Contact tracing for COVID-19 in a Swiss canton: analysis of key performance indicators.

BACKGROUND: Contact tracing (CT) has played an important role in strategies to control COVID-19. However, there is limited evidence on the performance of digital tools for CT and no consensus on which indicators to use to monitor their performance. We aimed to describe the system and analyse outcomes of CT with a partially automated workflow in the Swiss canton of Solothurn, using key performance indicators (KPIs). METHODS: We describe the process of CT used in the canton of Solothurn between November 2020 and February 2022, including forward and backward CT. We developed 16 KPIs representing CT structure (S1-2), process (P1-11) and outcome (O1-3) based on previous literature to analyse the relative performance of CT. We report the changes in the indicators over waves of SARS-CoV-2 infections caused by several viral variants. RESULTS: The CT team in Solothurn processed 57,363 index cases and 71,809 contacts over a 15-month period. The CT team successfully contacted 99% of positive cases within 24 hours (KPI P7) throughout the pandemic and returned almost all test results on the same or next day (KPI P6), before the delta variant emerged. Three-quarters of contacts were notified within 24 hours of the CT interview with the index (KPI P8) before the emergence of the alpha, delta and omicron variants, when the proportions decreased to 64%, 36% and 54%, respectively. The percentage of new symptomatic cases tested and interviewed within 3 days of symptom onset was high at >70% (KPI P10) and contacts started quarantine within a median of 3 days of index case symptom onset (KPI P3). About a fifth of new index cases had already been in quarantine by the time of their positive test (KPI O1), before the delta variant emerged. The percentage of index cases in isolation by day of testing remained at almost 100% throughout the period of analysis (KPI O2). CONCLUSIONS: The CT in Solothurn used a partially automated workflow and continued to perform well throughout the pandemic, although the relative performance of the CT system declined at higher caseloads. CT remains an important tool for controlling the spread of infectious diseases, but clearer standards should improve the performance, comparability and monitoring of infection in real time as part of pandemic preparedness efforts.

Population-Based Severe Acute Respiratory Syndrome Coronavirus 2 Whole-Genome Sequencing and Contact Tracing During the Coronavirus Disease 2019 Pandemic in Switzerland.

BACKGROUND: Testing and contact tracing (CT) can interrupt transmission chains of SARS-CoV-2. Whole-genome sequencing (WGS) can potentially strengthen these investigations and provide insights on transmission. METHODS: We included all laboratory-confirmed COVID-19 cases diagnosed between 4 June and 26 July 2021, in a Swiss canton. We defined CT clusters based on epidemiological links reported in the CT data and genomic clusters as sequences with no single-nucleotide polymorphism (SNP) differences between any 2 pairs of sequences being compared. We assessed the agreement between CT clusters and genomic clusters. RESULTS: Of 359 COVID-19 cases, 213 were sequenced. Overall, agreement between CT and genomic clusters was low (Cohen's κ = 0.13). Of 24 CT clusters with ≥2 sequenced samples, 9 (37.5%) were also linked based on genomic sequencing but in 4 of these, WGS found additional cases in other CT clusters. Household was most often reported source of infection (n = 101 [28.1%]) and home addresses coincided well with CT clusters: In 44 of 54 CT clusters containing ≥2 cases (81.5%), all cases in the cluster had the same reported home address. However, only a quarter of household transmission was confirmed by WGS (6 of 26 genomic clusters [23.1%]). A sensitivity analysis using ≤1-SNP differences to define genomic clusters resulted in similar results. CONCLUSIONS: WGS data supplemented epidemiological CT data, supported the detection of potential additional clusters missed by CT, and identified misclassified transmissions and sources of infection. Household transmission was overestimated by CT.