It's almost amusing to think of humans as part of a "herd" — a term that usually conjures up images of hoofed animals grazing in a field.
Yet herd immunity is an important concept when it comes to stopping the spread of disease among people. It’s also become the focus of intense debate during the coronavirus outbreak.
Sen. Rand Paul, R-Ky., and Dr. Anthony Fauci, the nation's top infectious disease specialist, sparred Wednesday over when "herd immunity" from the virus would be reached.
When Paul implied New Yorkers have "developed enough community immunity" to beat the spread, Fauci disagreed.
"I challenge that," Fauci said, noting about 22% of New York's residents have likely been exposed to the coronavirus. "If you believe 22% is herd immunity, I believe you're alone in that," he added.
Experts say at least 60% of a population needs to have been exposed to a virus to reach herd immunity, but the U.S. is nowhere near that.
The new coronavirus has infected up to 15-20% of the population in some states and less than 1% in others, Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, said Wednesday. That means more than 90% of the U.S. population overall remains susceptible to the virus, he noted.
Sweden hoped to achieve herd immunity before a vaccine for COVID-19 became available, resisting the kind of severe lockdowns other countries implemented as a way to slow down the virus.
So what is herd immunity and what’s the best way to achieve it?
TODAY asked Dr. Marybeth Sexton, an infectious disease expert, hospital epidemiologist and assistant professor of medicine at Emory University in Atlanta.
What is herd immunity?
Sexton: The idea is to reach a point in your population where enough people are immune to the disease — either because they’ve had it or because they’ve had a vaccine — so that it won’t spread very far if someone does get sick.
People protected by herd immunity are those who either shouldn’t have natural exposure to the disease because they’re at high risk for having complications, or those who can’t get a vaccine because they’re allergic to it or it’s contraindicated with some of their other medical problems.
If enough people around them aren’t susceptible to a disease, it’s not likely to spread and put them at risk.
How much of a population needs to reach that point to achieve herd immunity?
Sexton: It depends on how contagious a disease is.
For something like measles, which is airborne and highly contagious, herd immunity is usually thought of as over 90% of people being immune.
For COVID-19, which is less contagious, it may be more like 60-70% of the population would need to be immune.
How far away are we from that?
Sexton: We’re nowhere near that level. We’re a long ways away.
Sweden’s approach shows it’s a slow process — why?
Sexton: While COVID-19 is fairly contagious, it’s not going to spread as fast as something that's airborne, like measles.
I also think that despite the fact that Sweden didn’t lock things down to the same extent other countries did, individual people in Sweden don’t want to get this so they are likely taking some precautions. You probably don’t have completely normal daily activities going on and that slows the spread of this, too. That’s a good thing.
What are the concerns about getting to herd immunity?
Sexton: The problem with “natural” herd immunity — meaning you let the disease spread — is that if you assume COVID-19 has a mortality rate of about 1% and you think about 60-70% of the U.S. population getting it, that’s more than 200 million people, so that’s more than 2 million deaths to get to herd immunity.
There might be even more because if you let COVID-19 spread quickly, you could also overwhelm the health care system. That’s why we don’t advocate that as a strategy for addressing COVID-19.
The best way for us to get herd immunity for COVID-19 is going to be a vaccine.
What’s the strategy until we get a vaccine?
Sexton: That’s why people keep talking about social distancing because it’s the same general idea. Somebody who has COVID-19 doesn’t pass it on because they don’t get close enough to anybody to do that.
It puts more of a strain on people, the economy and the educational system because social distancing is hard to do. But when you look at the alternative of a lot more deaths that might be preventable, it’s still preferable.
How will we know herd immunity has been achieved for coronavirus?
Sexton: Technically, you’d measure people’s antibodies in a representative group in the population and know that you have 60-70% of people who have them.
The problem is we still don’t know if all of our antibody tests are created equal in terms of accuracy. We don’t know if having antibodies makes you immune — it probably does based on our prior experiences with coronavirus, but we don’t actually know that for sure. And if you do have immunity, we don’t know how long it lasts.
For the other coronaviruses, it’s been estimated anywhere from 40 weeks to two years, but this is not permanent protection. So even if today, you had 60-70% of your population that was immune, that might not be the case a year from now.
Herd immunity might not last if you achieved it through natural infection.
Are we on the right trajectory?
Sexton: We don't want to be on a trajectory to herd immunity for coronavirus if we’re talking about natural infection. A massive outbreak would be disastrous for our health care system, and at the rate it’s spreading currently, you’re not going to achieve herd immunity anytime soon.
Achieving immunity through vaccination is the much safer way to do this. While we wait for that to be available, our goal should be to continue practicing social distancing, mask wearing, hand washing, getting tested and not going out in public or to work if you feel sick.
This interview was condensed and edited for clarity. This story was updated in September 2020 to include relevant information about herd immunity in the U.S.