Table 3.

Specific contact tracing interventions achieving high (> 50% reduction in reported outcomes), intermediate (between 10% and 50% reduction in reported outcomes) and low effectiveness (< 10% reduction in reported outcomes): studies modelling the epidemic in a context other than the 2020 lockdown reopening

Specific intervention modelled	High effectiveness	Intermediate effectiveness	Low effectiveness
Manual forward tracing of primary contacts	- Isolation (cases + HH contacts) + tracing all contacts [87] *** R reduced by 64% - One-step tracing, identification and quarantining of social contacts of those testing + (with 10% of infected tested per day of illness) [39] *** peak number of infected reduced by 60% - Contact tracing with 100% trace and quarantine efficacy [44] *** Reduced peak in infectious prevalence by 75% - Contact tracing of symptomatic and variable tracing rate of asymptomatic (0-100%) [47] *** Infection density per 10,000 population reduced between 65-88% - Contact tracing of any infected individual with higher than 47% quarantine efficacy [61] *** epidemic size is reduced by 95% - Isolating cases, tracing and testing contacts with 40% efficacy [91] *** Deaths reduced by 99% - In the context of advanced epidemic with vaccination and with/without social distancing, tracing 40% of contacts [38] *** Deaths reduced by 71%/74%	- Manual tracing of HH (instant), School (0.5 days after case isolation), Work/ Causal contacts (within three days of case isolation) with moderate (50%) quarantine efficacy [81] *** R is reduced by 35% - Tracing and testing HCW with a two-day/ seven-day window and reduction in transmission due to PPE [53] *** Reduction in R of 32-37% - Isolation of infected (85%) + 10%/30% contact tracing coverage [64] *** Reduction of infections by 23%/41% - Tracing and testing 80% of HH/ Work/ School contacts with two-day test turnaround time [66] *** Reduction in infections 25.5% - Contact tracing without (with) mask-wearing with a shelter in place order [92] *** Reduction in reported cases 11% (30%)	- Tracing and testing 80% HH/ Work/ School contacts with eight-day test turnaround time [66] *** Infections reduced by 8%
Digital or hybrid (manual + digital) contact tracing	- Hybrid tracing of HH/ Work/ School/ Casual contacts within 3 days with moderate/ high quarantine efficacy and 75% app adoption [81] ***R reduced by 48%/53% - Manual tracing of acquaintances and digital contact tracing with 53% app adoption [87] *** R reduced by 61%	- Random testing with 10 tests per thousand per day and digital contact tracing (50% app adoption) [88] *** Infections reduced by 20%
Tracing of secondary contacts	- Tracing primary contacts with variable (0.3–0.9) probability of success and tracing their secondary contacts [11] *** Infections reduced by 78% - Perfect isolation of infected + 100% contact tracing coverage of primary and secondary contacts [64] *** Infections reduced by 82% - Tracing secondary contacts with a mild lockdown [37] *** Recovered reduced by 99%
Delays to contact tracing	- With 14-day isolation/ quarantine (57-67% adherence), reducing tracing delay from three to zero days [55] *** 58% of transmissions averted	- Testing 80% of symptomatic and contact tracing (80% coverage) with one-day delay to contact tracing [86] *** R reduced by 26%	- Testing 50% of symptomatic and contact tracing (50% coverage) with two-day delay to contact tracing [86] *** R reduced by 8%
Backward or bidirectional contact tracing	- Bidirectional contact tracing across a broad range of infectivity, tracing coverage and quarantine efficacy levels [9] *** Increased number of cases averted by 200-300%	- Hybrid bidirectional contact tracing with low/high uptake, a two-day tracing window and social distancing [10] *** R reduced by 21%/ 42% - Bidirectional contact tracing with 2-day window (70% coverage) or bidirectional contact tracing with 6-day window (50% coverage)  [70] *** R reduced by 10% - Hybrid bidirectional contact tracing with Bluetooth technology (20% adoption) [36] *** Infections reduced by 10%

* HH = households; HCW = health care workers