[S2-E23] Endemicity Is Meaningless

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Endemicity Is Meaningless

This one word has come to imply a single pandemic end point. Actually, we have no idea what happens next. By Jacob Stern and Katherine J. Wu

By now, we’ve all heard some version of how this ends. The same story has certainly been told often enough: We missed our chance to wipe the new coronavirus out, and now we’re stuck with it. Our vaccines are stellar at protecting against serious disease and death, but not comprehensive or durable enough to quash the virus for good. What lies yonder, then, is endemicity—a post-pandemic future in which, some say, our relationship with the virus becomes simple, trifling, and routine, each infection no more concerning than a flu or common cold. Endemicity, so the narrative goes, is how normal life resumes. (Some pundits and politicians would argue that we are, actually, already at endemicity—or, at the very least, we should be acting as if we are.) It is how a devastating pandemic virus ends up docile.

Endemicity promises exactly none of this. Really, the term to which we’ve pinned our post-pandemic hopes has so many definitions that it means almost nothing at all. What lies ahead is, still, a big uncertain mess, which the word endemic does far more to obscure than to clarify. “This distinction between pandemic and endemic has been put forward as the checkered flag,” a clear line where restrictions disappear overnight, COVID-related anxieties are put to rest, and we are “done” with this crisis, Yonatan Grad, an infectious-disease expert at Harvard, told us. That’s not the case. And there are zero guarantees on how or when we’ll reach endemicity, or whether we’ll reach it at all.

Even if we could be certain that endemicity was on the horizon, that assuredness doesn’t guarantee the nature of our post-pandemic experience of COVID. There are countless ways for a disease to go endemic. Endemicity says nothing about the total number of infected people in a population at a given time. It says nothing about how bad those infections might get—how much death or disability a microbe might cause. Endemic diseases can be innocuous or severe; endemic diseases can be common or vanishingly rare. Endemicity neither ensures a permanent détente nor promises a return “to 2019,” Abraar Karan, an infectious-disease physician and global-health expert at Stanford, told us. Its only true dictate—and even this one’s shaky, depending on whom you ask—is a modicum of predictability in the average number of people who catch and pass on a pathogen over a set period of time.

Endemicity, then, just identifies a pathogen that’s fixed itself in our population so stubbornly that we cease to be seriously perturbed by it. We tolerate it. Even catastrophically prevalent and deadly diseases can be endemic, as long as the crisis they cause feels constant and acceptable to whoever’s thinking to ask. In a rosy scenario, reasonably high levels of population immunity could bring the virus to heel, and keep it there; its toll would be roughly on par with the flu’s. As coronavirus cases drop from their Omicron highs in the United States and other countries, there’s at least some reason to hope things are bending in that direction. But at its worst, endemicity could lock us into a state of disease transmission that is perhaps as high as some stretches of the pandemic have been—and stays that way.

If endemicity contains a world of possibilities, not all of them good or even better, then it makes a poor goal, and an impractical conceptual framework for any action aimed at managing COVID in the months, years, and decades ahead. Simply declaring endemicity gets us nowhere. It doesn’t answer the real questions about what we want our relationship with this virus to be. And it doesn’t erase the difficult decisions we’ll need to make if we plan to shape that future, rather than risk letting the virus make our choices for us.

It is an unfortunate coincidence that the word endemic begins with end. The arrival of endemicity is actually the beginning—of a long and complicated relationship between a pathogen and its host population. En demos. In the people.

Exactly what kind of long and complicated relationship endemicity denotes, though, is impossible to say, even for experts. “It’s a very nonspecific notion,” Karan said. “There’s really no definition of endemic,” Emily Martin, an epidemiologist at the University of Michigan, told us. And the word is so “muddy and misused” that it’s “really hard to pin down why someone is using it wrong,” Ellie Murray, an epidemiologist at Boston University, told us. We spoke with more than a dozen experts for this article, and nearly every one of them explained endemicity differently.

