Future Tense

Most People Are Thinking of Herd Immunity All Wrong

Rows of white stick figures holding shields emblazoned with a coronavirus symbol. Interspersed among them are three yellow figures without shields.
Photo illustration by Slate. Photos by Getty Images Plus.

The phrase “herd immunity” has become our utopia—a promised land where we can live life as it was before the pandemic. But it’s appropriate that utopia means “nowhere,” because whether that’s even possible has now come under debate. This is hard to hear because scientific pessimism about herd immunity feels like giving up on the whole idea of returning to normal life. But we can still look forward to the pandemic’s end if we understand what herd immunity really is. Spoiler alert: It’s complicated.

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In the fantasy version of herd immunity, when we get to the end of this nightmare, it will be sudden. We will cross a line and the pandemic will be declared over. We’ll burn our masks in the kitchen sink, book vacations, and start processing our PTSD. COVID-19 will become a relic of history. The end. No gray areas.

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Unfortunately, this fantasy doesn’t map very well to what herd immunity really is: a scenario where so few people have a disease that even those who can’t be vaccinated will never get sick, simply because they’ll never have the chance. One of herd immunity’s hallmarks is that cases are so rare that they make news. It’s not a cutoff, it’s not a magic number, and it’s usually not even an ending.

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Among human diseases, smallpox is the only example of herd immunity taken all the way to its limit. Thanks to a sustained global vaccination effort, for the past 40 years running, the number of smallpox cases worldwide has been zero. Smallpox has been eradicated from planet Earth through vaccination. It took 200 years to get there. Like most people my age, I am not vaccinated for smallpox, and I’ll never get it because there is no smallpox left to get.

Eradication seems a very unlikely ending for COVID-19 at this point. If we do reach herd immunity, it will require continued work to keep it. This is herd immunity reality.

Measles outbreaks in the United States over the past few decades demonstrate the fragility of herd immunity. We reached a nadir in 2000, when the Centers for Disease Control and Prevention declared measles eliminated in the United States. But herd immunity for one of the world’s most contagious diseases erodes quickly. Though our individual immunity usually lasts for life, every year babies are born, and every year we need to vaccinate more 1-year-olds to keep up. If we quit vaccinating for measles today, it wouldn’t be long before the number of kids who never got a shot was enough to cross measles’s herd immunity threshold. Even a small fraction of parents who decide measles vaccines are not right for their children can lead to outbreaks—especially if their kids spend time together. Because, as herd immunity expert Carl T. Bergstrom has noted, disease transmission happens locally.

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The herd immunity threshold is the number of immune people it will take to maintain herd immunity once cases reach their nadir—not the number it will take for cases to start dropping, and not a number used to declare the pandemic ended.

Pundits complain that we’re “moving the goalposts” when scientists debate about what the threshold is, exactly. The truth is we just don’t know what the threshold is because it’s a moving target. It is very sensitive to small changes in both human and viral adaptation. Little things matter. A lot. Things like a change in the daily riders on the subway system, or one big party where 500 people are exposed. The threshold is also sensitive to small changes to the virus and how it infects humans. If it gains an adaptive advantage somewhere along its world tour, the threshold will shift in ways that no one can predict. We often don’t measure these things well, or even know what they are until after the fact. In addition, these effects are often not linear, so measuring the right things well wouldn’t necessarily lead to better predictions. In short, it’s complicated. It’s so complicated that the study of herd immunity is itself housed in the niche field of complexity science.

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But debates about the exact herd immunity threshold are to some extent missing the point. It doesn’t matter that we don’t know the exact threshold. The specifics are only important—or even knowable—after herd immunity is reached. It’s not the magic number for getting back to school without masks; it’s the target to keep the pandemic from returning once it’s over.
Even then, it’s not exact. It’s a policy goal, not a light switch.

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Scientists don’t agree about whether herd immunity for COVID-19 is even a possibility at this point largely because we can’t predict whether the virus will mutate in important ways. In order to end the pandemic for good and reach herd immunity, we need to stop giving SARS-CoV-2 the chance to experiment on us. Every new infection is a chance for the virus to improve on its strategy. Every replication is an opportunity for SARS-CoV-2 to come upon an adaptation that isn’t well described by the “wanted” poster provided to our immune systems via the vaccines or previous infection. This is why it’s crucial for even low-risk people to get vaccinated (yes, including kids).

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If herd immunity is possible, it’s a long way off. Widespread vaccine hesitancy and the fact that vaccines aren’t available to everyone yet put it out of reach for now, even in the countries that have bought up most of the world’s vaccine supply. For example, the New York Times recently republished federal survey data on vaccine hesitancy suggesting much of the United States will struggle to reach herd immunity thresholds anytime soon based on hesitancy among adults alone. Due to an apparent issue with double-counting people who are vaccine hesitant, the Times later revised its map to be slightly more optimistic—but it’s still grim when we add vaccine-hesitant adults to the total population still ineligible due to age.

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And a map of vaccine uptake in the United States doesn’t even touch the whole picture. A pandemic is by definition a global problem, and much of the world lags behind us by orders of magnitude. About 182 million people in the U.S. have still not received a vaccine, while about 4.6 billion in Asia have not yet received one dose. Many Americans are just now awakening to the fact that global inequality in vaccine distribution means the pandemic will not formally end for years to come.

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In the meantime, we’ll have something like a receding tide. As more and more people are vaccinated, those who are at highest risk for ending up in the ICU will be protected. We’ll all have growing protection from outbreaks as people who are high transmitters get vaccinated. (Looking right at you, young adults.) Cases will start to ebb overall, but we can expect some ups and downs. There will be more variants, and there will be more surges. We’ll soon have low enough transmission that we will be able to resume most normal activities. We will not be wearing masks forever. As expert Marc Lipsitch and others have said, herd immunity is neither necessary nor sufficient to end the pandemic.

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If we do reach it, it won’t be like crossing a finish line. There will be no moment when we know for sure that our risk of COVID-19 has returned permanently to zero and everyone runs outside at once to celebrate by kissing strangers. The end of the pandemic will be a slow fade. That will be frustrating. We need to let go of the idea that someday everything will be just like before.

And that’s not easy. We wish we could undo the horror of living through a pandemic. The anxiety and trauma are real. It’s also true we will not be living in a pandemic forever, and that vaccines are the way out of this mess, even if herd immunity isn’t.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.

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