Methods
This retrospective cohort study was conducted from January 20 to March 10, with follow up till April 10, 2020. All cured adult patients with COVID-19 who performed antibody test were enrolled in our study. Patients were followed up in Guangzhou Eighth People’s hospital, a government-designated hospital which admitted nearly 80% of the COVID-19 cases in Guangzhou, the capital city of Guangdong Province in southern China. This study was approved by the ethics committee of the Guangzhou Eighth People’s Hospital. Because of the retrospective nature of the study design and the grim scenario of COVID-19 pandemic, the Ethics Committee assented to exempt of all informed consents.
Definition: COVID-19 was diagnosed as per the World Health Organization’s interim guidelines [9]. High throughput sequencing or RT-PCR were only performed in subjects with the following features: 1. with a confirmed or suspected contact history of COVID-19; 2. presented with symptoms; 3. with abnormal chest computed tomography (CT) imaging related to COVID-19. A positive result on high throughput sequencing or RT-PCR assay together with at least two of the above three clinical features, confirmed the diagnosis of COVID-19. Criteria for cured and discharged to home were as follows: vital signs were stable for more than three days; the PCR test was negative two times consecutively 24 hours apart; and the acute exudative lung lesions were absorbed or cured on chest CT. Re-infection criteria were as follows: typical clinical symptoms; chest CT indicative of new infiltration; and two positive repeat PCR tests performed consecutively at an interval of more than 24 hours. All confirmed re-infection cases were reviewed by two senior COVID-19 experts. Person-to-person transmission criteria were as follows: New confirmed COVID-19 cases occurred after one with unprotected exposure to the cured within 2 weeks. Person-to-person transmission was assessed on the basis of the reports of the cured patients. Since Guangzhou CDC released the new diagnosed COVID-19 cases including the exposure to source of transmission daily, the reports from CDC were also screened.
Follow up: All recovered or cured patients with COVID-19 were quarantined at home for two weeks after being discharged. They were free to go anywhere after two weeks. The cured patients were followed up every two weeks. Follow-up consisted of assessing the general condition, symptoms, living area, PCR assay, and antibody test. Additionally, these recovered patients were required to report if people close to them had been diagnosed with COVID-19. For patients with a positive PCR test, a chest CT was performed immediately, and PCR test was re-performed consecutively at an interval of more than 24 hours. The PCR assay and antibody test were performed on the same day. If positive, IgM antibody test would be repeated within two weeks. During the study, the researchers screened the report from CDC in Guangzhou every day to determine whether there were any new confirmed COVID-19 cases linked to transmission by the cured patients.
IgM and IgG Testing: The serum SARS-CoV-2 antibodies (IgM and IgG) were detected using colloidal Gold-based Immunoassays (ELISA kits, Livzon Inc, Zhuhai, China). First, the kit was removed and kept for 30 minutes at room temperature. Subsequently, 10 μl of plasma sample and 20 μl of whole blood sample were added into the reaction pore until the liquid was fully absorbed. Lastly, two drops of sample diluents were added into the reaction hole until the liquid was fully absorbed. The result could be read in 15 minutes.
Statistical Analyses: Shapiro-Wilk normality test was used to assess for normal distribution of data. Continuous variables with normal distribution were expressed as mean ± standard deviations (SD), while those with non-normal distribution were expresses as median and inter quartile range (IQR). Categorical variables were summarized as counts and percentages. For continuous variables, Independent t-test or Wilcoxon rank sum test were used. For comparison of categorical variables, Chi-square test and Fisher’s exact test were used. Logistic regression analyses were performed to examine the relationship between independent variables and presence of IgG. Determinants with a Pvalue of 0.10 or less in univariate models were initially included in the multivariate model and were then discarded using backward selection. A P values < 0.05 means statistically significant. All data were processed with SPSS version 22.0 for Windows (SPSS, Chicago, IL, USA).