Asymptomatic infection in a proof-of-concept longitudinal study on SARS-CoV-2 vaccine recipients

Individuals with asymptomatic SARS-CoV-2 infection can unknowingly transmit the virus, yet identifying such infections in vaccinated populations remains challenging. We conducted a longitudinal study of 129 vaccine recipients immunized with various combinations of SARS-CoV-2 spike (S) protein vaccine platforms. Sera were collected before the first dose (v1), at 2 weeks (v7) and 6 months (v8) after the third dose. Taiwan’s first major COVID-19 outbreak occurred between v7 and v8. We measured anti-nucleocapsid (anti-N) and anti-S IgG antibody titers by ELISA and assessed virus-neutralizing activity using live virus and pseudovirus assays. By developing an iterative serial screening method, we identified asymptomatic infections among unconfirmed cases. Our v7-v8 paired cohort resolved into three distinct groups: confirmed cases (21%), asymptomatic infections (17%), and uninfected cases (62%). In normalized v8 sera, confirmed cases exhibited an anti-S+++/anti-N+++ phenotype, while uninfected cases showed an anti-S+/anti-N+ phenotype. Statistical analysis validated a distinct asymptomatic group characterized by intermediate anti-S but baseline anti-N antibody levels (anti-S++/anti-N+). This approach enables more accurate estimates of infection prevalence and vaccine efficacy.