In brief: Instagram founders Mike Krieger and Kevin Systrom have developed a Covid-19 tracker that measures how fast the virus is spreading in each state. Systrom believes Rt will be key to getting us through the next few months. "Without knowing Rt, we are simply flying blind," he said.

As Systrom highlights, epidemiologists have argued that one of the only true ways to combat Covid-19 is to understand and manage the Rt – the effective reproduction number. In the simplest of terms, Rt is a measure of the number of people who become infected by an infectious person. If the Rt is above 1.0, the virus will spread rapidly. Values under 1.0 suggest the virus will stop spreading.

Systrom said it is impossible to measure Rt directly, so it must be estimated.

Tracking Rt can help us better understand how effective various stay-at-home strategies are and provide insight into when we might be able to loosen restrictions and send people back to work. For example, states like California have been under strict lockdown for weeks while other states like Nebraska and Iowa have been far more lax in their response. As a result, California’s Rt is under 1.0 while both Nebraska and Iowa are over 1.0.

Those interested in digging deeper are invited to download the Jupyter notebook for how they are calculating Rt as well as calculated Rt values on a per-state basis. Case count data from The Covid Tracking Project is also available for independent analysis.

Masthead credit: OSORIOartist