Chinese authorities have put protective measures in place as the new coronavirus spreads.Credit: Kevin Frayer/Getty

Scientists and health authorities around the world are racing to halt the spread of a deadly virus that emerged in the Chinese city of Wuhan in December. Thousands of people have already contracted the new coronavirus, which causes respiratory illness. The death toll is at 213, and is rising daily. On 30 January, the World Health Organization (WHO) declared the outbreak a “public-health emergency of international concern” — an alarm it reserves for events that pose a risk to multiple countries and which requires a coordinated international response.

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Crucial details about the virus and how it spreads are still unknown, but experts are considering best- and worst-case scenarios on the basis of previous epidemics and what scientists already know.

How many people will the virus infect?

Chinese authorities have locked down cities at the centre of the epidemic, and researchers were quick to share data on the virus with the World Health Organization and researchers. But the case numbers have been rising, and surged past 9,000 in the past day, mostly in China. This has led to one prediction that the virus could infect about 39,000 of the 30 million people living in the region of Wuhan. “It seems like the virus has got out of hand in China, spread too far, too quickly to really be contained,” says Ian Mackay, a virologist at the University of Queensland in Brisbane, Australia.

In the best case, fewer people will be infected because the effects of the control measures will start kicking in, says Ben Cowling, an epidemiologist at the University of Hong Kong. But it’s too early to tell whether efforts to quarantine people, and the widespread use of face masks, are working. The incubation period for the virus — up to 14 days — is longer than most control measures have been in place, he says.

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In a worst-case scenario, some 190,000 people could be infected in Wuhan, according to another prediction model. Scientists are particularly concerned about fresh outbreaks emerging outside China. The virus has already spread in small, localized clusters in Vietnam, Japan, Germany and the United States, but authorities have been quick to isolate the people affected. Fewer than 100 cases have been recorded outside China as of 30 January.

Is the virus here to stay?

When a virus circulates continuously in a community, it is said to be endemic. The viruses that cause chicken pox and influenza are endemic in many countries, but outbreaks can be controlled through vaccination and keeping people at home when they are ill.

One big question is whether the coronavirus is also here to stay. If efforts to contain it fail, there’s a high chance that it will become endemic. As with influenza, this could mean that deaths occur every year as the virus circulates, until a vaccine is developed. If the virus can be spread by people who are infected but don’t have symptoms, it will be more difficult to control its spread, making it more likely that the virus will become endemic.

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There have been several cases of infected people displaying no symptoms, but it’s still unclear whether such asymptomatic or mild cases are common, and whether or how infectious they are. “We’re possibly looking at a virus that’s going to be with us for a long time, possibly forever,” says Mackay.

Asymptomatic cases set the new virus apart from the related coronavirus that causes severe acute respiratory syndrome (SARS). There was a global outbreak of this virus in 2002–03, but it usually spread only once people were ill enough to need hospital care. Once outbreaks in hospitals were brought under control, SARS was contained. There is no evidence that the virus is still circulating in humans, says Mackay.

If control measures are effective, and transmission slows so that each infected person infects no more than one other person, the current outbreak could simply peter out, says Cowling.

Is the virus likely to change?

Some researchers are worried that as the China coronavirus spreads, the pathogen could mutate so it can spread more efficiently, or become more likely to cause disease in young people. Currently, the virus has caused severe illness, and death, mainly in older people, particularly those with pre-existing conditions such as diabetes and heart disease. A 36-year-old Wuhan man with no known pre-existing health conditions is the youngest victim reported so far.

Kristian Andersen, an infectious-disease researcher at Scripps Research in La Jolla, California, is not concerned about the virus becoming more virulent. He says that viruses constantly mutate as part of their life cycle, but those mutations don’t typically make the virus more virulent or cause more serious disease. “I can’t think of any examples of this having happened with an outbreak pathogen,” he says.

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In situations where a virus jumps from one animal host to another species — which is probably how the new coronavirus began to infect humans — there might be a selection pressure to improve survival in the new host, but that rarely, if ever, has any effect on human disease or the virus’s transmissibility, says Andersen. Most mutations are detrimental to the virus or have no effect, he says. A 2018 study2 of SARS in primate cells found that a mutation the virus sustained during the 2003 outbreak probably reduced its virulence.

Researchers have shared dozens of genetic sequences from strains of the new coronavirus, and a steady supply of those sequences will reveal genetic changes as the outbreak progresses, says MacKay. “Viruses don’t change behaviour unless they change sequence, and we need to see constant or consistent virus change,” he says.

How many people will it kill?

The fatality rate for a virus — the proportion of infected people who die — is difficult to calculate in the middle of an outbreak because records on new cases and deaths are constantly being updated. With 213 deaths so far out of nearly 10,000 infections, the new coronavirus has a death rate of 2–3%. This is significantly lower than SARS, which killed around 10% of the people it infected. The known death rate for the new coronavirus is likely to decrease as mild and asymptomatic cases are identified, virologist Mark Harris at the University of Leeds, UK, told the Science Media Centre in London.

There is currently no effective drugs against the virus. Two HIV drugs thought to target a protein that helps coronaviruses to replicate are being tested as a treatment. Scientists have also identified other existing medications that target this function, and several international research groups are working on a vaccine.

The number of deaths will also depend on how China’s health system copes with the high number of cases. Putting people on drips and ventilators can ensure that they get enough fluids and oxygen while their body’s immune system fights the virus. China is building two new hospitals in Wuhan to accommodate the people infected, but if the virus spreads to parts of the world with fewer resources, such as low-income regions of Africa, their health systems could struggle, says Sanjaya Senanayake, an infectious-disease specialist at the Australian National University in Canberra.

In declaring a global health emergency, the WHO’s director-general, Tedros Adhanom Ghebreyesus, said his main concern was that the outbreak could spread to countries with fragile health systems.

If the virus spreads throughout the world, the number of deaths could be substantial. The current death rate of 2–3% — while not as high as for SARS — is still quite high for an infectious disease, says Adam Kamradt-Scott, a global health-security specialist at the University of Sydney, Australia. The 1918 influenza outbreak, known as the Spanish flu, infected around half a billion people, one-third of the world’s population at the time, and killed more than 2.5% of those infected; some have estimated that as many as 50 million people died.

The China coronavirus probably won’t trigger such an apocalyptic scenario, because it isn’t typically infecting or killing young, healthy people, says Kamradt-Scott.