Hidden asymptomatic breakthrough infection in a proof-of-concept longitudinal study on SARS-CoV-2 vaccine recipients.

Publication date: Jul 22, 2026

Individuals with asymptomatic SARS-CoV-2 infection can unknowingly transmit the virus, yet identifying such subclinical infections in post-vaccinated populations remains challenging. We conducted a longitudinal study of 129 infection-nacEFve 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 breakthrough (post-vaccination) infections among unconfirmed cases. Our v7-v8 paired cohort resolved into three distinct groups: confirmed cases (21%), asymptomatic breakthrough infections (17%), and uninfected subjects (62%). In normalized v8 sera, confirmed cases exhibited an anti-S (high) /anti-N (high) phenotype, while uninfected subjects showed an anti-S (low)/anti-N(baseline) phenotype. Statistical analysis validated a distinct asymptomatic group characterized by an antibody profile anti-S (intermediate) /anti-N (baseline). This approach may enable more accurate estimates of vaccine efficacy and infection prevalence. In a spike-vaccinated population, anti-N antibody is more a potential specific marker for COVID-19 symptomatic disease than an ideal marker for SARS-CoV-2 infection. To our knowledge, this is the first preliminary report of identification of asymptomatic breakthrough infection from a well-vaccinated population using self-matched longitudinal pairs of serum samples.

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