We’re Not at Endemicity Yet
Too many people still have no protection against the coronavirus.
No one knows exactly what endemic COVID will look like, but whatever it looks like, this—gestures at the current situation—ain’t it. COVID is not yet endemic. There is little doubt that the coronavirus will get there eventually, when almost everyone has been vaccinated or infected or both, but right now we are still living through a messy and potentially volatile transition period. Cases are ticking up again. A new variant is afoot. The challenge ahead is figuring out how to manage the transition to endemicity, however long it takes.
COVID is not yet endemic because too many people still lack any immunity from either vaccination or infection, here in the United States and globally. Europe is a cautionary tale in this regard: Countries such as Germany and Austria that have slightly better vaccination coverage than the U.S.—68 percent and 66 percent, respectively, compared with 60 percent here—are nevertheless seeing their cases and hospitalizations soar in yet another wave. Even with most people vaccinated, there isn’t enough immunity to blunt big and fast surges of Delta. Just 15 percent of the population without immunity is still a huge absolute number in a country with millions of people, says Lloyd Chapman, an infectious-disease modeler at the London School of Hygiene and Tropical Medicine. Chapman and his colleagues have estimated the number of unvaccinated and unexposed people who could still be hospitalized for COVID in Europe based on each country’s age structure. (He is planning to do a similar analysis for the U.S.) “The main headline point would be that,” he says, “there’s still a long way to go.” And that was before Omicron. The new variant could be even better at evading previous immunity than Delta, and its spread might push endemicity further off into the future.
Endemic is now often used to describe the point where the virus’s danger fades to the levels of the flu or, better yet, the common cold. In its technical definition, though, endemic describes an equilibrium, a point where the immunity gained in a population is balanced by the immunity lost. Immunity can be gained through vaccination or infection, and it can be lost through waning immune response, new variants, or population turnover as susceptible babies are born. A pathogen’s impact becomes a lot more predictable and stable when it’s endemic. During their long coexistence with us, the viruses that cause the common cold and flu have all found this equilibrium with some seasonal fluctuation; we are first infected or vaccinated as young children and then frequently reinfected as immunity fades and viruses evolve. The coronavirus that causes COVID is new, though; it is still trying to infect large swaths of adults for the very first time.
So we might approximate the start of endemic COVID as the point where nearly everyone has been vaccinated or infected. Reinfections or breakthroughs will happen, but we hope they will be milder, which seems to be true so far. This blanket of immunity might be enough to head off big surges that overwhelm hospitals. But whether the endemic COVID actually becomes as benign as the common cold, or as bad as the flu, or worse, depends on both our changing immunity and the virus’s continued evolution. “We just don’t know,” says Rustom Antia, an evolutionary biologist at Emory University. And we don’t know how long it will take to reach endemicity.
A dose of humility: We are not very good at predicting the future of this virus. If you were reading COVID news back in March 2020, you may remember graphs of projected COVID cases that looked like a steep mountain. This is the classic epidemic curve. Cases rise exponentially until they hit a peak—the point of supposed herd immunity—and they start falling exponentially. Then, the pandemic is over.
This is obviously not what happened. Instead, COVID has come in multiple waves and plateaus. Some of these peaks and troughs were probably seasonal, as people spent more or less time indoors. But Americans also clearly changed their behavior in response to the threat of the coronavirus itself. In spring 2020, people stopped going out. Schools closed. We later started wearing masks and socializing more outdoors. Traditionally, models haven’t really incorporated behavior “because we haven’t altered our behavior in drastic ways to respond to pathogens,” says Virginia Pitzer, an epidemiologist at Yale. The behavioral shifts due to COVID were so profound that they’re forcing epidemiologists to reconsider how to model infectious disease. In particular, they are trying to understand how people may keep modulating their behavior as cases rise or fall: When the local news reports that hospitals are overwhelmed, does that prompt people to take more precautions in response? Could that explain why the summer Delta surge in the South fell off without drastic interventions?
