Study Population and Ethics Statement
We included 180,000 inbound overseas travelers in Xiamen, China, from March 2020 to December 2020. The participants’ temperature and respiratory symptoms were recorded twice daily. SARS-CoV-2 RT-qPCR and/or tests for total antibodies against SARS-CoV-2 were performed using a defined process. Participants who tested positive for SARS-CoV-2 via RT-qPCR were diagnosed with COVID-19 based on epidemiologic and clinical evidence (12). Subsequently, they were admitted to the hospital for further observation and management. We recorded the participant’s age, continent of origin, nationality, comorbidities, and SARS-CoV-2 RT-qPCR and total antibody results.
The Testing Strategy of Inbound Overseas Travelers
All inbound overseas travelers were subjected to a 14-day government quarantine, with the arrival date being considered as day 1. Two testing strategies were performed during different periods. From March 2020 to April 2020, a PCR testing strategy was applied. Briefly, each participant was placed in a separate room and tested for SARS-CoV-2 via RT-qPCR on days 1, 7, and 14. From May 2020 to December 2020, the strategy for combining PCR and antibody testing was applied. Briefly, each participant was placed in a separate room and tested for SARS-CoV-2 via RT-qPCR and total antibodies via chemiluminescence microparticle immunoassay on day 1 and day 7. Participants who tested positive for RT-qPCR (regardless of the antibody test results) were diagnosed with COVID-19. Among the remaining participants, participants who tested negative in the antibody tests underwent PCR tests on days 7 and 14, while those who tested positive were assigned as a key screening population and underwent PCR tests at 2-day intervals. All participants with negative PCR test results were discharged on day 14 (Figure 1). The PCR test rounds were considered as the number of PCR tests performed for an individual during the 14-day quarantine period.
Nucleic Acid Amplification Tests
Nasopharyngeal and oropharyngeal swabs were obtained using the Tellgen platform (Tellegen, Shanghai, China) and tested via RT-PCR using the Daan 2019-nCoV RT-PCR Kit (Daan gene, Guangzhou, China) for the ORF1ab and N genes. The detection limit of the reagent was 500 copies/mL. The threshold cycle values for both the ORF1ab and N genes were ≤ 40 cycles. Samples positive for both genes were considered positive for SARS-CoV-2 RNA. Samples with either ORF1ab or N gene positivity were reexamined, with repeated positivity for the same gene indicating positivity for SARS-CoV-2 RNA.
Antibody Measurement
The total antibodies (Ab) against SARS-CoV-2 in plasma samples were tested using chemiluminescence microparticle immunoassay kits supplied by Beijing Wantai Biological Pharmacy Enterprise Co., Ltd, following the manufacturer’s instructions. Briefly, the reagent was developed based on a double-antigen sandwich immunoassay, with the receptor-binding domain of the spike SARS-CoV-2 protein as the immobilized and horseradish peroxidase-conjugated antigen. The antibody titer was calculated based on the cutoff and was recorded as the cutoff index (COI). A COI < 1.00 and ≥ 1.00 was considered negative and positive, respectively.
Statistical Analysis
The Mann-Whitney U test was used for continuous variables with skewed distribution, while a χ2 test or Fisher’s exact test was used for categorical variables. A two-sided P-value of < 0.05 was considered statistically significant. All statistical analyses were conducted using SPSS statistics version 20 (SPSS Inc., Chicago, Illinois, USA).