmissible, like measles, requires that a very large portion has gotten the disease or been vaccinated with a very effective vaccine to achieve herd immunity," Dr. Kessler says. COVID-19 is less transmissible than measles, so we'll need slightly fewer people to get vaccinated against it than we do for measles.

What Are the Roadblocks to Herd Immunity?

With vaccine supply ramping up quickly and millions of people getting the vaccine each day, we're building immunity quickly. But two big obstacles currently stand in the way of reaching herd immunity—kids and people who refuse the vaccine. With vaccines still being tested for the under-16 crowd, and between 20 and 25 percent of the population saying they'll refuse the vaccine, we won't reach herd immunity until one group (or both!) get the vaccine.

"Hopefully, the needle could be moved a bit on those with vaccine hesitancy," Dr. Kessler says. "A lot of that 25 percent are waiting to hear more about whether it's safe—but some won't be moved at all." Children should have an approved vaccine by late summer or early fall, Dr. Kessler says, so we may be able to move into the holiday season that looks a lot more like "normal."

What Happens When We Get Herd Immunity?

It won't be as simple as flipping a switch—we won't go from wearing masks one day to burning them the next. Instead, expect a slow and steady return to normalcy.

"It isn't going to come all at once," Dr. Kessler says. "There will be a slow unwinding—first the least risky activities, while some of the activities that are most risky remain out of reach."

For instance, schools may reopen full time in the fall, but kids will still be socially distanced and wearing masks, Dr. Kessler predicts. "I'm not sure we're going to be having large indoor concerts—or if we do, everyone will have to confirm they're vaccinated, and probably wear masks at the same time. There won't be a day we declare End of Covid Day."

What About COVID Variants?

Variants could also be a roadblock in our path toward herd immunity—especially if a new variant crops up that can outwit the immune response we've developed due to vaccines. Expect that you'll be rolling up your sleeve for another COVID shot sometime in the future.

"COVID has changed and mutated, but not [with] the same frequency and intensity as influenza," Dr. Kessler says. "I don't know that we'll be getting an annual COVID vaccine, but I don't think it's a one-time event. We'll likely need intermittent vaccines—and the frequency is not clear yet."