Fourth, R0 is not easy to calculate. That’s especially true in the early days of an epidemic, when it’s not even clear how many cases there have been. Some people might have been infected without showing symptoms. Others might not have reported their symptoms to health authorities. Absent clear data on who has the disease, let alone how they’re moving around and interacting with other people, scientists have to calculate R0 by doing complicated simulations using a variety of possible methods. That’s why early estimates can vary so wildly, and why they should be taken with a grain of salt.

Photos: The 1918 flu pandemic

Fifth, R0 is not some magical, immutable property of the virus itself. It depends on how likely someone is to be infected after contact with an infectious person, and how often such contact occurs—and these quantities are also affected by how societies deal with a virus. When SARS first emerged, transmission dynamics played out very differently in China and Canada, which is why the virus’s R0 values cover a wide range, from 2 to 5. “In places with good infection control, where you can isolate cases as soon as they happen, you’ll see a lower R0 than, say, in places where an outbreak initially took off,” Majumder says.

The current R0 estimates for the new coronavirus are specific to Wuhan, and mostly to the era before people knew about the virus. New estimates will emerge as the virus spreads to places that are now aware of and watching for it. “Likely, these will all be significantly lower,” says Kristian Andersen, a virologist at the Scripps Research Translational Institute.

Sixth, R0 is not destiny. It is a measure of a disease’s potential. And once nations realize that a new disease exists, they can actively screen for it, check that health-care workers are using proper protection, and instigate quarantines. Even simple steps such as hand-washing might make a difference. All these measures could potentially lower the chances that the virus will spread and ensure that its actual transmission rate—the quantity known simply as R—is less than R0, and ideally less than 1. There are a few reassuring signs: One study suggests that patients are now being isolated just one day after showing symptoms, as opposed to six days after at the start of the outbreak.

Read: How to misinform yourself about the coronavirus

None of this should be cause for complacency. The new virus is a serious threat, and the world should absolutely start considering what to do if containment measures fail. But at a time of great uncertainty, people grasp for solid answers, and numbers seem to offer them.

This new virus has emerged at a time when scientists have more avenues than ever for publishing their data and comparing notes. That can be a good thing, since fast and open communication can help bring diseases to heel more quickly. The risk is that a complicated number is released without context into a world that doesn’t know how to think about it. “Getting these R0 values out very rapidly is super important,” Andersen says. “But the way that some people and news outlets have interpreted what they mean … that part is problematic.”

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