Accepted for/Published in: JMIR Public Health and Surveillance
Date Submitted: Jun 4, 2021
Date Accepted: Sep 19, 2021
Date Submitted to PubMed: Sep 30, 2021
Characterization of unlinked cases of COVID-19: Implication on contact tracing measures
ABSTRACT
Background:
Contact tracing and intensive testing programmes are essential for COVID-19 control. However, conventional contact tracing is unable to trace all the cases due to a huge demand of resources. So far, few studies have reported the epidemiological features of cases who were not identified in the tracing (“unlinked cases”) or described their roles in seeding community outbreaks in-depth.
Objective:
Here we characterized the role of unlinked cases in the epidemic by comparing their epidemiological profile with the linked cases and estimated their transmission potential across different settings.
Methods:
We obtained the rapid surveillance data containing the line-listing of COVID-19 confirmed cases accounting for the first three waves in Hong Kong. We compared the demographics, history of chronic illnesses, epidemiological characteristics, clinical characteristics, and outcomes between linked and unlinked cases. Transmission potentials at different settings were assessed by fitting a negative binomial distribution to the observed offspring distribution.
Results:
Time interval from onset to hospital admission was longer among unlinked cases than the linked (median: 5.00 vs 3.78 days; P<0.001), with a higher proportion experiencing critical or serious conditions (13.0% vs 8.2%; P<0.001). The proportion of unlinked cases was associated with the increase of the weekly number of local cases (P=0.049). Cluster transmissions from the unlinked cases were most frequently identified in household settings, followed by eateries and workplaces, with the estimated probability of cluster transmissions being around 0.4 for households and 0.1-0.3 for the latter two settings.
Conclusions:
The unlinked cases were positively associated with the time to hospital admission, severity of infection, and epidemic size – implying a need to promote digital tracing methods on top of current conventional testing and tracing. To avoid cluster transmissions, digital tracing approaches should be effectively enforced in high-risk social settings, and risk assessment conducted to review and adjust the policies. Clinical Trial: NA
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