How will the coronavirus outbreak end?

A woman with a face mask at an airport.
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Despite worldwide efforts to contain the new coronavirus, hotspots continue to emerge, and the number of cases is on the rise.

It's been only a couple of months since the virus, called SARS-CoV-2, made its first public appearance in a seafood-and-live-animal market in Wuhan, China. Since then, the virus has hopped borders, infected over 82,500 people and killed 2,810. But as with all past outbreaks, this one will eventually come to an end.

So, how will this story wrap up? Experts say one possibility is that cases of the disease will start decreasing when enough people develop immunity, either through infection or vaccination. Another possible scenario is that the virus will continue to circulate and establish itself as a common respiratory virus.

The possibility of containment

At this point, it's very unlikely that the outbreak will be contained to a few locations experts say.

"It is increasingly likely that this virus will spread worldwide," said Aubree Gordon, an associate professor of epidemiology at the University of Michigan. "We still may have some chance to contain it, but that window does appear to be closing."

The outbreak already meets two criteria of a pandemic: SARS-CoV-2 can infect people and cause illness, and it can spread easily from person to person, the U.S. Centers for Disease Control and Prevention (CDC) said at a news conference Tuesday (Feb. 25). As the virus begins to infect communities in more countries, it is coming closer to meeting the third criterion: worldwide spread of the virus.

Around 95% of the cases of COVID-19, the disease caused by SARS-CoV-2, are in mainland China. However, spikes are occurring in other countries, including South Korea, Iran, Italy, Singapore, Taiwan, Thailand and Japan. On Wednesday (Feb. 26), the CDC announced the first case of likely "community spread" of the virus, in a northern California patient with no ties to a hard-hit region.

Related: The 9 deadliest viruses on Earth

The best way to control the virus or slow its spread is through containment measures such as quarantines and travel restrictions, Gordon told Live Science. Indeed, there's been a worldwide effort to stop the spread of this new coronavirus. Some efforts, such as the quarantines on the Diamond Princess cruise ship, might have had limited success, Live Science previously reported

But containing the spread of the virus will be extremely difficult, Gordon said. There are a few reasons for that: SARS-CoV-2 spreads very easily, most cases of COVID-19 aren't severe and thus may not be identified, and the virus has a long incubation period, or the time between when a person is infected and when they start showing symptoms, Gordon said. 

What's more, all containment efforts are based on what we know about the virus, and there's still much we don't know. Quarantine periods are typically 14 days, based on early studies that suggested that was the longest possible incubation period for the virus. But there's some evidence that the incubation period might be much longer. 

For instance, local news reports from China's Hubei province (where the first human cases of the disease appeared) claimed that a 70-year-old man who was infected with the coronavirus didn't show symptoms until 27 days after infection, according to The Washington Post. The most common ways the virus is thought to spread are through respiratory droplets and via contact with an infected person, but it's still unclear if the virus can spread before symptoms begin.

And there's still a possibility that the coronavirus began to spread before we even knew it existed. "I think it is already a pandemic," said Dr. Amesh Adalja, an infectious-disease specialist and a senior scholar at the Johns Hopkins Center for Health Security in Baltimore. "I do believe that there likely have been cases in the U.S. and other places that went unrecognized, especially since the vast majority are mild and indistinguishable from other causes of the common cold."

If containment efforts fail and this indeed becomes a pandemic, "there are two ways that it could end," Gordon told Live Science. Either enough people will develop immunity — either through infection or vaccination — that the virus will stop transmitting and is eliminated, or the virus will continue to circulate and establish itself as a common respiratory virus.

A natural trickle-off

Pandemics end when the virus doesn't have enough susceptible people to infect. 

The catastrophic 1918 Spanish flu pandemic is thought to have infected 500 million people worldwide, many of them soldiers living in close quarters fighting in World War I. Once the war ended and people dispersed, the spread slowed as people had less contact. But the flu was ultimately halted in part because those who survived it had immunity and the virus didn't hop as easily as it did at the beginning. 

If the virus comes into contact with another person but that person isn't susceptible to the disease, then that chain of transmission is snuffed out. If one person infects two, those people together infect four and so on, and eventually, the virus runs out of susceptible people to infect, said Joshua Epstein, a professor of epidemiology at New York University. "What happens typically is that enough people get the bug that there just aren't enough susceptible people to keep the chain going."

Related: 10 deadly diseases that hopped across species

If SARS-CoV-2 is like common strains of the flu (or like other coronaviruses that cause the common cold), there's also a chance that the number of infections may die down as the weather warms up. It could then have a resurgence this fall and winter.

But "I think it's premature to assume that," Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said at a news conference on Feb. 12. 

