RR:C19 Evidence Scale rating by reviewer:
Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.
In this manuscript Endo et al. attempt to quantify the relative improvement in efficacy of contact tracing strategies in terms of cases averted in the 3rd generation of cases. The authors set up a simple branching process model, and assuming a negative binomial offspring distribution, present the G3 cases averted under a combination of parameters. The aim of the study is to compare forward contact tracing with forward and backward contact tracing.
The effectiveness of contact tracing strategies is an important scientific and public health question. Given the over-dispersion in the transmission of COVID-19, it seems intuitive that a combination of backward and forward contact tracing would be better at breaking up clusters than forward contact tracing along. Figure 1 in the manuscript explains the issue in a succinct manner. Through some basic analysis and simulations, the authors attempt to quantify this intuition and show that introducing backward tracing in addition to forward tracing increased the effectiveness of contact tracing by a factor of 2-3. This is a useful first step.
While the manuscript is technically sound, I recommend adding a discussion about situations or parameter ranges where forward+backward tracing either did not improve the efficacy as compared to forward tracing alone, or improved it only marginally so that the extra cost/effort may not be worthwhile. While the authors explore a range of parameter values, it would be useful to note (if only in the discussion) which combinations are closer to the “real world”. Finally the results section is a bit sparse, and there should be full discussion of the results presented in the figures to help the reader interpret the evidence.
There are several real-world complexities that have been (rightly) left out of the model e.g., example, cost-effectiveness, operational challenges, simplifying assumptions etc. These caveats have been noted in the discussion. Future development of the methods incorporating these elements will likely add nuances to the conclusions of the study.
I noted one typo in the study: "Backward tracing first identifies the primary case (G0) that infected the index case and then applies forward tracing to those infected by the primary case (G1)." Presumable the authors meant G0 instead of G1.