A hospital-related outbreak of SARS-CoV-2 associated with a novel variant Cal.20C (B.1.429) in Taiwan : transmission potential and outbreak containment under intensified contact tracing, January–February 2021

Objectives A hospital-related cluster of 22 COVID-19 cases occurred in Taiwan in January–February 2021. Rigorous control measures were introduced and could only be relaxed once the outbreak was declared over. Each day after the apparent outbreak end, we estimated the risk of future cases occurring, to inform decision-making. Methods Probabilistic transmission networks were reconstructed and transmission parameters (the reproduction number R and overdispersion parameter k) were estimated. We estimated the reporting delay during the outbreak (Scenario 1). We also considered a counterfactual scenario with less effective interventions characterized by a longer reporting delay (Scenario 2). Each day, we estimated the risk of future cases under both scenarios. Results The values of R and k were estimated to be 1.30 (95% credible interval: 0.57,3.80) and 0.38 (0.12,1.20), respectively. The mean reporting delays considered were 2.5 days (Scenario 1) and 7.8 days (Scenario 2). The inferred probability of future cases occurring declined more quickly in Scenario 1 than Scenario 2. Conclusions Following outbreak containment, rigorous control measures allowed the outbreak to be declared over quickly. This highlights the need for effective interventions, not only to reduce cases during outbreaks but also to allow outbreaks to be declared over with confidence