Editor’s note, April 2: There have been significant developments in the coronavirus pandemic since this story was last updated, and some details may no longer apply. For our most up-to-date coverage, visit our coronavirus hub.

In late January, I posed a simple question to several experts in public health and epidemiology: How does the Covid-19 coronavirus outbreak end? Back then, the virus was still mainly just spreading in China, and the scientists I spoke with outlined a hopeful scenario: containment.

The idea is that through identifying and isolating the sick, the virus could be kept from spreading in communities around the globe. It seemed reasonable: Containment was how the 2003 SARS outbreak — also caused by a member of the coronavirus family — ended.

Now, many experts tell Vox, that scenario seems impossible. “Two or three weeks ago, we were still hoping for containment,” says Tara Smith, an epidemiologist at Kent State University. “We’re really past that. ... The horse is out of the barn.”

One reason has to do with what we’ve learned about the virus itself: There’s now evidence that people who do not show severe symptoms can spread it silently. Another reason is the slow rollout of diagnostic tests in the United States and other countries like Italy and Iran: We don’t have a precise case count or know where the virus might be spreading.

Currently, the World Health Organization reports there are more than 120,000 confirmed cases of Covid-19 across the globe, and more than 4,700 deaths. There could be many more undetected and unconfirmed both here and abroad.

Given this new, uncertain phase, I decided to go back to some of the same virologists, immunologists, and epidemiologists (and a few new ones) with my question: How does this outbreak end?

The most uncomfortable answer they gave is the possibility that Covid-19 keeps spreading at a high rate and becomes endemic — regularly infecting humans, like the common cold.

“Without an effective vaccine, I don’t know how this ends before millions of infections,” Nathan Grubaugh, an epidemiologist at the Yale School of Public Health, says.

To be sure, much is uncertain about the virus and how it will spread. There’s still no single, accurate death rate for the illness. Little is known about the susceptibility of children. So much can still change. But I asked these experts to weigh in with the best available evidence in mind.

Just because the virus isn’t being contained doesn’t mean we’re powerless to prevent serious illness and deaths among the most vulnerable. There’s still a lot communities can do to slow the spread, save lives, and buy crucial time for either a cure or a vaccine to be developed. There are many forking paths on the way from outbreak to endemic. Lives can still be saved, and the worst-case scenario can still be avoided.

Why scientists think the containment scenario is now unlikely

What the epidemiologists and virologists told me is that containment, in the US at least, so far isn’t working. And the longer containment efforts fail, the harder they become to implement.

The biggest failure is the slow rollout of diagnostic testing. The Centers for Disease Control and Prevention reports as of March 11 that it has counted 938 cases of Covid-19 in the US. (Johns Hopkins researchers are reporting more than 1,300, including those who were infected overseas, as of March 12.)

Epidemiologists fear the actual case count is a lot higher. The CDC has been slow to get Covid-19 diagnostic testing out to labs (due in part to a production error). And initially, testing was restricted to small numbers of people who had known travel to affected countries.

This all means “we don’t know what the prevalence actually is” in the US, Angela Rasmussen, a Columbia virologist, says. Two weeks ago, she says, “I probably would have said that there’s a possibility that this will become endemic.” Now, “I think given our government’s public health response, I’m much more alarmed that this probably will become endemic.” And still, this week, the federal government is struggling to produce tests.

As cases of Covid-19 in China were increasing dramatically in January and February, a lot of the US response was focused on travel restrictions and travel-focused testing. In retrospect, there should have been more planning for a pandemic.

“Once it was established this virus was spreading efficiently between people... we immediately should’ve realized that this was not going to be containable,” Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, writes in an email. “At that point every country in the world should [have begun] pandemic preparation. This would include scaling up diagnostic testing, preparing hospitals, and crafting public health messages.”

Meanwhile, the virus spreads. Genetic detective work from Washington state suggests the virus has been circulating there for at least six weeks.

The biology of the virus also makes it hard to contain, since it’s possible to spread the virus before showing symptoms of it. “I don’t think we quite know the extent of how often that happens, but it is happening,” Grubaugh says. With SARS in 2003, infected people did not spread the virus without symptoms. They also tended to get sicker, often contracting pneumonia. That made cases easier to detect and isolate.

