Thread on why the launch of @CovidWatch in Arizona yesterday is different from other launches. This GAEN app is superior re risk scoring, re rollout strategy, and re customizability (the last two related) blog.covidwatch.org/en/covid-watch… 1/10
Other apps try to reproduce 6 feet (or 1 meter or 2 meters) for 15 minutes. But how infectious someone is also varies ~10-fold as function of timing relative to the day their symptoms begin 2/10 
@CovidWatch integrates timing, duration, and Bluetooth signal (a noisy correlate of distance) to estimate infection risk. 7 feet for 8 hours at most dangerous time is much riskier than 5 feet for 15 minutes at edge of infectious period 3/10
Using infection risk to recommend quarantine, rather than 6 feet plus 15 minutes, focuses quarantine where it can do the most good to break the chain of transmission. Because quarantine causes financial and psychological harm, maximizing the benefit is a matter of ethics. 4/10
The threshold for quarantine can be adjusted. We are starting by notifying only those at highest risk, and offering them testing. If testing capacity can keep up, we will lower the threshold and recommend more quarantine. Note the diagonal structure of quarantine thresholds. 5/10
A region can switch from the current "Basic" setting to an "Advanced" version that also makes flexible the length of quarantine. Coming soon under Advanced: adjustment of quarantine given negative test results, taking into account their time-dependent sensitivity. 7/10
Where thresholds are strict to maintain containment success, incorporating negative test results prevents overly long quarantine. Given a long right tail of incubation periods, and asymptomatic infections, 14 days quarantine is not conservative in the absence of testing. 8/10
Because @CovidWatch is built around customizable regional config files, it can easily be adapted for a new jurisdiction. They think they can get new entries on board in 4 weeks. covidwatch.org/public-health 9/10
Rollout requires ground operation so that those issuing positive test results also issue secure codes. But staged rollout isn't just a logistic concession: it's the best way to achieve long-term high uptake. It worked for facebook, whatsapp, and uber hbr.org/2020/07/how-to… 10/10
Thanks @DiseaseEcology for nudging me to write this!
