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.
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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.