All of the above is a recipe for an outbreak to become endemic, or a disease that sticks around. Humans haven’t seen this virus before, which means we’re not immune. It’s also a recipe for millions of potential infections in a pandemic — a worldwide outbreak of a new disease.

What might happen: A huge portion of the world could become infected

You might have seen an alarming headline in the Atlantic recently: You’re likely to get the coronavirus.

The assertion was based off an estimate from Harvard epidemiologist Marc Lipsitch, who predicted some 40 to 70 percent of all adults around the world would catch the virus within a year. Lipsitch has since revised that estimate downward and with a greater range: He now estimates it’s “plausible” that 20 to 60 percent of adults will catch the disease. (If this comes to pass, while being bad, it’s not apocalyptic: Most cases of Covid-19. are mild. But it does mean millions could die.)

In an email, Lipsitch says his model “assumes that the transmission in the rest of the world is at least fairly similar to that in China.” But “projections should be made with humility,” he adds, as there’s a lot still to uncover that will impact the forecast (like the role children play in spreading the disease).

The bottom line of his modeling, though, is that a sizable portion of the human population is at risk of catching this virus. It might not come to pass — especially if a vaccine or other treatment is developed. But it is possible.

If the virus cannot be contained, Lipsitch says, the only way for this to get under control is for 50 percent of people to become immune to it.

If enough people get Covid-19, and develop an immune response, “essentially it creates its own herd immunity,” Grubaugh says. “But that’s after causing, you know, millions of worldwide infections.”

Obviously, that’s far from an ideal situation. (It’s also possible, hypothetically, that the virus becomes less deadly over time, through evolution: The most lethal versions of the virus essentially kill themselves when they kill their hosts.)

There is still a ways to go from the current outbreak to the numbers projected above. Some of the paths are worse than others. The risk is high, and we may not be able to contain the virus. But we do have tools to slow it down.

The nightmare scenario: A sudden huge spike in cases

The worst-case scenario for the outbreak in the United States is if there are sudden spikes in infections among many communities across the country. A spike could overwhelm our health care system.

“That’s one of the most dangerous things about this,” Ron Klain, who led the response to the 2014 Ebola epidemic under the Obama administration, said in February. “What if all of a sudden 10,000 sick people needed hospitalization in a major city? There’s no 10,000 extra beds sitting around someplace.”

Hospitals are already worried about equipment shortages. And as sick people rush into the hospital system, they could infect health care workers — as well as other vulnerable patients, particularly the elderly — leaving the system in even more dire straits.

“We saw in Wuhan 1,000 health care providers get sick and we had at least 15 percent severely ill and in ICUs,” Peter Hotez, a vaccine expert with the Baylor College of Medicine, testified before a House committee Thursday. “And that is very dangerous, because not only do you subtract those people out of the health care workforce, but the demoralizing effect of colleagues taking care of colleagues ... the whole thing can fall apart if that starts to happen.”

But this nightmare scenario isn’t inevitable.

“If we slow it so that infections happen over 10 or 12 months instead of over one month, that’s going to make a big difference as far as how many people seriously infected, how many people may end up hospitalized, and how many they end up dying,” Smith says. “We talk about it as ‘flattening the epidemic curve’ — so that it’s not a big, sudden peak in cases, but it’s a more moderate plateau over time.”

And that’s the current goal: to flatten the curve.

The better scenario: Public health measures slow the spread and buy scientists time to work on treatments

New cases in China are now declining thanks to the government’s dramatic measures to contain the virus — mainly case finding, contact tracing, and suspension of public gatherings — as WHO epidemiologist Bruce Aylward, who led a recent mission there, told my colleague Julia Belluz.

In the US, there’s still time to put those efforts in place to flatten that curve.

In Washington state, health officials are now asking community groups to cancel events that bring together more than 10 people, and are recommending that people telework if they can. Pregnant people, people older than 60, and those who have underlying health conditions are being asked to stay home. More states and communities may have to impose such measures in the coming weeks. So be prepared for them.

