Here’s what the oft-cited R0 number tells us about the new outbreak—and what it doesn’t.
R0 is important because if it’s greater than 1, the infection will probably keep spreading, and if it’s less than 1, the outbreak will likely peter out. So it offers vital information to organizations and nations as they consider how to respond to an outbreak—such as the one the world is currently experiencing.
In December, a previously unknown coronavirus, now called 2019-nCoV, emerged in the Chinese city of Wuhan. There have been more than 4,500 confirmed cases, the majority of which have been in mainland China. But several dozen cases have been detected in more than 15 other countries, and as the outbreak has spread, so has fear. Public-health researchers have sped to estimate the R0 of the new disease, and as they have shared their findings, this number has fueled several alarmed missives on social media.
In the intervening time, however, some observers seized upon the 3.8 number, with one Harvard doctor describing it as “thermonuclear pandemic level bad” in a tweet that has since been retweeted more than 16,000 times. That’s a dubious interpretation, and here are six reasons why.
First, the R0 estimates for the new coronavirus are in line with those for many other diseases. They’re similar to those for SARS (2 to 5) and HIV (also 2 to 5), and considerably lower than those for measles (12 to 16).
Second, a bigger R0 doesn’t necessarily mean a worse disease. Seasonal flu has an R0 that hovers around 1.3, and yet it infects millions of people every year. SARS had an R0 of 2 to 5 and infected just over 8,000 people. The number is a measure of potential transmissibility. It does not actually tell you how fast a disease will spread.
Why? Because third, R0 is an average. Let’s say the virus has an R0 of 2. This could mean that every single infected person passes the virus to two other people. It could also mean that one infected person is a “super-spreader” who infects 100 people, while 49 infected people infect no one. These two scenarios have radically different implications for what will happen during an outbreak.
Super-spreader events are dangerous for health-care workers, but counterintuitively, they can sometimes be a good sign. They suggest that most infected people won’t actually perpetuate the outbreak, while the most problematic cases “may be more likely to be recognized due to their dramatic nature,” writes David Fisman of the University of Toronto. This attention can mean that control measures are put in place more readily, he explained in a posting to the ProMED email list. Other coronaviral diseases, such as SARS and MERS, involved super-spreader events, although it’s still too early to say if 2019-nCoV will have the same.
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.
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.”