from February 17, 2020

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It’s time to introduce another concept from epidemiology, the variable R0, also called R naught. It represents the reproduction rate of a virus spreading disease from one host to another. The simplest way to think of it is if R0=1, that means that each person infected spreads it to one other person. If the value is less than one, the disease is diminishing in the population, if it’s greater than one, it’s multiplying. At R0=2 or greater, we see exponential growth.

To put it in numbers, if a disease had a perfect R0=2 rate, we would see the numbers of cases like this increase at this rate over a set interval periods of time: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, etc.

If you look at the incidence of cases since about Feb 9 (with the exception of Feb 12 due to the change of diagnostic criteria in Hubei province), the number of new cases has remained relatively flat. Granted, it’s not a very long number of days to draw solid conclusions, but to me this is a pretty good indicator that R0 is pretty close to 1, which is a good sign. That could mean that the isolation, quarantine, and PPE measures that have been put in place are doing their job effectively.

On a broader sense, the longer we can delay the start of an outbreak, the better off we are. It provides time to get PPE measures in place, education of the public and healthcare providers, and time for vaccine development. For these reasons, I remain optimistic at present.

Disclaimer: This commentary is my own interpretation and does not represent the analysis by the government or my employer. The data is from the Johns Hopkins University’s Center for Systems Science and Engineering.

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