What Is R₀ of Coronavirus and Why Does It Matter?
If you want to describe how an infectious disease spreads, one convenient number is what epidemiologists call R 0 (“R no”), the disease’s main reproductive number. At the moment, the R 0 estimates of the coronavirus make it more infectious than influenza, but less so than for many other infectious diseases.
What is R 0 ?
R 0 is one of the numbers used by epidemiologists to describe how an infectious agent spreads through a population. But it’s important to remember that these are just statistics describing some of the numbers we see.
The basic idea is this: the average sick person from a perfectly susceptible population must be able to infect at least one other person (R 0 = 1) for the disease to spread. If the disease spreads to less than one person on average, there will be no outbreak.
So all epidemics are associated with something with an R 0 greater than 1. Seasonal flu has an R 0 of about 1.2 . Measles has one of the highest R 0 values, although the exact number is difficult to give: usually it is from 12 to 18 . Many diseases, from horrific to mundane, range from 2 to 7: Ebola, HIV, the common cold. The Guardian has a spreadsheet comparing the R 0 of several well-known infectious agents, if you’re wondering.
There are several important caveats to this number. First, he imagines what is happening in a completely vulnerable population. Nobody has been vaccinated; no one had time to develop immunity. Second, it says nothing about how quickly the disease will spread, but only about how many people it will eventually infect.Here is a short video from the University of Pennsylvania Epidemic Course that describes R 0 and how it relates to other information we need about the spread of disease.
What is the R 0 coronavirus?
The virus that causes COVID-19 is still fairly new, and scientists still understand its R 0. Much information is lacking: for example, how many people have had a mild form of the disease and have not been tested? Given the widespread lack of tests, and studies showing that unreported infections contributed to the rapid spread , there are still many unknowns.
Thus, the researchers who tried to calculate R 0 for the current flare had to make many assumptions. Over time and with the spread of COVID-19, we began to better understand what R 0 is . However, this is still a rapidly changing situation and the numbers are likely to change as we learn more.
Maya Majumder, one of the scientists working on the R 0 calculation for the coronavirus, has an excellent Twitter thread where she explains some of the assumptions her team used and discusses how their scores compare to those of other teams. Keep in mind that her research was closer to the beginning of the outbreak, when we didn’t know as much as we do now.
However, I know you are behind the numbers here, so here it is:
According to current estimates by the World Health Organization, the coronavirus has an R 0 of 2.0 to 2.5 . This is an estimate based on data provided in March 6th , which, at pandemic time, was about five years ago. Some noteworthy studies include an analysis of the Diamond Princess cruise ship , published in the International Journal of Infectious Diseases, which estimates an R 0 range of 2.06–2.52 with an average rating of 2.28. Another study published in the journal Science that analyzed an early outbreak in China estimates an initial R 0 of 2.38 before travel restrictions were imposed.
So far, these studies estimate R 0 to be somewhere in the 2.0–2.5 range. It is important to remember that this number depends on the situation and depends on the environment as well as the population of infected people.
How important is R 0 ?
R 0 is a useful number for understanding things like setting goals for vaccination (the higher the R 0 , the more people need to be vaccinated to stop the spread of the disease). But work to contain the outbreak could begin even before we hit R 0 .
R 0 depends on several things, including how long a person has been infectious, how many susceptible people they tend to interact with, and how much the infectious agent is transmitted.
This means that we can reduce the likelihood of an epidemic spreading by acting on these specific factors. We can make fewer people susceptible to the virus; that’s what the vaccine does. ( There is no vaccine for the coronavirus yet , but it may someday be available.) You can reduce the chances of the virus spreading by practicing social distancing, quarantining yourself if you become infected, and isolating yourself if you develop symptoms. You can reduce transmission by taking measures such as hand washing and physical distancing.
Editor’s Note: This article, which was originally published in January 2020, has been updated by Rachel Fairbank to include additional research, information and guidance on the coronavirus.