Tracking the COVID-19 Reproduction Number

The basic reproduction number, represented by R0 (and pronounced “R naught”), is an epidemiologic statistic that is used to quantify the transmissibility of an infectious pathogen. R0 represents the average number of people who are infected by a person with the infection. If R0 > 1, it means the overall number of people with the infection is growing. If R0 < 1, it means the overall number of people with the infection is shrinking, and the infection will eventually end. The effective reproduction number can also be specified at a particular time t, and it is represented as R(t) or Rt. Much more is written about the reproduction number in this article.

Instagram founders Kevin Systrom and Mike Krieger introduced Rt COVID-19, a website that presents Rt values state by state.



In the main chart, you can view current and past values of Rt for all states and also filter by the ten largest, no shelter in place, northeast, west, midwest, and south. There are also state-specific charts where you can see current and historical Rt values.

Case count data from The COVID Tracking Project are used for these visualizations, and Kevin Systrom explains his methodology for calculating Rt. This related article provides more information. A limitation to any statistic related to COVID-19 is that we don’t truly know the number of actual cases due to the fact that we still have limited testing capacity, but the numbers are interesting nonetheless. How is your state doing?