How efficient is contact tracing?

One of the challenges in predicting future evolution of the COVID-19 pandemic is figuring out what the impact of contact tracing will be. Contact tracing at such a large scale will be new for most states and to our knowledge there are no studies that can inform us about the effectiveness of the practice in the US. In our analysis, we made some assumptions on contact tracing effectiveness based on data from China. (What we mean by contact tracing effectiveness is the degree with which infected individuals become more likely to be detected through contact tracing.) Admittedly, however, there is significant uncertainty around this assumption. Therefore, we investigated how our results would change with changes in contact tracing effectiveness. The following figure illustrates our main observation. In the figure, CTEP stands for Contact Tracing Effectiveness Parameter. In the figures we posted so far for North Carolina, CTEP was set to 5. The figure shows how things would look like if contact tracing is less effective (CTEP = 2) and if contact tracing is more effective (CTEP =10) under the assumption the state stays in Phase 2 until the end of the year.

As we can see from the figure, the effectiveness of contact tracing will have a significant impact on how bad things will get and so it is possible that (if for instance contact tracing is so effective that CTEP would be even larger than 10) future evolution of the pandemic may not be as disastrous as we predicted. However, this is really a big if not only because, as suggested by some news articles, contact tracing so far is not working as well as hoped but also contact tracing capacity is far below the levels it needs to be to meet the anticipated workload. We clearly see this from our analysis. I will explain this in more detail in my next post.