Short Wave - It's Okay To Let Go Of Herd Immunity

Episode Date: June 2, 2021

Researchers say the concept of achieving herd immunity threshold isn't the right finish line to end the pandemic. It's an elusive number to define in the first place, and it changes under various circumstances. Science correspondent Geoff Brumfiel talks with Maddie about the complexities in even defining the number and what the public should focus on instead.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. NPR science correspondent Jeff Brumfield. I understand you want to have a little talk with us today. That's right, Maddie. So back in August of last year, you and I chatted about something called herd immunity. Yes, herd immunity, this idea that if enough people are immune to the coronavirus, either through vaccination or infection, this disease will run out of places to spread. It'll burn out.

Yeah, that's right. I don't know how much of that conversation can. you remember. Well, I certainly remember you getting very excited once you learned about it from a YouTube video of a guy in his backyard. Oh, those were some good times. We spent a lot of time talking about what made herd immunity hard to achieve and how we probably weren't going to get there. And after that conversation, I kind of thought it might go away. But it turns out, the weight is on for enough doses to achieve herd immunity. How worried are you? How worried are you? about the U.S. reaching herd immunity, do you think it's still possible to get the vaccine before we can say

herd immunity? Do scientists not think we will actually reach herd immunity? How do we achieve herd immunity? When do we go back to normal? So the question now is when will we reach herd immunity? People are still talking about herd immunity, and I'm here to tell you today, Maddie, that some scientists want it to stop. They think the public is spending too much time worrying about it. I think we're focusing too much of our time, our effort on quibbling over a number. So today on the show, herd immunity, why are we still talking about it?

Do we actually need it and what we should be focused on instead? This is Shortwave, the Daily Science Podcast from NPR. Okay, Maddie, just to remind everyone, you're the biologist and I'm the math guy. So maybe you should get us started with a little definition of what we're talking about when we talk about herd immunity. Sure. So herd immunity is an idea that's been around for at least a century, a phenomenon observed in an actual herd of cows. And very basically, it works like this. If you introduce a pathogen, like a virus, it will spread. But as individuals recover, they become immune. So eventually you reach a point where the virus can't spread because it's surrounded by a lot of immune individuals. And that's herd immunity. Even the very few in that population who don't have immunity yet, or protected. Right. And the math part is there's this mathematical turning point where the virus really finds itself in trouble. And that's called the herd immunity threshold. It's basically the point at which the virus can't find enough new people to infect. It varies from disease to

disease. And we'll talk more about why in a second. But for now, just know that the threshold is the point at which the virus starts to flame out. And from that point on, eventually it disappears. Okay. So that's the definition. But over the past year, herd immunity has gone from something that relatively few people knew about to something that a lot of people are talking about. So how did that happen, Jeff? Well, as far as I can tell, it started back in February and early March. The coronavirus was spreading through Asia. It had just hit Italy.

And across Europe, modelers, those are people who build computer models of disease outbreaks, were really worried. And these modelers ran projections, which showed that this was unstoppable, uncontrollable. That's Devi Schreeder. She's from the University of Edinburgh. And so this led to this approach towards herd immunity, which is just let the virus go, let nature take its course. Right. And Jeff, I remember this is what we spent a lot of time talking about last time, this idea of letting a lot of people catch COVID in order to reach this, like, quote, natural, I guess you'd call it herd immunity, which is just a really bad idea in all of the ways. Yeah, it's such a bad idea. But it appealed to conservative policy. politicians at the time because they were really worried about lockdowns and the economy. Perhaps you could sort of take it on the chin, take it all in one go.

British Prime Minister Boris Johnson toyed around with the idea for a while. So did the government of Sweden. But then a computer model from Imperial College in London showed it would cost hundreds of thousands or even millions of lives if you just let the virus go. Yeah. So herd immunity without vaccines as a policy, terrible idea. But herd immunity as a more general. concept seems to have stuck around in the public discourse. I mean, can you explain why that is? I'm not actually entirely sure I can. But that scientist you heard at the top saying we're

spending too much time on one number, her name is Lauren Ansel Myers. And she's at the University of Texas at Austin, where she builds computer models of COVID. She says for her, this pandemic has felt really different. I don't remember the public looking at models during the H1N1 pandemic, during, you know, Ebola during... In the age of COVID-19, it feels like computer models and forecasts are everywhere. They were on dashboards. They were on TV. Presidents were citing them.

And so I think as the models became more public, I think some of the concepts did as well. And that herd immunity threshold we talked about in particular, after it got into the public mind, it was just really hard to shake. Because whether that threshold is 50% of the population immune or 80% or whatever, it's basically one number that spells the end of the pandemic, and it feels really concrete. It gives us an anchor or it gives us a goal. Right. And another factor was in November and December, when we learned just how effective the vaccines are. Because vaccination is the actual way we've reached herd immunity with other diseases.

It kind of lets you build up the immune population quickly without much illness or death. Totally, totally. Yeah, when the vaccines began to roll out in December, you heard public health officials talking about it a lot again. Here's Anthony Fauci, morning edition. If you get that level of herd immunity, you could essentially crush this outbreak in this country. And Fauci's right. If we could reach that level of herd immunity, it could crush the outbreak. But like we've talked about it before, it's unlikely that we'll actually get there.

