Surveillance of Imported Cases for Alerting Community-Acquired Outbreak by SARS-CoV-2 Variants From D614G to Omicron

Background: Global transmission from imported cases to domestic cluster infection is often the main route for the resultant community-acquired outbreaks facing the emerging SARS-CoV-2 variants. It is so important to monitor imported cases as to foretell outbreaks given various containment measures. Methods: We used Taiwanese COVID-19 epidemic data, mainly covering D614G and three VOCs (Alpha, Delta, and Omicron) from Jan 2020 to Jan 2022 to estimate the increased risk of domestic cluster infection per one imported case by type of variants given NPIs, test, and vaccination using an extra-Poisson regression model. The upper limit of predicted value one week before is used as the threshold for alerting community-acquired outbreak.Findings: An increase in one imported D614G case prior to one week led to 9·54% (95% CrI 6·44% to 12·59%) higher risk of domestic infection, yielding five clusters with the observed cases beyond the 95% upper limit but they did not lead to any outbreak due to effective contact tracing of infectives and the elevation to NPI level two. The risk of domestic cluster infections in Jan 2021 was gradually elevated to 14·14% (95% CrI 5·41% to 25·10%) until the end of April. The failure of timely contact tracing together with the loose of NPI led to the outbreak of Alpha VOC in mid-May 2021. After stamping out the outbreak with raising level three of NPI and rolling out of vaccination, surveillance of imported cases of Delta VOCs prevented any outbreak until Nov 2021. During Omicron pandemic from mid-Dec 2021 onwards, surveillance of imported cases found the observed cases exceeding the alert threshold around early Jan 2022, leading to a small-scale community-acquired outbreak.Interpretation: The proposed model for monitoring imported cases can be used as a global surveillance tool for forestalling large-scale community-acquired outbreak once SARS-CoV-2 VOCs emerges.Funding Information: This study was funded by Ministry of Science and Technology, Taiwan (MOST 108-2118-M-002-002-MY3; MOST 108-2118-M-038-001-MY3; MOST 108-2118-M-038 -002 -MY3; MOST 109-2327-B-002-009). Declaration of Interests: All authors declare no competing interests.Ethics Approval Statement: This study uses publicly available case line list without any private and identifiable information and does not require IRB approval.