Fig 9. Test-trace-and-isolate (TTI) policies allow for greater freedom (quantified by the gross reproduction number R_t) while observing the same reproduction number ${\widehat{R}}_t^{\text{obs}}$.

Systematic efforts to slow down the spread of the disease, such as mass testing (random screening) and contact tracing, allow decreasing the observed reproduction number of the disease. For observing the same outcome in ${\widehat{R}}_t^{\text{obs}}$, the gross reproduction number R_t would increase, or, in other words, individuals would be allowed to increase their potentially contagious contacts. Therefore, we extrapolate the R_t allowed in a full TTI setting at low case numbers and determine the equivalent R_t trends required to reach the same ${\widehat{R}}_t^{\text{obs}}$ in different regimes, starting from the raw value considering no TTI (red curve). Assuming that the relationship between R_t and ${\widehat{R}}_t^{\text{obs}}$ is exponential (Eq (35)), we can obtain the expected R_t trends in the low-case numbers TTI regime. Starting from the raw R_t curve (red, 1), we can obtain R_t in all the other possible regimes: under strict testing criteria (yellow, 2), self-reporting (green, 3), or full TTI (blue, 4). Adapted from [18].