Like certainly if we're talking about national level herd immunity. That's right, that's right. You know, things like hesitancy, access, and a lot of other reasons. But, you know, as I've reported on this story, there's something else I've learned you have to think about. And that's the herd immunity threshold itself. It's determined by computer models. But those computer models are much simpler than the real world. We make a bunch of assumptions that we know aren't true. So that's Sam Scarpino, who's at Northeastern University. So, for example, he says, to figure out how a disease spreads, you've got to model how people interact.

And in a lot of those models, those interactions are just randomized. It just makes things simpler. So the way I decide who I'm going to have lunch with is I put everybody in Boston in a bag, and I shake the bag up, and I draw somebody out at random, and that's who I have lunch with. And, of course, that's not how the real world is. In real life, we mostly have lunch with our friends, our family. And how you model those social networks has a huge impact on the herd immunity threshold. So different computer models can give very different numbers for the threshold.

Right. Plus people can change their behavior, right? Like if they see an outbreak in their city, they might cancel lunch and stay home. Exactly. Or it could go the other way. They think the outbreak is over, and so they invite a hundred or so of their closest friends for a house party. That could become a super spreader event. The point is all of these real-world behaviors are hard to build into computer models. And then there's the problem of who you count as part of the herd. Right. I mean, herd immunity is always local. There's always a herd, but then there's people outside of the herd. Exactly. So Lauren Ansel Myers gives this example of imagining a city. You know, you may hear numbers like 50% of a population are immunized, but is that really 50% in every single neighborhood? Or do we have some pockets of very high levels of immunity and other pockets of low levels of immunity? If the east side of a city is completely vaccinated and the west side doesn't have a single dose, well, then you can't.

you've still got a big problem for that city's hospitals. They could still fill up. Right. And if we're talking about the entire United States, there are already big regional differences in vaccinations and caseloads. Exactly right. And regional outbreaks will matter, especially going into the fall and winter. And that brings us to the final ding against using this herd immunity threshold. It's a finish line. And that is, it moves around, Maddie. People talk about herd immunity as if it's a sort of endpoint. You either have it or you don't. don't, and once you have it, you keep it. And that's not true either. Mark Lipsich is at Harvard. And his point is that the herd immunity threshold is changing all the time.

So in the wintertime, for example, people move indoors where there's less air circulation, and that makes it easier for the virus to spread. And then there are the variants of the virus itself. The U.K. variant and quite possibly others of the major variants are more contagious, more transmissible. And that means that the herd immunity threshold for them, even if the vaccine works the same, is higher. And things like reinfections matter, too. If immunity fades, that would mean people who had been vaccinated or had COVID could get it again and spread it. Exactly. And it's probably a little bit early to say how big a factor that will be, but it will change things, you know. And look, we've seen this every single year with the flu, right? The flu comes in the fall. It gets worse in the winter and it fades in the spring. And what's actually happening there is there's this constantly shifting mix of vaccines and human behavior and virus mutations that is causing the herd immunity threshold for the flu to move around multiple times every single year. Okay. So, Jeff, I think we've made a pretty good argument for why we shouldn't focus on nationwide herd immunity as our way out of this.

It's a tough thing to model. The threshold moves around all the time. And it probably won't be how we stop this virus in the States. Right. And to be clear, that doesn't mean we won't get it under control. We will. But we have to let go to this idea that the coronavirus is just going to burn itself out and disappear. The reality is this virus is going to be with us for a long time, hopefully at low levels, kind of like the flu. And we'll just have to deal with it every year, you know, adapt our vaccines, continue to track it, continue to test for it and live with it. Right. I mean, we will have outbreaks. And that means we need to be able to test people, isolate them, prevent that outbreak from getting out of control when people do get sick, which, to be honest, Jeff, is something we have really struggled with as a country, but we'll have to nail down going forward. And right this moment, scientists I talked to told me the same thing. We don't need to worry about this theoretical mathematical threshold. We don't need to argue about it. We've got a real solution in our hands. We've got a real solution in our hands. highly effective vaccines. And Devi Schreter says the vaccines are the message. I think we need to make this a vaccine preventable disease. I think that's the term to use, that how we've dealt with other things like measles, moms, diphteria, tetanus, rubella.

We've had many infectious diseases that have caused major devastation for our societies, and we have vaccinated our way through them. And this is what we want to be doing now. All these scientists agree. It is really simple. Just get vaccinated. Everybody doing. it. The more people who do it, the better things get. This pandemic ends when enough people are protected from severe illness, and selfishly, you want to be protected from severe illness. And doing that helps you, and it helps your community. Just ignore the herd immunity threshold. All the computer models show vaccination is going to make a huge difference. Things are going to get better. And the more people who are

vaccinated, the better things will get. All right, Jeff Brumfield. We appreciate you bringing this to us today. You're welcome, Maddie. This episode was produced by Thomas Liu, edited by Giselle Graysson, and fact-checked by Tyler Jones. I'm Maddie Safaya. You're listening to Shortwave, the Daily Science Podcast from NPR.