“We also need to stop panicking and stigmatizing people of different ethnicities— this will only make people more hesitant to speak out and seek care,” says Abraar Karan, a physician at Brigham and Women’s Hospital and Harvard Medical School. “Disease should show us that we are all connected and need to help each other, not divide us.”

Individuals can help slow the spread of the virus through measures like staying home when you feel just a little bit sick (even when it is inconvenient), washing hands often, and following public health official recommendations when it comes to avoiding large crowds of people.

That’s the optimistic take: We will slow the spread, and help out our communities and the most vulnerable in the process.

The pessimistic view: Because of the lag in testing, the outbreak might be further along — and therefore harder to contain — than authorities currently realize. “There certainly is a window to [impose social distancing measures], but whether or not we’re still in that window, we have no idea because we have no idea what’s happening without the testing,” Grubaugh warns.

The lucky scenario: Covid-19 naturally stops spreading as fast during the summer

Another factor that could potentially slow the spread of coronavirus is the changing of the seasons.

For a variety of reasons, some viruses — but not all — become less transmissible as temperatures and humidity rise in the summer months. The viruses themselves may not live as long on surfaces in these conditions. Also, human behavior changes, and we spend less time in confined spaces. “A lot of how the outbreak ends or at least how things progress in the next few months really depends on if this is seasonal,” Grubaugh says.

That’s still a big unknown. “Just because some respiratory diseases, like flu, demonstrate seasonality doesn’t mean that Covid-19 will,” Maimuna Majumder, a Harvard epidemiologist, says. She and colleagues recently published an early version of a paper (which has not been peer-reviewed) that found that changes in weather across China did not seem to impact the course of the outbreak.

The study suggests that humidity — which appears to be correlated with the seasonality of the flu — is not correlated with transmissibility of Covid-19, she says. (She also stresses to treat the data as “provisional,” and that her group is still studying the potential effect of temperature on transmissibility.)

But if it is seasonal, it doesn’t mean Covid-19 goes away after the summer. “It likely isn’t just going to magically go away,” Grubaugh says. “Next winter might end up being the big winter.”

And if it is seasonal, it’s still dangerous. It would be like the flu, “except potentially with a higher case fatality rate,” Rasmussen says. “Which is definitely a problem because the seasonal flu kills 30,000 to 60,000 Americans every year. And even if it’s the same case fatality rate of seasonal flu, that still presents a substantial public health burden.”

How this outbreak could truly end: With a vaccine

To end this outbreak, for good, we’ll need antiviral treatments or a vaccine. Those are currently being produced, and at record speeds. Researchers are working on new vaccine technologies — like mRNA vaccines that don’t use viruses at all in their production process — as well as cutting-edge therapeutic antibodies.

That said, it still could be a year or more before the safety and efficacy of these pharmaceuticals are proven. In medicine, effectiveness is not guaranteed.

Related A simple guide to the vaccines and drugs that could fight coronavirus

But even if it takes a year or more to produce, those treatments could still prove useful.

“We don’t know what’s going to happen with this virus,” says Barney Graham, the deputy director of the Vaccine Research Center at the National Institute of Allergy and Infectious Diseases (NIAID). “So our job is to try to develop interventions that could be used if it gets worse. ... We need ways of protecting ourselves.”

Remember: Outbreaks harm more than the sick

It’s also important to remember that outbreaks don’t just affect those who get sick with the illness and die — there can also be collateral damage.

Outbreaks economically impact the people who have to take off from work for a quarantine, those who cannot afford medical care, and the groups that are unfairly targeted and stereotyped as being disease carriers. As the outbreak progresses, it will expose the cracks in our society and our preparedness for future outbreaks. We need to remember the lessons we learn over the next several months.

“I think that it’s going to end probably the way the 2009 H1N1 [flu] pandemic ended, which is that shortly after it’s over, people will lose memory of it and not worry about it,” Rasmussen says. “But it’s going to have tremendous — really negative and lasting effects — for the most vulnerable people who are either medically or economically vulnerable in our society.”

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