While most people sheltered in place over the weekend baking bread or streaming movies and TV shows, the co-founders of Instagram launched a new website that calculates how quickly the coronavirus is spreading in each state.
Kevin Systrom and Mike Krieger, both graduates of Stanford University, created rt.live, which tracks the effective reproduction rate (Rt) of the novel coronavirus. Rt is similar to the basic reproduction number (R0), but Rt is adjusted to give a more real-time estimate of how fast a disease is spreading through the population.
“I was thinking about the metric I’d need to make decisions during times of uncertainty and this seemed like the right fit,” Systrom told Recode.
Tracking Rt over time in each state, it is possible to see the effects of actions, such as sheltering in place, taken by the local, state and federal government, and whether those steps are working.
An Rt value of one means that, on average, a person with the coronavirus infects one other person. If the value of Rt is less than one, that means the spread of a disease is slowing down and will eventually die out. If the Rt value is more than one, it means a disease is spreading through the population. The higher the value of Rt, the faster a disease is spreading.
As of late Monday evening, the Rt value for California was 0.44, with a peak Rt of 3.2 on March 16-17. California issued a statewide shelter-in-place order on March 19 that went into effect on March 20. Since then, the Rt value in California has trended downward.
According to the latest figures on, California has the sixth-lowest Rt value in the United States, with Nebraska, which has yet to issue an order to shelter in place, at No. 1 with an Rt of 2.0, meaning a resident of Nebraska with the coronavirus is, on average, infecting two other people. Three weeks ago, all 50 states had an Rt value over 1.0.
“Without the use of a clear metric on our ability to contain the coronavirus pandemic, it’s difficult to imagine that we’ll manage a return to normalcy anytime soon,” Systrom said in his blog, where he describes how they calculated Rt. “I hope by sharing this work with you, you will consider Rt as the metric that can guide our analysis and decision-making.”