The path to endemicity might also be bumpy because of how a virus spreads through social networks. A virus is inherently self-limiting in the short term; it induces immunity in those it sickens and eventually runs out of people to infect in a particular social circle. In a recent paper in eLife, scientists call this “transient collective immunity”—the virus hits this wall and cases fall even without the entire population reaching herd immunity. But the protection “has an expiration date,” says Sergei Maslov, a computational biologist at the University of Illinois at Urbana-Champaign and an author of the paper. Cracks form in this wall as we start interacting with new people. Perhaps Joe Shmoe stayed at home awhile and then attended a slew of weddings over the summer, where he got exposed. This constant rewiring of our social networks lets the virus find new susceptible people and can lead to new waves.
As we continue on the path toward endemic COVID, we may see more local surges every time the coronavirus finds a pocket of susceptible people, other epidemiologists have told me. But it may be hard to predict exactly when. You might think of it like a fire: The dry fuel is out there, though precisely when a spark of the coronavirus will find it depends on chance. “You can get lucky for so long with this virus, and you can get unlucky,” says Jessica Metcalf, a demographer at Princeton who studies infectious disease. “There’s something very erratic about transmission going on.”
The more inherently transmissible the virus is though, the quicker it will find the rest of the susceptible population and reach endemicity. The coronavirus has already significantly upped its transmissibility from the original Wuhan strain to Alpha to Delta. We don’t yet know where Omicron sits. The emergence of new variants has been hard to forecast. Early in the pandemic, scientists thought the coronavirus mutated rather slowly—until these variants with a huge number of mutations suddenly appeared and rewrote the rulebook. “Nobody had predicted that. That’s totally out of the box,” says Elizabeth Halloran, an epidemiologist at the Fred Hutchinson Cancer Research Center and a co-author, with Antia, the evolutionary biologist at Emory, of a paper modeling the transition to endemicity.
How our immunity changes over time will also influence the length of this transition period, Antia adds. So far, immunity to infection is waning, but immunity to severe disease still looks rather durable. Will immunity to severe disease ever wane? Will multiple exposures to the virus, either through boosters or infections, strengthen immunity permanently? All this affects the speed at which we reach equilibrium, and where that equilibrium lands. Further complicating matters, the virus is also adapting to evade the immune system. Delta has some ability to do this; Omicron might be even better at it, given its 30-plus mutations in the spike protein. Considering all of the complexity here, the final endemic equilibrium of COVID is hard to describe clearly. We might know we’ve technically reached endemicity only in retrospect, Antia says, when we’ve seen COVID follow a regular seasonal pattern year after year.
Instead of trying to gauge how far we are from this still-hazy future, we might turn instead to figuring out how to live through this uncertain transition period. We may be stuck here for a while yet. Even as the long-term picture remains unclear, we can make decisions for the short term based on what’s happening today. This requires a willingness to alter our behavior, switching precautions on and off as needed. We have a precedent for this, says Bill Hanage, an epidemiologist at Harvard. When cases were low over the summer, he felt comfortable going maskless indoors—knowing full well that masks might be needed again if cases went back up. “Let’s just be straight with people,” he says. “Rather than saying, ‘Oh, we just need to get to this, and then it’ll be over.’ Just say, ‘You know what, this is a marathon, not a sprint.’” Given how quickly COVID fortunes have shifted with Delta and might now shift again with Omicron, our strategies also need to evolve to suit the situation at hand. It’s not flip-flopping to change course. It’s facing reality.
Although this virus and our immunity shape the possible futures of endemic COVID, the ultimate burden of the number of cases and deaths we tolerate is up to us. Will we permanently alter our behavior to suppress respiratory illnesses? Wear masks in winter? Improve building ventilation? Isolate at the slightest sign of illness and be allowed to take sick days from work and school, no questions asked? Like herd immunity, endemicity is a bit of technical jargon that has been refashioned as shorthand for the threshold when science supposedly says we can stop worrying about COVID. But that isn’t up to science alone. We decide when we stop worrying about COVID. How far we are willing to go to prevent how many more cases is a question with social and economic trade-offs.
For now, we are truly living through unprecedented times. SARS-CoV-2 is the first virus modern science has ever seen emerge and march toward global endemicity. We’ve never watched this process play out before in such detail. We won’t know what endemic COVID looks like until we get there. But we do have to figure out how to live with the coronavirus, now and into the future.