Seroprevalence of COVID-19 in Taiwan revealed by testing anti-SARS-CoV-2 serological antibodies on 14,765 hospital patients

Abstract

Background: Coronavirus Disease 2019 (COVID-19) has become a worldwide pandemic and affected more than 227 countries or territories, resulting in more than 25 million cases with over 0•85 million deaths, as of September 2, 2020. Taiwan has been successful in countering the COVID-19 outbreak, however, the potential risk for asymptomatic infections and the prevalence rates remain unknown. We aimed to estimate the seroprevalence of COVID-19 in Taiwan via serologically testing hospital patients with neither symptoms indicative of nor positive nucleic acid test for SARS-CoV-2 infection.

Methods: Residual specimens from laboratory blood tests for outpatient and emergency department patients visiting a medical centre in Taipei, Taiwan, within one week in May and another in July, 2020, were collected. We used Elecsys Anti-SARS-CoV-2 Assay to screen and further validated cases with high cutoff index by a confirmatory ELISA assay. We also analysed antibody responses against SARS-CoV-2 along disease progression in four nucleic acid test confirmed COVID-19 patients.

Findings: Blood samples from a total of 14,765 patients were tested. The unweighted seroprevalence of anti-SARS-CoV-2 antibodies was 0•07% [95% CI, 0•04%-0•13%]; after weighting with the population demographics of Taiwan, the estimated overall seroprevalence was 0•05% [95% CI, 0•02%-0•10%]. Furthermore, based on data of the four COVID-19 cases, the seroconversion dates for IgM were as early as 9 days and that for IgG 11 days after symptoms onset.

Interpretation: We screened the anti-SARS-CoV-2 antibodies in a small-scale population-based study and observed an approximately 0•05% seroprevalence of COVID-19, indicating that the current containment protocols emphasising mask wearing, hand washing, social distancing and mandatory quarantine for all incomers are effective in Taiwan.

Funding: Taipei Veterans General Hospital, Taipei, Taiwan.

Keywords: COVID-19; Pandemic;Taiwan; Seroprevalence; anti-SARS-CoV-2 serological antibodies.

Fig. 1

Comparison of the results of the Elecsys Anti‑SARS‑CoV‑2 Assay and Academia Sinica ELISA assay in 25 Elecsys Assay positive cases. Among 25 individuals tested with COI ≥ 1•00 by Elecsys Assay, seven were tested positive for SARS-CoV-2 anti-spike RBD IgG antibodies (IgG+ / IgM-), four were tested positive for SARS-CoV-2 anti-spike RBD IgM antibodies (IgG- / IgM+), and the other 14 were tested double negative (IgG- / IgM-) by ELISA assay. Results of the qualitative Elecsys Assay and ELISA assay are amended as positive (red) or negative (green).

Fig. 2

Longitudinal assessment with viral RNA and serological antibodies testing for four COVID-19 patients. Shown is the variation of presence of viral RNA and antibodies against SARS-CoV-2 in four confirmed COVID-19 patients: male, 59 year-old (Patient 1; A), female, 75 year-old (Patient 2; B), female, 39 year-old (Patient 3; C), and female, 22 year-old (Patient 4; D). The results of Elecsys Assay are presented as COI values (purple), and the results of ELISA assay are presented as OD values (IgG: dark blue; IgM: light blue) measured by 450 nm. Results of the qualitative viral RNA testing, Elecsys Assay and ELISA assay are amended as positive (red), weakly positive (pink), negative (green), and n/a (white). The dash lines denote the cut-off value for ELISA IgG assay (dark blue, 0•134), ELISA IgM assay (light blue, 0•168), and Elecsys Assay (purple, 1•00).

Fig. 3

Titers of antibodies against SARS-CoV-2 in four COVID-19 patients at different days after symptoms onset. Levels of IgG (A) and IgM (B) antibodies against SARS-CoV-2 at different time periods after symptoms onset in four COVID-19 patients are plotted. The IgG and IgM antibody titers gradually increased up to the third week and decreased thereafter with a more dramatic decrease back to the titers of the first week in the IgM antibodies. The boxplots show medians and first and third quartiles, and the whiskers show minimal to maximal range below and above the box.