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We’re wearing masks. We’re social distancing. We’re avoiding large gatherings, especially inside. At least, most of us are. As officials endeavor to underline the importance of these and other measures in containing the coronavirus pandemic, how can we tell if our collective efforts are working?

Cases per 100k, positivity rate, hospital admittance and deaths are some of the commonly tracked stats. Health experts around the globe have varying interpretations of each one, and they mean different things amid different conditions.

There’s one more metric, and it focuses specifically on the rate of viral spread.

Referred to as Rt, it estimates how fast or slow the pathogen that causes COVID-19 is propagating among a community. In broad strokes, Rt can be thought of as measuring how many other people each infected person will go on to infect. Keeping this number down is what contact tracing aims to accomplish.

A variety of sites have popped up where you can track this curve for various nations or different U.S. states, but finding Rt for Philadelphia alone was difficult — until now.

(If you’re viewing on mobile, hold your phone sideways to view the full curve.)

The chart above is calculated using data from the Harvard Chan School of Public Health. It will stay updated on this page as researchers refresh their information online; this happens multiple times each week. Each point shows a 7-day average for a smoother approximation of trends.

What does Rt actually mean?

Popularized by German Chancellor Angela Merkel in a viral video back in April, the statistic is complicated, and is less useful for predicting the future than for taking an assessment of current and past conditions.

The number epidemiologists call R0 provides a base-level approximation of how fast cases reproduce within any specific community, and it varies according to herd immunity and other factors. Rt — what’s shown in the graph above — is what researchers call the statistic as it evolves over time during an active epidemic.

If Rt = 1, each positive case is causing one other case. Not great, but not horrible. Horrible is when Rt is greater than 1, because that means an infection is spreading exponentially — exactly what happens to cause an epidemic, or in the case of COVID-19, a global pandemic. When Rt is less than 1, it means the infection is being contained.

If you’re interested in a longer explanation, science journal Nature has a good piece on this here.

How contact tracing works in Philly

The Philadelphia Department of Public Health created a new division to build a contact tracing program, which Health Commissioner Dr. Tom Farley has called “even more important as case counts go down.”

As of late July, the city had hired about 110 people to work on the project, including interviewers, tracers and supervisors.

The group is ethnically diverse: 57% are African American, 25% are Caucasian, 6% are Latino/Latina and 11% are Asian, per Farley. There’s an entire Spanish-speaking team, and a quarter of the staff speaks two languages; a sixth were born outside the U.S.

Staffers were recruited from local communities with help from neighborhood associations, because trust is paramount in the job. These folks have to convince random strangers — some of whom may know they were in contact with a COVID carrier, many of whom do not — that a) it’s not a spam robocall and b) people should discuss their personal habits and specific whereabouts.

Privacy is a huge part of the training, and Farley said data is never shared outside the health department, not with law enforcement or anyone else.

Still, about a third of people called by Philly’s contact tracers are not answering the phone or providing any information about their contacts. If you or anyone you know sees a call from 215-218-XXXX, pick up. It can help keep Rt down and get the city closer to full reopening.

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Danya Henninger

Danya Henninger is director of Billy Penn at WHYY, where she oversees the team, all editorial decisions, and all revenue generation — including the...