Australians flown home from Wuhan, China, will be quarantined on an island for two weeks. Americans, also evacuated from Wuhan, will be “temporarily housed” on an air base in California. And in South Korea, the police have been empowered to detain people who refuse to be quarantined.
For countries outside China, the time to prevent an epidemic is now, when cases are few and can be isolated. They are trying to seize the moment to protect themselves against the coronavirus outbreak, which has reached every province in China, sickening more than 7,700 people and killing 170.
More than a dozen nations with a handful of cases — including the United States — are isolating patients and monitoring their contacts, as well as screening travelers from China and urging people to postpone trips there.
But whether this virus can be contained depends on factors still unknown, like just how contagious it is and when in the course of the infection the virus starts to spread.
China, with nearly 1.4 billion people, is the most populous nation on Earth, and it has taken extreme measures to try to stop the disease, first reported in December in Wuhan, a city of 11 million. The government has stopped travel in and out of that city and surrounding ones, effectively locking down tens of millions of people.
“The fact that to date we have only seen 68 cases outside China and no deaths is due in no small part to the extraordinary steps the government has taken to prevent the export of cases,” Dr. Tedros Adhanom Ghebreyesus, the director-general of the World Health Organization, said at a news briefing on Wednesday.
But the disease has spread far and wide inside China and, with extensive worldwide travel by its citizens, especially during the celebration of the Lunar New Year, countries everywhere are bracing for the arrival of more new cases.
Person-to-person transmission is occurring, and cases have turned up in several countries with people who have not visited China. Mentioning those cases, Dr. Tedros said the potential for further global spread was one of the reasons he had called on the W.H.O.’s emergency committee to meet again on Thursday to decide whether to declare the epidemic a public health emergency of international concern. The committee met twice last week but was split about whether to declare an emergency, saying it did not have enough information to decide.
“I think things are going to get worse before they get better,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in the United States, said in a podcast posted on Tuesday by the medical journal JAMA.
If China can somehow contain its outbreak, and if other countries with cases can prevent sustained transmission, Dr. Fauci said that it might be possible to end the outbreak, just as the coronavirus that caused the SARS epidemic in 2003 was stamped out.
“But it’s going to be a real kind of tightrope walk, because if it gets so expansive then it’s not going to just disappear the way SARS did,” he said. “I think the next four to five weeks are going to be critical. It’s either going to start peaking and go into a downturn, or it’s going to explode into a global outbreak.”
Dr. Thomas R. Frieden, a former director of the Centers for Disease Control and Prevention, said in an interview that he thought it would become clear in a few days whether the outbreak could be contained.
“If we’re seeing widespread transmission, thousands or tens of thousands of infections in the community, I don’t see how this gets controlled,” Dr. Frieden said. “On the other hand, if we see a SARS-like situation, where with incredible effort they were able to isolate people, tamp down the spread, then we’re in a containment situation.”
A particular concern is the possibility that the virus could wreak havoc in Africa, where possible cases are being investigated.
“We are very concerned about Africa because some of the least prepared countries for outbreaks are in Africa,” Dr. Frieden said, adding, “We know the systems there to find it and stop it are weaker there than elsewhere.”
If efforts to contain an outbreak fail, public health authorities will focus on “mitigation”— dealing with the disease and trying to minimize the harm it does to people and communities.
“It’s a worrisome situation in China,” Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said in an interview on Wednesday. “It seems like more of a mitigation strategy than a control strategy that China has moved to.”
The United States still has a chance to avoid China’s fate, she said.
“We still only have five cases, and we can really do aggressive measures around those cases,” Dr. Messonnier said. “We’re trying to contain the disease, and by being aggressive we’re hoping to learn more about what it takes to contain it.”
The five patients have hundreds of contacts. Some are being tested for the virus, and the results may help researchers understand how the disease is transmitted. The C.D.C. is also monitoring more than 100 “patients under investigation” — some with cough or fever who have been to Wuhan, or have had contact with a patient.
But if the case count were to increase exponentially, Dr. Messonnier said, it would be hard to continue the concerted containment efforts. The C.D.C. is already gearing up should the approach need to evolve to mitigation strategies like closing schools, preventing public gatherings and helping hospitals prepare for a surge of cases.
One troubling question is whether infected people can start spreading the virus before they themselves get sick. Chinese health officials have said they believe such transmission has taken place. If it happened often, it could make stopping an outbreak much harder.
The reason is that the first step in halting outbreaks has traditionally been to identify people who are ill and then stop them from infecting others, usually by isolating them. But that approach will not work as well if people without any symptoms are already transmitting the disease.
Health officials in the United States said they had not seen data from China to support that claim, nor any evidence that people without symptoms had spread the disease in the United States. The five patients in the United States had all visited Wuhan, and so far none of their contacts have become ill or tested positive for the virus.
Dr. Fauci said that epidemics are fueled by people with symptoms — like sneezing and coughing, which help spray the virus around — and not by those without symptoms, even if some of them can spread the virus. Dr. Frieden said if asymptomatic people do transmit some virus, sick patients are likely to spread a lot more.
People with colds or flu can spread viruses for a day or two before they become ill, but how big a role that plays in outbreaks is not known, researchers say.
In any case, flu spreads worldwide every year, infecting tens of millions of people.
And the spread of the new coronavirus is starting to resemble that of seasonal flu, said Dr. Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
“I think we have to revisit which model we’re really using, and I think we really over the past week and a half have come closer to the influenza model,” Dr. Osterholm said. “Trying to stop influenza in a community without vaccine is like trying to stop the wind. I don’t know how we’re going to stop this.”
He added, “The only thing operating in our favor is at least it doesn’t appear to be as severe as SARS or as MERS.”
Among patients with the Wuhan coronavirus, about 20 percent have become seriously ill, and the rest have a mild illness, the W.H.O. said at a news briefing on Wednesday. So far, the death rate appears to be about 2 percent, but that is not certain yet. Those who die tend to be older people with underlying ailments. The median age of the first 425 patients in China was 59, and a little more than half were male, according to a report published in The New England Journal of Medicine on Wednesday.
Diseases caused by related coronaviruses are deadlier: SARS killed 10 percent, and MERS about 35 percent.
But if the new virus were to spread even more widely and a 2 percent mortality rate continued, the death toll could be considerable. Seasonal flu, with a much lower overall death rate of 0.1 percent, kills more than half a million people worldwide every year. At least 8,200 people in the United States have died during this flu season, according to estimates from the C.D.C.
Roni Caryn Rabin contributed reporting for this article.