What happens next?

The novel coronavirus that emerged in Wuhan has exploded worldwide. What happens if it goes pandemic? Debora MacKenzie reports

WITH more than 17,000 confirmed cases -and probably far more undiagnosed -the 2019-nCoV coronavirus that emerged in Wuhan, China, seems poised to go global. As New Scientist went to press, the epidemic was still centred on the province of Hubei, but the virus had travelled to 23 countries, and further epidemics seemed possible.
Genetic analysis suggests that the virus isn't changing much in humans, becoming neither more nor less harmful. So where is the outbreak likely to go from here?
There are three options, says Eric Toner at Johns Hopkins University in Baltimore. One, viruses new to humans that don't adapt quickly can simply peter out after they have spread to several successive people, as another coronavirus from animals, MERS, seems to.
But the rocketing number of cases in China mean 2019-nCoV shows no sign of doing this.
Two, we could block transmission of the virus enough for it to die out. One way would be with drugs or vaccines (see "How soon will a treatment be ready?", page 11), but it may take a year to develop anything effective. Or we could quarantine infected people and block the virus. That worked for the related SARS virus in 2003, but early signs suggest that it might not be so easy this time.

Global spread
"Options one and two seem unlikely," says Toner. Instead, the virus may simply spread, like flu does, until most people have been exposed to it and either died or recovered and become immune. Then it may burn out for lack of hosts, or become a disease that mostly affects children who haven't yet encountered it.

Special report: Coronavirus
Understanding the outbreak

What happens next?
The novel coronavirus that emerged in Wuhan has exploded worldwide. What happens if it goes pandemic? Debora MacKenzie reports The full impact of the virus will depend on its death rate, which we don't yet know (see "Understanding the Wuhan virus", right). It seems to spread more readily than SARS, making it harder to contain with quarantine.
One problem is that some cases of the new virus are mild. No public health systems are equipped to test everyone with flu-like symptoms, then quarantine all those who have the virus -and everyone they have been in contact with -to prevent spread. However, this is what we would need to do to stop it spreading, says David Heymann at the London School of Hygiene and Tropical Medicine, who organised the WHO's SARS campaign.

Preparedness plans
In theory, the virus could be largely kept out of other countries and burn out in China. But that would be hard: models show that even testing all travellers at exit and entry would fail to catch 75 per cent of those incubating the virus, says Pasi Penttinen at the European Centre for Last year, Toner led a pandemic management exercise in Baltimore, in which industry and health leaders discussed options as a computer model of a pandemic involving a fictional coronavirus played out. After 18 months, the spread of the virus started to slow down, as people either died or became immune. However, by then, the fictional virus had killed 65 million people.
Toner stresses that the simulation was just that: a model of a hypothetical virus slightly different to 2019-nCoV. But unless the new virus self-destructs or we find ways to stop it, it is likely to follow a similar route, circulating until the pandemic slows. This is especially bad news for older people and those with pre-existing health conditions. Like SARS, the virus seems to kill by triggering out-of-control inflammation. In China, people with severe cases of the virus are more likely to be older or have chronic inflammatory diseases like diabetes, says Sylvie Briand at the World Health Organization (WHO). Disease Prevention and Control. Temperature monitoring in airports had no effect on SARS.

The coronavirus's spread
It therefore seems likely that the virus will go pandemic, circulating in multiple cities worldwide. "Independent self-sustaining outbreaks in major cities globally could become inevitable because of substantial exportation of pre-symptomatic cases," Gabriel Leung at Hong Kong University and colleagues wrote in a recent modelling study (The Lancet, doi. org/ggjvr7). "Preparedness plans and mitigation interventions should be readied for quick deployment globally." Countries were supposed to invest in such plans under 2005 WHO rules. But an independent report last year found "severe weaknesses in [countries'] abilities to prevent, detect, and respond to " The virus may spread until most people have been exposed to it and either died or recovered" FEATURE CHINA/BARCROFT MEDIA VIA GETTY IMAGES health emergencies". Fewer than 5 per cent of countries got high marks for their ability to rapidly respond to an epidemic.
Heymann says one lucky break with SARS was that it never invaded a chaotic, impoverished megacity, which would have had particular trouble organising the contact tracing and quarantine required to stop a virus this way.
That break seems unlikely with the Wuhan virus, which has already reached India and the Philippines. The Gates Foundation has earmarked $10 million to fight the virus -half for China, the rest to help African countries prepare. African health agencies will hold a workshop in Senegal this week.
Even if the virus remains largely in China, there would be global consequences. According to Michael Osterholm at the University of Minnesota and his team, 153 crucial drugs, from blood pressure pills to stroke treatment, are mostly made in China, and there are fears the virus could affect their production and export.
Osterholm is worried about further impacts. So far, he says, "people aren't saying how bad it can get". ❚

Debora MacKenzie
Pathogens obey simple rules of transmission, and these can be used to predict how fast the number of infections will rise and fall, how hard it will be to stop a disease and how bad things are likely to get.
Measuring the necessary parameters in a virus that is new to humans can be difficult, though. First you need the basic reproduction number, R0.
If everyone is susceptible to an infection -and we are, as the Wuhan virus is new to humans -R0 is the average number of other people that any individual with the virus will infect. Research teams have calculated 12 different estimates for the virus's R0 so far. Together, these suggest it is between 1.8 and 3.3.
To stop an epidemic, the average number of new people that each case infects needs to be below one. This can happen as people become immune, get a vaccine or change their behaviour to prevent transmission. In the case of the Wuhan virus, people in China are already avoiding crowds and public transport.
The second parameter is the number of cases. Because the illness looks like flu and can be mild, many cases are thought to be going undiagnosed.
But long-distance travellers are coming under close scrutiny, and assuming that they aren't vastly more likely to be infected than people who don't travel, this can give us an indication of case numbers. In this way, two research groups have calculated that case numbers in Wuhan could be between 10 and 40 times higher than official numbers.
When a virus has jumped over from animals, as in the case of this coronavirus, one question is whether it is still jumping. So far, analysis suggests very little genetic variation in the virus, which implies it has only crossed into humans once.
Genetic analysis also suggests that the virus appears to be stable: it doesn't seem to be changing or adapting to us.
Another crucial parameter is the fatality rate, which we cannot yet know. The number of deaths so far make up about 2 per cent of all known cases. But according to the China National Health Commission, only 5 per cent of known cases have either died or recovered so far. Some of the rest will die, so the real death rate must be higher.
One study of hospitalised people with a known outcome suggests that the fatality rate has been 14 per cent, but these are only cases that were severe enough to be hospitalised -we don't know how many people had milder illness and lower death rates from the virus.
To fill these gaps in our understanding will take massive testing of exposed populations. ❚

Understanding the Wuhan virus
An illustration of the structure of the new coronavirus