I wanted to give a basic picture of why #TestTraceIsolate is so important to the control of the #COVID19 epidemic and why we need to move quickly when testing and contact tracing in order to create an effective intervention. 1/14
The core idea of why this is so important is that we have evidence that a substantial fraction of transmission occurs in the ~2 days before symptom onset. Figure from @lucaferrettievo et al (science.sciencemag.org/content/early/…). 2/14 
Similarly, He et al (nature.com/articles/s4159…) estimate 44% (95% confidence interval from 25% to 69%) of transmission events occur presymptomatically, with these generally occurring between 0 and 2 days before symptom onset. 3/14
If someone is infected, but not yet symptomatic, there's no way for them to know that they may be contagious and so they won't know to take appropriate quarantine measures. 4/14
Identifying and isolating presymptomatic individuals is key to limiting transmission in a #TestTraceIsolate intervention. And for this to happen, everything needs to move very quickly from the alerting of a confirmed case to the quarantine their contacts. 5/14
Here is a median scenario in which Alice is infected at day 0, but does not show symptoms until day 5, despite being infectious at day 3. This results in transmission to Bob at day 4 and Bob, in turn, infecting Carol at day 8. 6/14
In order for #TestTraceIsolate to be truly effective, Bob needs to be alerted by day 7 to quarantine. This is a tight timeline. It requires that Alice get tested on day 5 when she develops symptoms and rapid turnover of lab assay and alerting. 7/14
Thus, we often have a ~48hr window after an index case develops symptoms to test and alert contacts. This turnaround is highly challenging, but if done in time and Bob is alerted to quarantine we can break the transmission cycle. 8/14
Challenges here lie in getting someone with symptoms immediately tested as well as getting contacts rapidly alerted. However, the basic strategy of #TestTraceIsolate as routinely conducted by public health has demonstrated efficacy. 9/14
We need a huge push to increase the speed and scale of contact tracing, but this doesn't necessarily require "digital" solutions. statnews.com/2020/04/15/cor… 10/14
Going forward, we should be empowering local and state public health to do their job with contact tracing, while simultaneously thinking about how new technologies can be brought to bear. 11/14
With @nexttrace, we're trying to thread this needle and think about strategies that can empower speed and scale of #TestTraceIsolate, while adhering to the fundamental process that we know works. 12/14
To this end, we're prototyping a survey-based contact tracing platform that can be deployed by public health officials around the United States. Our current thinking is available in the GDoc here: nexttrace.org/resources 13/14
Huge thanks to public health colleagues and to the @nexttrace team including @alliblk, @rebeccaegger, @thefreemanlab, @marcprecipice, @colinmegill, @dbsmasher, @miguelp1120, @trs, @dylanbgeorge and Leah Alpert for help thinking through this problem. 14/14
