"Herd immunity," in which the vast majority of a population has immunity, has been cited as the key to ending the COVID-19 pandemic. But public health experts are split on whether it can be achieved.

Transcript

AILSA CHANG, HOST:

Herd immunity - that is, having the vast majority of the U.S. population immune to the coronavirus - has been cited over and over again as the key to ending this pandemic. But now, more than a year in, public health experts are split on whether it can ever be achieved. Dr. Ali Mokdad of the University of Washington's Institute for Health Metrics and Evaluation joins us now.

Welcome.

ALI MOKDAD: Thank you for having me.

CHANG: Thank you for being with us. So can you just explain why are people saying now that we might not ever achieve herd immunity?

MOKDAD: Herd immunity is when you have a certain percentage of the population immune to a disease - in this case, COVID-19. So there is no room for the virus to circulate, and you're able to contain the virus. You'll have an outbreak here and there, but it's not anymore an epidemic. And from what we know from COVID-19, it would require a high number of people to be vaccinated and immune in order to reach herd immunity.

CHANG: Right.

MOKDAD: Compared to other viruses, it's somewhere between 75 to 85%. We in the United States are not going to get to herd immunity before this winter simply because right now the vaccine is only authorized for adults, not for children...

CHANG: Right.

MOKDAD: ...Fifteen or younger. So you have about 25% of your population not eligible for the vaccine.

CHANG: OK. But down the road, what is your view? Is herd immunity within reach?

MOKDAD: Yes, definitely. Down the road, we can get to herd immunity if we have a vaccine that we can provide to two years or older and if Americans are willing - who are eligible to take the vaccine are willing to go ahead and get the vaccine.

CHANG: You're talking about vaccine hesitancy at the moment. How significant do you think vaccine hesitancy is as an obstacle to ultimately achieving herd immunity?

MOKDAD: It's the only obstacle that we have right now between us and herd immunity simply because we have many Americans who are not willing to take the vaccine. Otherwise, we have vaccines that are safe and effective, and they will provide herd immunity. And we know children are going to get the vaccine. Twelve to 15 is coming very soon and, after that, 6 to 11.

CHANG: If we don't reach this so-called herd immunity, though, this 75 to 85% of people who are immune to the coronavirus, does that mean, Dr. Mokdad, that we will always live with some COVID in the U.S. just like we have all resolved to live with the possibility of the flu every year because we never reach herd immunity when it comes to any particular flu strain, right?

MOKDAD: Exactly. That's what will happen to all of us. We will have to live with COVID-19 for a long time. And depending on the number of people who are susceptible out there, how many people are refusing to get the vaccine, we may have a new variant that will make us, those who got the vaccine, face a new variant that the vaccines are not as effective. And herd immunity will also, for all of us who have been vaccinated - will require us to take a second shot, or a booster, we call it. That way, we're always trying to catch up with a new virus that's circulating every now and then.

CHANG: If we do end up eventually reaching herd immunity in the U.S., if the rest of the world doesn't, it doesn't make our achievement as effective.

MOKDAD: No, definitely not. It means if there is a new variant that will make the vaccines less effective, we're starting the clock again and counting again. And we need to be ready to develop a new vaccine and booster for order to stay ahead of that virus.

CHANG: That is Dr. Ali Mokdad of the Institute for Health Metrics and Evaluation.

Thank you very much for joining us today.

MOKDAD: My pleasure. Thank you. Transcript provided by NPR, Copyright NPR.