Kevin Systrom and Mike Krieger, Instagram’s founders, teamed up to launch their first product since leaving Facebook. Rt.live is an up-to-date tracker of how fast COVID-19 is spreading across each state in the U.S.


“Rt” has been used as a metric during the COVID-19 pandemic. It measures the average number of people who become infected by an infectious person. The higher the Rt is above 1 for a state, the quicker the virus is spreading.


“If Rt is above 1.0, the virus will spread quickly,” the Rt.live website explains. “When Rt is below 1.0, the virus will stop spreading.”


(via Vox): While the numbers of cases and deaths are important metrics, these figures lag behind the present situation since it takes a while before symptoms show and people are tested. That’s why Systrom and Krieger zeroed in on a different metric: the effective reproduction rate (Rt) of Covid-19. Their new site presents this Rt metric in a series of charts showing how the spread of coronavirus in the United States shifts over time.


You might be more familiar with the basic reproduction number (R0), which represents the number of infections caused by a single infection of a given disease. Rt, which is sometimes referred to as Re, uses statistical adjustments to create a more real-time and dynamic estimate of how fast the disease is spreading. Plotted out over the past few weeks, the Rt metrics of every state in the country can show the spread of the novel coronavirus being affected by government actions like stay-at-home measures.


Krieger explained some of the motivation behind launching the Rt.live dashboard in an interview with TechCrunch.


“As states decide whether and how to open back up, they’ll have to manage their infection rate carefully, and we hope dashboards like rt.live will be helpful in doing so,” Krieger said.


But while Rt is able to track the spread of the virus, it alone is not enough to reason to end shelter-in-place measures.


“If people are using it for permission to open things back up, it’s not the full story because there are other considerations, like what systems do you have in place to prevent this from shooting back up again,” said Natalie E. Dean, an assistant professor of biostatistics at the University of Florida. “If you just open everything back up but are not testing widely, isolating cases, and contact tracing, it’s just going to shoot back up.”


At the moment, it is not yet clear how Rt.live will factor into public health decisions.