The name Rt comes from the metric which the site tracks, which the site describes as “the effective reproduction rate of the virus calculated for each locale. It lets us estimate how many secondary infections are likely to occur from a single infection in a specific area. Values over 1.0 mean we should expect more cases in that area, values under 1.0 mean we should expect fewer.”
Instagram co-founders Kevin Systrom and Mike Krieger left the company in 2018, but they have now reunited to launch a website for tracking the spread of coronavirus, officially called COVID-19, across the U.S. The Rt.Live website shows how fast the virus is spreading with data for each state.
The reproduction rate, or R0, measures how many other people an infected person is likely to infect, and is a key metric which countries and regions use to determine whether the epidemic will continue or peter out. Getting this rate below 1 is being used as one essential prerequisite for re-opening public life in many countries. But this new site tracks the effective reproduction rate, or Rt, which is an estimation of whether the spread of the virus is continuing to accelerate, or whether is spread is lessening.
In support of the use of this particular metric, the site links to a blog post by programmer and entrepreneur Kevin Systrom, who argues that R0 is of limited use because it is static, while Rt can track changes over time.
At the time of writing, the data from Rt.Live shows six states which have an Rt below 1 including confidence intervals. A large number of states are hovering around the rate of 1 to 1.2, while 10 states still have an Rt of over 1.2, with Georgia having the highest rate of 1.5.
As well as a chart showing the overall Rt for all states, the website has graphs for each individual state that show how the rate has changed over time between March 15 and April 17.
You can also sort the all-states graph by various categories, to see selected states such as the 10 largest states, the states which have not instituted a shelter-in-place order, and states by area of the country.