An observational study on imported COVID-19 cases in Hong Kong during mandatory on-arrival hotel quarantine

Background Hong Kong has enforced stringent travel restrictions particularly for inbound travellers since the emergence of SARS-CoV-2. Understanding the characteristics of imported COVID-19 cases is important for establishing evidence-based control measures. Methods We conducted a retrospective cohort study to summarise the characteristics of cases classified as imported cases that were detected on or soon after arrival into Hong Kong from 13 November 2020 through to 31 January 2022, when all arriving persons were required to quarantine in a hotel or a designated quarantine facility. We analysed individual demographics, and clinical information including symptoms and disease severity, virus variants, and Ct values. Results There were 2269 imported COVID-19 cases aged 0-85 years identified in Hong Kong. Almost half (48.6%) of the imported cases were detected on arrival. A shorter median delay from arrival to isolation was observed in Delta and Omicron cases (3 days) than cases infected with the ancestral strain and other variants (12 days; p<0.001) while lower Ct values at isolation were observed in cases infected with Omicron than the ancestral strain or other variants. No Omicron cases were detected beyond 14 days after arrival, and the cases (n=58, 2.6%) detected after 14 days of quarantine more frequently presented without symptoms at isolation and had a higher RT-PCR Ct-value during isolation. At least some of these cases were post-arrival infections. Conclusions Testing inbound travellers at arrival and during on-arrival quarantine can detect imported cases early although it may not be sufficient to prevent all introductions of COVID-19 into the community. Public health measures should be adjusted in responses to the emergence of new variants of SARS-CoV-2 based on the epidemiologic evidence from continuous surveillance. ### Competing Interest Statement BJC consults for AstraZeneca, Fosun Pharma, GlaxoSmithKline, Moderna, Pfizer, Roche and Sanofi Pasteur. SGS has served (unpaid) on advisory boards for Sanofi and Seqirus. The authors report no other potential conflicts of interest. ### Funding Statement This project was supported by the Health and Medical Research Fund, Health Bureau, Government of the Hong Kong Special Administration Region, the Collaborative Research Scheme (Project No. C7123-20G) of the Research Grants Council of the Hong Kong SAR Government, and the National Foundation for Australia China Relations Doherty Sino-Australia COVID-19 Partnership Seed Funding (SACOV-05). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics committee/IRB of the University of Hong Kong gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.