Seroprevalence of SARS-CoV-2 antibodies among blood donors in Taiwan during the initial COVID-19 surge, April-July 2021.

To understand the extent of SARS-CoV-2 transmission and immunity during Taiwan's first major COVID-19 outbreak in 2021, we conducted a seroepidemiological investigation using blood donor specimens. A total of 5000 residual blood samples were randomly selected from donors aged 17-65 years across six geographic regions during April 25-July 3 (week 17-week 26), 2021. This time period spanned from the beginning of the surge to six weeks after its peak. Serologic testing using Roche Elecsys® assays measured antibodies against nucleocapsid (N) and spike (S) proteins. Infection-induced seropositivity, defined as having both anti-N and anti-S antibodies, was detected in only one individual (0.02%, 95% confidence interval (CI): 0-0.06%). Blood having only anti-S antibody, indicating vaccine-induced immunity, was observed in 5.2% of samples (95% CI: 4.6-5.8%). The findings suggest that SARS-CoV-2 infection prevalence remained low in the community at the time, with seropositivity primarily reflecting early stages of vaccine rollout. This study demonstrates the value of blood donor-based serosurveillance for estimating infection and vaccine-induced immunity, and informs future policy for public health monitoring.