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What was the prevalence of unidentified SARS-CoV-2 infection in April 2021 after 3 major pandemic waves in Hong Kong, a city without complete lockdown?
In this cross-sectional study of 4198 participants of the general public in Hong Kong, 6 were identified as positive for anti-SARS-CoV-2 IgG after 3 major waves of COVID-19. The adjusted prevalence of unidentified infection was 0.15%, with fewer than 1.9 unidentified infections for every recorded case.
The findings suggest that stringent isolation and quarantine policies even without complete city lockdown are successful in minimizing SARS-CoV transmission.
Seroprevalence studies inform the extent of infection and assist evaluation of mitigation strategies for the COVID-19 pandemic.
To estimate the prevalence of unidentified SARS-CoV-2 infection in the general population of Hong Kong.
Design, Setting, and Participants
A prospective cross-sectional study was conducted in Hong Kong after each major wave of the COVID-19 pandemic (April 21 to July 7, 2020; September 29 to November 23, 2020; and January 15 to April 18, 2021). Adults (age ≥18 years) who had not been diagnosed with COVID-19 were recruited during each period, and their sociodemographic information, symptoms, travel, contact, quarantine, and COVID-19 testing history were collected.
Main Outcomes and Measures
The main outcome was prevalence of SARS-CoV-2 infection. SARS-CoV-2 IgG antibodies were detected by an enzyme-linked immunosorbent assay based on spike (S1/S2) protein, followed by confirmation with a commercial electrochemiluminescence immunoassay based on the receptor binding domain of spike protein.
The study enrolled 4198 participants (2539 [60%] female; median age, 50 years [IQR, 25 years]), including 903 (22%), 1046 (25%), and 2249 (53%) during April 21 to July 7, 2020; during September 29 to November 23, 2020; and during January 15 to April 18, 2021, respectively. The numbers of participants aged 18 to 39 years, 40 to 59 years, and 60 years or older were 1328 (32%), 1645 (39%), and 1225 (29%), respectively. Among the participants, 2444 (58%) stayed in Hong Kong since November 2019 and 2094 (50%) had negative SARS-CoV-2 RNA test results. Only 170 (4%) reported ever having contact with individuals with confirmed cases, and 5% had been isolated or quarantined. Most (2803 [67%]) did not recall any illnesses, whereas 737 (18%), 212 (5%), and 385 (9%) had experienced respiratory symptoms, gastrointestinal symptoms, or both, respectively, before testing. Six participants were confirmed to be positive for anti-SARS-CoV-2 IgG; the adjusted prevalence of unidentified infection was 0.15% (95% CI, 0.06%-0.32%). Extrapolating these findings to the whole population, there were fewer than 1.9 unidentified infections for every recorded confirmed case. The overall prevalence of SARS-CoV-2 infection in Hong Kong before the roll out of vaccination was less than 0.45%.
Conclusions and Relevance
In this cross-sectional study of participants from the general public in Hong Kong, the prevalence of unidentified SARS-CoV-2 infection was low after 3 major waves of the pandemic, suggesting the success of the pandemic mitigation by stringent isolation and quarantine policies even without complete city lockdown. More than 99.5% of the general population of Hong Kong remain naive to SARS-CoV-2, highlighting the urgent need to achieve high vaccine coverage.
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CME Disclosure Statement: Unless noted, all individuals in control of content reported no relevant financial relationships. If applicable, all relevant financial relationships have been mitigated.
Accepted for Publication: September 6, 2021.
Published: November 15, 2021. doi:10.1001/jamanetworkopen.2021.32923
Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2021 Boon SS et al. JAMA Network Open.
Corresponding Author: Paul K. S. Chan, MBBS, MSc, MD, Department of Microbiology, Faculty of Medicine, Prince of Wales Hospital, The Chinese University of Hong Kong, 32 Ngan Shing St, Shatin, New Territories, Hong Kong SAR, China (email@example.com).
Author Contributions: Drs Boon and M.C.S. Wong contributed equally to this study. Dr Chan had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Concept and design: Chan.
Acquisition, analysis, or interpretation of data: All authors.
Drafting of the manuscript: Boon, Ng, Ho, Chan.
Critical revision of the manuscript for important intellectual content: M.C.S. Wong, Leung, Chen, Lai, Huang, B.K.C. Wong, Fung, Chan.
Statistical analysis: M.C.S. Wong, Huang.
Obtained funding: Chan.
Administrative, technical, or material support: Boon, M.C.S. Wong, Ng, Lai, B.K.C. Wong, Fung, Chan.
Supervision: M.C.S. Wong, Chan.
Conflict of Interest Disclosures: Dr M.C.S. Wong reported receiving grants from Food and Health Bureau during the conduct of the study. Dr Chan reported receiving grants from Food and Health Bureau, Hong Kong SAR Government during the conduct of the study. No other disclosures were reported.
Funding/Support: The study was supported by the Health and Medical Research Fund–Commissioned Research on the Novel Coronavirus Disease (COVID-19) (reference No. COVID190108) from the Food and Health Bureau, Hong Kong SAR Government.
Role of the Funder/Sponsor: The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
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