Table 1.
Summary of included studies of contact tracing* for COVID-19
| Country | Study year | Design | Setting | Sample | Intervention | Control | Outcome measured | Results† | Risk of bias | |
|---|---|---|---|---|---|---|---|---|---|---|
| Fetzer and Graeber (2021)20 | UK | 2020 | Quasi-experimental design, with difference-in-differences regression | Lower tier local authorities (districts, boroughs, and city councils) around the country (urban and rural) | 15 841 COVID-19 cases from Sept 25 to Oct 2, 2020, with contact tracing accidentally not performed on an estimated 48 000 contacts due to a computer error; areas were compared based on the degree they were affected by the error, which was seemingly random | Timely contact tracing for close recent contacts (not fully described) including instructions to self-quarantine; other social distancing policies in place not fully described | Delayed contact tracing for close recent contacts (not fully described), including delayed instructions to self-quarantine (computer error); other social distancing policies in place not fully described | Forward transmission: cases not prevented, deaths not prevented | Accidental failure to conduct timely contact tracing for 15 841 index patients (around 20% of index patients during the one-week period) was estimated to have led to an additional 126 836–185 188 additional COVID-19 cases during the 6 weeks after the error was discovered (22·5–32·8% of all cases during the period), as well as an additional 1521–2049 deaths over the 6 weeks (30·6–41·2% of all deaths during the period); ie, there were an estimated 18·6 additional COVID-19 cases per late referral and 0·24 additional deaths per late referral | Low |
| Kendall et al (2020)21 | UK | 2020 | Retrospective cohort study | Isle of Wight compared with rest of the UK (urban and rural) | All COVID-19 cases in the UK from March 28 to June 29, 2020 (exact number not specified) | Isle of Wight: traditional contact tracing (not fully described; starting May 5, 2020) and digital contact tracing with a Bluetooth-powered mobile application (starting May 7, 2020), with related advertising and community discussions (not fully described); pre-existing social distancing policies simultaneously in place (not fully described) | Rest of UK: initially no contact tracing, followed by traditional contact tracing only (not fully described; starting May 28, 2020); pre-existing social distancing policies simultaneously in place (not fully described) | Forward transmission: R0 | After intervention initiation, the Isle of Wight had a decrease in R0 (1·3 to 0·5) between May 5 and June 29, 2020, and also had a lower R0 than the UK as a whole at the same timepoint (p<0·0001) | Low |
| Liu et al (2021)22 | 130 nations | 2020 | Country-level cohort study | 130 nations (urban and rural) | All COVID-19 cases in 130 nations from Jan 1 to June 22, 2020 | Contact tracing with intensities rated as comprehensive, limited, or none (not fully described; different countries likely to have employed different approaches); 12 other NPIs were also assessed and controlled for | No contact tracing or decreased intensity of contact tracing (not fully described) | Forward transmission: Rt | Contact tracing was associated with an increase in Rt (exact effect size not specified) at 10 days; this increase might have reflected an increase in case detection, or other temporally associated NPIs in the model (eg, testing policies) | Low |
| Malheiro et al (2020)23 | Portugal | 2020 | Retrospective cohort study | Eastern Porto (urban) | All COVID-19 cases in eastern Porto from March 1 to April 30, 2020: 551 index patients, 1627 close contacts | Identification as a potential COVID-19 case through contact tracing (interview with patient or caregiver) or travel history, with mandatory quarantine prior to testing positive; social distancing policies (limiting movements and business activities) simultaneously in place starting March 22, 2020 | Identification as a COVID-19 case through testing, with no mandatory quarantine prior to testing positive; social distancing policies (limiting movements and business activities) simultaneously in place starting March 22, 2020 | Forward transmission: secondary attack rate | There was no significant difference in the secondary attack rate for index cases between the intervention group (16 of 132, 12·1% [95% CI 7·1–18·9]) and the control group (138 of 1495, 9·2% [7·8–10·8], p=0·13), including when stratifying by the presence or absence of social distancing policies (p=0·72) | Some |
| Park et al (2020)24 | South Korea | 2020 | Pre–post design | Seoul (urban) | All COVID-19 cases in Seoul from Jan 24 to May 2, 2020: 637 index patients, 16 176 contacts | Post-period (March 9 to May 2, 2020): widespread testing of contacts associated with clusters of COVID-19 cases (identified via interviews, global positioning system data, credit card histories, drug utilisation review [not fully described], closed-circuit television records, and possibly other methods) and individuals with COVID-19 symptoms; all positive-testing individuals moved to health-care settings for quarantine; other social distancing policies in place not fully described | Pre-period (Jan 24 to March 8, 2020): widespread testing of individuals with COVID-19 symptoms only; all positive-testing individuals moved to health-care settings for quarantine; other social distancing policies in place not fully described | Forward transmission: Rt | The intervention was associated with a decrease in Rt across Seoul (1·3 during the pre-period, 0·6 during the post-period) | Some |
| Wymant et al (2021)25 | UK | 2020 | Retrospective cohort study | All of England and Wales (both urban and rural) | All COVID-19 cases in England and Wales from Oct 8 to Dec 31, 2020: 1 892 000 index patients (32 500 deaths) | Digital contact tracing with a Bluetooth-powered mobile application, with simultaneous manual contact tracing and additional restrictive and lockdown measures (not fully described) in place | No digital contact tracing, but with simultaneous manual contact tracing and additional restrictive and lockdown measures (not fully described) in place | Forward transmission: cases prevented, deaths prevented | With matched-neighbours regression, use of the contact tracing application by 16·5 million (27·8% of 59·4 million people in England and Wales) was estimated to prevent 594 000 COVID-19 cases (95% CI 317 000–914 000) and 8700 deaths (95% CI 4700–13 500) between Oct 8 and Dec 31, 2020; each percentage point increase in application usage was associated with a 2·26% (95% CI 1·50–3·00) reduction in cases for the total period (1·09% [0·04–2·14] reduction from Oct 8 to Nov 6 and 2·66% [1·75–3·56] reduction fromNov 7 to Dec 31 following application improvements released on Oct 29 that were not fully described) | Low |
R0=reproduction number. Rt=time-varying reproduction number. NPI=non-pharmaceutical intervention.
*
Provider-initiated contact tracing for all studies listed.
†
Significance or non-significance of result is not stated when it was not specified in the study.