By Sarah Zhang

The coronavirus that causes COVID-19 has sickened more than 16.5 million people across six continents. It is raging in countries that never contained the virus. It is resurging in many of the ones that did. If there was ever a time when this coronavirus could be contained, it has probably passed. One outcome is now looking almost certain: This virus is never going away.

The coronavirus is simply too widespread and too transmissible. The most likely scenario, experts say, is that the pandemic ends at some point—because enough people have been either infected or vaccinated—but the virus continues to circulate in lower levels around the globe. Cases will wax and wane over time. Outbreaks will pop up here and there. Even when a much-anticipated vaccine arrives, it is likely to only suppress but never completely eradicate the virus. (For context, consider that vaccines exist for more than a dozen human viruses but only one, smallpox, has ever been eradicated from the planet, and that took 15 years of immense global coordination.) We will probably be living with this virus for the rest of our lives.

Back in the winter, public-health officials were more hopeful about SARS-CoV-2, the coronavirus that causes COVID-19. SARS, a closely related coronavirus, emerged in late 2002 and infected more than 8,000 people but was snuffed out through intense isolation, contact tracing, and quarantine. The virus was gone from humans by 2004. SARS and SARS-CoV-2 differ in a crucial way, though: The new virus spreads more easily—and in many cases asymptomatically. The strategies that succeeded with SARS are less effective when some of the people who transmit COVID-19 don’t even know they are infected. “It’s very unlikely we’re going to be able to declare the kind of victory we did over SARS,” says Stephen Morse, an epidemiologist at Columbia University.

If not, then what does the future of COVID-19 look like? That will depend, says Yonatan Grad, on the strength and duration of immunity against the virus. Grad, an infectious-disease researcher at Harvard, and his colleagues have modeled a few possible trajectories. If immunity lasts only a few months, there could be a big pandemic followed by smaller outbreaks every year. If immunity lasts closer to two years, COVID-19 could peak every other year.

At this point, how long immunity to COVID-19 will last is unclear; the virus simply hasn’t been infecting humans long enough for us to know. But related coronaviruses are reasonable points of comparison: In SARS, antibodies—which are one component of immunity—wane after two years. Antibodies to a handful of other coronaviruses that cause common colds fade in just a year. “The faster protection goes away, the more difficult for any project to try to move toward eradication,” Grad told me.

This has implications for a vaccine, too. Rather than a onetime deal, a COVID-19 vaccine, when it arrives, could require booster shots to maintain immunity over time. You might get it every year or every other year, much like a flu shot.

Even if the virus were somehow eliminated from the human population, it could keep circulating in animals—and spread to humans again. SARS-CoV-2 likely originated as a bat virus, with a still-unidentified animal perhaps serving as an intermediate host, which could continue to be a reservoir for the virus. (SARS also originated in bats, with catlike palm civets serving as an intermediate host—which led officials to order the culling of thousands of civets.) Timothy Sheahan, a virologist at the University of North Carolina at Chapel Hill, wonders if, with SARS-CoV-2 so widespread across the globe, humans might be infecting new species and creating new animal reservoirs. “How do you begin to know the extent of virus spread outside of the human population and in wild and domestic animals?” he says. So far, tigers at the Bronx Zoo and minks on Dutch farms seem to have caught COVID-19 from humans and, in the case of the minks, passed the virus back to humans who work on the farm.

The existence of animal reservoirs that can keep reinfecting humans is also why scientists don’t speak of “eradication” for these viruses. The Ebola virus, for example, probably comes from bats. Even though human-to-human transmission of Ebola eventually ended in the West African epidemic in 2016, the virus was still somewhere on Earth and could still infect humans if it found the right host. And indeed, in 2018, Ebola broke out again in the Democratic Republic of the Congo. Ebola can be contained through contact tracing, isolation, and a new vaccine, but it cannot be “eradicated.” No one is quite sure why SARS has never reemerged from an animal reservoir, but this coronavirus could well follow a different pattern.

In the best-case scenario, a vaccine and better treatments blunt COVID-19’s severity, making it a much less dangerous and less disruptive disease. Over time, SARS-CoV-2 becomes just another seasonal respiratory virus, like the four other coronaviruses that cause a sizable proportion of common colds: 229E, OC43, NL63, and HKU1. These cold coronaviruses are so common that we have likely all had them at some point, maybe even multiple times. They can cause serious outbreaks, especially in the elderly, but are usually mild enough to fly under the radar. One endgame is that SARS-CoV-2 becomes the fifth coronavirus that regularly circulates among humans.

In fact, virologists have wondered whether the common-cold coronaviruses also got their start as a pandemic, before settling in as routine viruses. In 2005, biologists in Belgium studied mutations in the cold coronavirus OC43, which likely evolved from a closely related coronavirus that infects cows. Because genetic mutations accumulate at a somewhat regular rate, the researchers were able to date the spillover from cows into humans to the late 1800s. Around this time, a highly infectious respiratory disease was killing cows, and even more curiously, in 1889, a human pandemic began killing people around the world. The older people were, the more susceptible they were. This illness, which produced “malaise, fever, and pronounced central nervous system symptoms,” was linked to influenza based on the antibodies found in survivors half a century later. But the cause was never definitively proved from tissue samples.

Could it have been a coronavirus that jumped from cows to humans? This is all speculative, and the possible links between the other three cold coronaviruses and past pandemics are even less clear, says Burtram Fielding, a coronavirus researcher at the University of the Western Cape. “But,” he says, “I wouldn’t be surprised.” It would also be good news, in a way, because it would suggest that COVID-19 could become less deadly over time, making that transition from pandemic to common cold.

With a virus, there is a general trade-off between how contagious it is and how deadly it is. SARS and SARS-CoV-2 are illustrative points of comparison: The earlier virus killed a much higher proportion of patients, but it also did not spread as easily. And what a virus ultimately wants to do is keep spreading, which is much easier to do from a live, walking host than a dead one. “In the grand scheme of things, you know, a dead host doesn't help the virus,” says Vineet Menachery, a coronavirus researcher at the University of Texas Medical Branch. The other four coronaviruses may also be less deadly because we have all encountered them as children, and even if our immunity does not prevent us from getting them again, it may still prevent severe disease. All of this, along with immunity from vaccines, means that COVID-19 is likely to become far less disruptive down the line.

Influenza might be another useful point of comparison. The “flu” is not one virus but actually several different strains that circulate seasonally. After pandemics like 2009’s H1N1 flu, also known as swine flu, the pandemic strain does not simply disappear. Instead, it turns into a seasonal flu strain that circulates all year but peaks during the winter. A descendent of the 2009 H1N1 pandemic strain is still the seasonal flu today. The seasonal peaks never quite reach pandemic heights because of building immunity in the population. Eventually, a new strain, against which people have no immunity, comes along and sparks a new pandemic, and then it becomes the new dominant seasonal strain.

In this way, the long-term outlook for COVID-19 might offer some hope for a return to normal. “I think this virus is with us to the future,” Ruth Karron, a vaccine researcher at Johns Hopkins, told me. “But so is influenza with us, and for the most part, flu doesn't shut down our societies. We manage it.”