For some, endemicity entails a disease with stability, constancy. For others, it means one that concentrates in a specific geography. Some think a degree of predictability is a prerequisite; some do not. Others still adhere to a more technical definition: Endemicity refers to a state in which over, say, a year, each person who catches an infection will on average transmit it to one other person, so that the overall case burden neither rises nor falls. Much of the population has at least some immune protection, and the spread of the disease is limited by the rate at which vulnerable people are introduced (or reintroduced) into the population, by birth or waning immunity. Think of a bathtub with water flowing in and draining out at the same rate. But some experts think that notion’s too strict: Any amount of sustained spread, however turbulent, can qualify as endemicity.

What experts do agree on is that endemicity is not monolithic. The water in that tub might be hot or cold; the level it plateaus at can be very high or very low. The world’s pathogens run the gamut. Viruses such as herpes simplex 1, which causes cold sores and, less commonly, genital herpes, are considered endemic throughout the world. In the United States, HSV-1 affects, by some estimates, at least half of Americans, though most of the infections are asymptomatic or not terribly severe, especially among adults. Malaria, meanwhile, sickens more than 200 million people a year, and kills at least 400,000, most of them under the age of 5. That, too, is endemicity.

Then there are flu viruses—so often held up as the paragon of endemicity, but actually a better example of just how absurdly confusing endemicity can get. In most places, flu viruses are seasonal, surging in the fall and winter, then subsiding in the warmer months. (They circulate year-round in parts of the tropics.) But they can also erupt into pandemics, as they did in 1918, 1968, and 2009, then tick back down. Flu is one of many examples that show why endemic can’t be thought of as the inverse of pandemic; the two terms are not opposite ends of a spectrum. Endemic doesn’t mean the virus is “suddenly not going to hurt us,” Murray said.

Flu viruses actually present such a bizarre case of boom and bust that many researchers don’t consider them to be endemic at all. The experts we spoke with were pretty much evenly split among saying Flu is endemic, Flu is not endemic, and some version of Who knows? or It depends. This set of viruses, the not-endemic camp argues, are just too erratic to warrant the label, even when flu doesn’t reach pandemic proportions. The seasonality seems reliable, but that may not be enough to count as stable. The magnitude and severity of these annual-ish cycles can vary widely; some strains will play nicer with humans than others. One year, a flu virus will kill about 10,000 Americans. Another year, it will kill six times that. The question of the flu’s endemish nature takes on an almost existential cast: What does it mean to expect something?

Others in the not-endemic camp contend that, in addition to being too unpredictable, the flu is also too global. An endemic pathogen, they say, must be restricted to a population in a specific geographical region, rather than “just everywhere,” Seema Lakdawala, a flu virologist at the University of Pittsburgh, told us. (The CDC agrees.) The Emory University virologist Anice Lowen, meanwhile, isn’t so bothered by the flu’s ubiquity. “I would call it endemic to humans,” she said. Martin, of the University of Michigan, doesn’t put herself in either camp. “Things get wiggly,” she said, “when you’ve got something like the flu.”

Pretty much all we can say for sure about the flu is that—as Malia Jones, a population-health expert at the University of Wisconsin at Madison, told us—it is “a huge pain in the butt, but also not a global pandemic, most of the time. Unfortunately, there is not a single word for that.”

Endemic or not, flu might still represent our best benchmark for what post-pandemic COVID will look like.

Yes, okay, it remains true: COVID is not the flu, especially not while the pandemic’s still raging, so many people around the world lack solid immunity to the new coronavirus, and variants burst out at blistering speeds. In the past two years, COVID has already killed more people than any flu pandemic we have on record. But the comparison becomes less fraught when we project a lot further—a lot further—into the future. Flu, fundamentally, is another respiratory virus that’s enmeshed itself quite messily into our population. Which makes it, “with caveats, an excellent model” for what might happen next, Martin told us.

Such familiarity might feel comforting, because flu has come to seem pretty normal to us—most people can visualize, maybe even shrug off, its threat. We name a season in honor of the flu; we de