LAUSR

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. 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. 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. 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.