Theoretically, environmental conditions may affect the transmission of a virus, and that's why some viruses have seasonality, Gordon said. "However, it really isn't clear, as viruses like influenza have just as big of a burden in tropical areas as temperate ones." 

Another possibility is that SARS-CoV-2 will mutate in a potentially beneficial way, making it more difficult for the virus to infect people. Back in 2002, a similar coronavirus in the Guangdong province of southern China first hopped to humans from animals and caused an outbreak of an infection that became known as severe acute respiratory syndrome (SARS).

SARS spread to 26 other countries, including Canada, sparking fear that the disease would become widespread in North America. "When SARS came to Canada, we were worried that this was going to be another epicenter," said Dr. Eric Cioe-Peña, an emergency room physician and the director of global health at Northwell Health in New York. 

But the outbreak ended up petering out, in part due to good public health containment, he said. But it also was because SARS randomly mutated — as viruses commonly do — and became much more severe but harder to transmit to humans, he said. 

A vaccine for eradication

But health officials aren't just sitting back and waiting to see how the virus will behave; researchers worldwide are racing to find a vaccine and treatment for SARS-CoV-2. Because the virus has "established itself in human populations, a vaccine is the only way to eradicate it," Adalja said. 

Recently, a group of researchers mapped out the detailed structure of the so-called spike protein that the new coronavirus uses to latch onto and infect human cells, potentially opening the door to a vaccine. The idea is that if people were to be injected with a spike-protein-based vaccine, their bodies would make antibodies against it so that, if they were ever exposed to the real virus, they would have immunity, Live Science previously reported.

But the problem with vaccines is that "it takes so long to go from the potential vaccine to the animal model to the types of clinical trials [needed], and there's only so many steps you can cut out without jeopardizing safety," Cioe-Peña said. And so a vaccine isn't going to help with the spread of the virus in epicenters right now, he added. 

Still, experts think a vaccine is likely 18 to 24 months away, which is still expedited from the 10 years it typically takes, according to the Live Science report. But the fact that experts are continuing to develop a vaccine means that they don't think SARS-CoV-2 will disappear anytime soon, Epstein said. If health officials thought the virus would vanish soon, "then building a vaccine for the future would not make much sense," Epstein told Live Science. "There's a sense in which it may not end."

A lingering virus

"I think it's unlikely that this coronavirus — because it's so readily transmissible — will disappear completely," said Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University in Tennessee. 

Eradication of a disease is "difficult and rarely achieved," according to the World Health Organization. For that to happen, there must be an available intervention to interrupt transmission, there must be diagnostic tools to detect cases that could lead to transmission and humans must be the only reservoir for the virus, they wrote. 

Even if the coronavirus is eradicated among humans, if the virus continues to survive in its natural form in animal reservoirs, those reservoirs can put the virus back into circulation, Epstein said. "I think it's always possible the thing has one cycle, we nail it, it doesn't mutate and it's eradicated," he added. "But I think the most likely prospect is that we don't entirely eradicate it."

There's a chance that, even if we manage to quench this virus, it might turn into a seasonal disease, making a comeback every year like other seasonal diseases such as the flu or colds, Epstein said. If that happens, there's a chance it could have less of an impact during subsequent circulations because more people will have built up immunity, Adalja said. But it's not clear if humans can become reinfected with this virus yet, Gordon said.

People can get reinfected by the other circulating coronaviruses because our immunity to them wanes over time. Immunity doesn't wane with every virus, however. With viruses such as the one that causes measles, once someone has it or has been vaccinated against it, they won't become reinfected, Schaffner said.

Though waning immunity is the most likely cause for reinfection, it's also possible that viruses might mutate just enough to evade the immune system. "But right now, there is no evidence that the coronavirus is mutating in any significant way," Gordon said. "The viral sequences that are available are nearly identical."

It's difficult to predict how the virus will behave. 

"Some of them reoccur; some of them don't," Epstein said. The virus that caused SARS had a relatively low mutation rate, so it didn't reoccur;  influenza, by contrast, has a really high mutation rate and thus reoccurs every year. If SARS-CoV-2 manages to mutate substantially in the coming months, the vaccines scientists produce now might not be well matched by the time it reoccurs, he added.

But even if you can't predict what will happen with the virus, you can prepare for it, Epstein said.

It's impossible to block people from all affected countries, he said. But people can take steps to reduce the number of infections in their area — such as screening and aggressively testing for the virus, isolating cases, canceling mass gatherings, partaking in household isolation and observing normal public health practices and good hygiene — "in the hopes that we won't have a big outbreak," he said. 

In the worst-case scenario, we might have a "substantial epidemic in the United States," he added. "I don't think anybody's prepared to put an actual number on it."

Originally published on Live Science.

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Yasemin Saplakoglu
Staff Writer

Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.