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 anticipation during the coronavirus outbreak.
One of the most optimistic predictions comes from Dr. Martin Makary, a public health expert at Johns Hopkins University, who believes the U.S. will reach herd immunity in early spring.
"As more people have been infected, most of whom have mild or no symptoms, there are fewer Americans left to be infected," Makary wrote in The Wall Street Journal on Feb. 18.
"At the current trajectory, I expect COVID will be mostly gone by April, allowing Americans to resume normal life."
Experts have said at least 60% of a population needs to have been exposed to the virus — either through vaccination or a previous infection — to reach herd immunity, but Dr. Anthony Fauci has been raising that number. "I think the real range is somewhere between 70 to 90%," he told The New York Times in December.
Fauci has predicted that the U.S. could achieve herd immunity against COVID-19 by late spring or early summer. In January, President Biden said the U.S. would be "well on our way toward herd immunity" by this summer.
Dr. Scott Gottlieb, the former U.S. Food and Drug Administration commissioner, estimated about 40% of Americans have antibodies right now — either from prior infection or inoculation — or enough immunity in the population for the virus not to be able to transfer as readily, he told CNBC on Feb. 22.
"I do think that infection levels are going to come down dramatically over the course of the spring and summer," Gottlieb said.
But some experts worry the recent extreme winter weather caused enough disruption to vaccinations that herd immunity may take slightly longer to reach. Another "wild card" could be COVID-19 variants — changes in the virus that may affect its ability to spread.
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.
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 most people are vaccinated?
Sexton: 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.
But 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.
This interview was condensed and edited for clarity. This story was updated in February 2021 to include relevant information about herd immunity in the U.S.