Belgium's SARS-CoV-2 contact tracing effectively curbed transmission, yet challenges persist in terms of delays, scalability and population adherence

Abstract

Background Through contact tracing, researchers could reconstruct chains of linked cases by linking High-Risk Exposure Contacts (HREC) testing positive to their index case. We present chain-level descriptives, delays and an individual-level analysis of the events that determine chain progression. Methods We used data from contact tracing outside of collectivities between September 2020 and March 2022 in Belgium. We present chain-level generation counts and structure. The individual-level analysis focusses on the events necessary to establish transmission chains from index cases: inclusion, successful contact with the call center, reporting HREC, identifying HREC and testing of HREC. We adjusted for age, sex, household, calendar time, region, vaccination, laboratory-confirmed prior infection, viral load, symptoms and the place in the transmission chain: we differentiated Cases Identified As HREC (CIAH, index cases that had been identified as HREC of another index case) from primary cases. Results The percentage of CIAH over all cases was 14% and most CIAH (63%) were household-members of the index case. Unsuccessful contact of cases (34% of all index cases) and reporting no HREC (53% of successfully contacted index cases) ended most chains. Among identified HREC, 75% had at least one test result. Persons age 65 and over were less likely successfully contacted (OR 0.59 (95%CI 0.58-0.59)), reporting HREC (OR 0.55 (95%CI 0.54-0.56)) or tested as HREC (OR 0.87 (95%CI 0.85-0.89), reference 25-44 year old). Compared to primary cases, CIAH were associated with less HREC and lower HREC test positivity. Delays however were comparable. The delay from symptom-onset to test result exceeded 4 days for 29% of CIAH. Conclusions While contact tracing effectively reduced transmission within identified chains, over 80% of cases remained unlinked due to, amongst others, contact failures and non-reporting of HREC. Future efforts would benefit from operational improvements and enhanced population participation.