Biology / Epidemiology / Health / Microbiology / Science

What is R0?

Given the new coronavirus which has emerged in Wuhan, China, there’s a lot of panic spreading around the internet. This hasn’t been helped by the irresponsible and factually incorrect fearmongering of a Harvard epidemiologist, Dr Eric Feigl-Ding. Without linking to him, so as to prevent amplifying him further, I’ll give you the gist of what he said: he said that the R0 of nCOV [novel coronavirus] is 3.8, which is something he has never seen in his entire career, and is basically apocalyptic-level bad.

He’s either lying, or Harvard should be checking his credentials. An R0 of 3.8 is small potatoes: norovirus strains have been recorded with an R0 of 3.72; mumps has an R0 of 10; measles has an R0 of up to 18 in an unvaccinated population. The common cold has an R0 of 2-3 and — particularly damningly for Dr Feigl-Ding’s catastrophising — SARS had an R0 of 4. Nobody is yelling that these viruses are going to devastate humanity right now, even though they still exist in human populations.

But this comparison doesn’t really help if you don’t understand what R0 is, so that’s my job today!

Note: I have referred to the R0 of 3.8 mentioned by Dr Feigl-Ding throughout. There have been other estimates both higher and lower since his tweets. None of the articles are peer-reviewed and the information available is changing rapidly.

What is it, then?

First of all, the R0 is the “reproduction number” of a pathogen. It estimates how many people a single infected person will infect, on average, in a homogenous and susceptible population without any measures in place to prevent further infection. So an R0 of 18 means that from one child’s case of measles, 18 new cases will arise. Someone with the common cold (R0 of 2-3) will infect two to three other people before they recover.

The things you need to know in order to calculate R0 include the probability a susceptible person will be infected when they come into contact with an infected person, the likelihood of an infected and a susceptible person meeting, and the length of time an infected individual can pass on the pathogen to people they have contact with. It’s a rule of thumb, not a hard and fast rule, because it will change according to the prevailing conditions: lots of immune individuals in the population make it less likely an infected person will meet a susceptible person to infect in the first place.

For example, if you’re not infectious until you’re showing symptoms of a disease, and the first symptom of that disease is not being able to stand up, you’re unlikely to infect many people — you’ll stay at home, and perhaps only infect your family. If they’re good about their handwashing or the disease is only transmitted by sexual contact, then you might not even infect them.

As a real-world example, polio used to spread like wildfire among children. It has an R0 of 5-7, but a case in your neighbourhood should not be a cause for panic. Firstly, you’re most likely vaccinated, and the vaccine is pretty effective. Secondly, because polio infects via the faecal-oral route, modern hygiene and sanitation mean it’s no longer such a problem. Although the actual virus is as infectious as it ever was, you’re less likely to come across it because we wash our hands regularly and sewage is kept separate from most people’s daily lives.

In other words, the R0 isn’t an oracle.

Does a high R0 mean a disease is dangerous?

No. If there was a common disease with an R0 of 40, but we knew from history that all it did was change the pigment in your irises so people who were infected had purple eyes for a week, we wouldn’t be too concerned (providing that’s really all it did).

The real issue with any disease is the severity of symptoms and the mortality rate. There are some figures on that coming out of Wuhan; the most reliable one I’ve seen in the Lancet reports fairly high mortality, but it is discussing a really small number of cases in a homogenous population. It could be a sampling error; it could be due to closer contact with the virus or particular living conditions or even the genetics of those infected.

It is true that a disease with a high R0 is harder to control, though. That hypothetical violet-eye disease would sweep the world in weeks. An R0 of 3.8 is certainly not good news when it comes to a disease that’s already of concern. The bottom line is, there’s definite cause for concern with nCOV, but it’s still possible that improved control measures will bring it under control.

Should we worry about nCOV?

As a species, yes. There are a number of concerning factors about it, and if we get this under control, it’s important to remember that we will do so because we worked to understand the disease and to react appropriately. One thing that may have enabled its rapid spread is the fact that people can be infectious without showing symptoms. The initial checks for people exiting China and entering other countries were based on looking for symptoms like fever; this obviously wasn’t sufficient, in light of the people who can be infectious but appear perfectly healthy. This knowledge is going to be really important going forward!

Sometimes people mock the public health scares around swine flu and SARS. This is wrong-headed: those diseases were contained and controlled because of the public health efforts to educate people and contain the disease. The panic was an unfortunate side effect, and probably counter-productive. That’s why I’m particularly angry with Dr Feigl-Ding’s actions; if we do control nCOV, his wolf-crying may hurt future efforts to control disease by numbing people to real dangers.

Even if nCOV does sweep the world and there is significant mortality, Dr Feigl-Ding’s tweets created panic and spread misinformation, instead of level-headedly making people aware of the facts.

Yeah, but should I be losing sleep over it?

No, unless you are a medical professional likely to come into contact with nCOV, an epidemiologist working on nCOV, or a public health official. There is probably very little you as an individual can do at this point, but do keep an eye out for public health announcements from your government or local authorities. Practice good hygiene (wash your hands regularly, especially before touching your face, eating or preparing food), avoid close contact with people with respiratory symptoms, and make sure you stay otherwise healthy by sleeping, eating and drinking properly. In other words, the same rules which will always help you to stay healthy do apply here.

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