RACHEL MARTIN, HOST:
How will the pandemic end? The answer sometimes gets boiled down to two words - herd immunity. This is a tipping point when enough people are either infected or vaccinated, and the virus has nowhere left to spread. But as NPR's Geoff Brumfiel reports, some scientists think the fixation with herd immunity needs to come to an end.
GEOFF BRUMFIEL, BYLINE: The idea of herd immunity has been around for over a century in academic circles, but it really didn't hit the public until the spring of 2020. The coronavirus was spreading in Europe, and politicians like British Prime Minister Boris Johnson started talking about it on television.
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PRIME MINISTER BORIS JOHNSON: We'll see what the scientists advise. The best thing I find - you know, these guys are brilliant.
BRUMFIEL: The group of scientists Johnson was listening to were mainly modelers, according to Devi Sridhar, a professor of public health at the University of Edinburgh in Scotland. Modelers build computer simulations or models of the future.
DEVI SRIDHAR: And these modelers ran projections, which showed that this was unstoppable, uncontrollable. And so this led to this approach towards herd immunity, which is just let the virus go, let nature take its course.
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JOHNSON: Perhaps you could sort of take it on the chin, take it all in one go and allow...
BRUMFIEL: Of course, we now know that COVID can be stopped with things as simple as cloth masks and that the deaths from this herd immunity strategy would have been even more staggering than they were with the lockdowns and other protective measures that governments ended up taking.
SRIDHAR: I think if you could go back in time and they were completely honest, both the public health advisors and the leadership would say that was the wrong approach.
BRUMFIEL: But the idea of herd immunity stuck around because there's another way to get there, and that is vaccination. When the first vaccines rolled out in December and looked incredibly effective, experts like presidential adviser Anthony Fauci began to talk a lot about it.
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ANTHONY FAUCI: If you get that level of herd immunity, you could essentially crush this outbreak in this country.
BRUMFIEL: The appeal is clear. The herd immunity threshold represents a simple goal - more or less one number that spells the end of the pandemic. It feels really concrete, sort of something to grab onto in a time filled with so much uncertainty, until you speak to the modelers who actually calculate herd immunity.
SAMUEL SCARPINO: We make a bunch of assumptions that we know aren't true.
BRUMFIEL: Samuel Scarpino runs the Emergent Epidemics Lab at Northeastern University. For example, he says computer models often drastically oversimplify the way people interact with each other.
SCARPINO: 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.
BRUMFIEL: In the real world, people only have lunch with their social contacts. And that changes the herd immunity threshold.
LAUREN ANCEL MEYERS: It's also complicated by the fact that we may not have an even distribution of immunity.
BRUMFIEL: Lauren Ancel Meyers is at the University of Texas at Austin. She says the herd immunity threshold is usually presented as a single overall percentage of a population. But in a given city...
MEYERS: 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?
BRUMFIEL: If the east side of a city is immunized and the west side isn't, then the hospitals could still be overwhelmed. Finally, herd immunity is often described as a finish line to be crossed. But that is an illusion, says Marc Lipsitch at Harvard University.
MARC LIPSITCH: People talk about herd immunity as if it's a sort of endpoint - you either have it, or you don't, and once you have it, you keep it. And that's not true, either.
BRUMFIEL: Things like new variants or the time of year can cause huge swings in how many people need to be immune to reach herd immunity. In the months since December, there have been real world complications. Data out of Asia and Brazil suggests reinfection may be more common than thought. Vaccine hesitancy has emerged as an issue, as has the rise of more transmissible variants. All of this changes whether we can get to the herd immunity threshold.
LIPSITCH: Based on the best calculations I know how to do, it will be impossible or very difficult to reach in many parts of the United States.
BRUMFIEL: But it could all change again in the future. And it's that squishiness that makes all of these scientists say it's time to stop talking about herd immunity.
MEYERS: I think we're focusing too much of our time, our effort on quibbling over a number.
BRUMFIEL: Instead, Lauren Ancel Meyers says all the computer models show a very clear way forward.
MEYERS: Every vaccination gets us a step closer. Every vaccination makes our community, our society a safer, healthier place.
BRUMFIEL: There's no magic finish line, but as long as vaccinations continue, things can and will get a lot better than they are now.
Geoff Brumfiel, NPR News.
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