Early detection of immunoglobulin A against a vaccine backbone protein enables fast, durable, and objective identification of individuals vaccinated against Ebola virus

Background rVSVΔG-ZEBOV-GP vaccine is recommended for prevention of Ebola virus (EBOV) infection and disease (EVD). However, breakthrough EBOV infections have been observed in vaccinees. Currently available diagnostic and research assays cannot reliably distinguish individuals vaccinated against Ebola virus from those previously infected with the virus. This limitation complicates interpretation of vaccine-effectiveness studies, seroepidemiologic investigations, and estimates of infection or attack rates. The ability to differentiate vaccine-induced immune responses from those resulting from natural infection remains an important unmet public health and research need. Methods We developed a multiplex sero-diagnostic assay simultaneously detecting IgA, IgG, and IgM antibodies against vesicular stomatitis Indiana virus nucleoprotein (VSV–N), a vaccine-specific marker absent in natural infection, and EBOV-GP, the vaccine's target antigen. We evaluated longitudinal samples from 30 individuals after vaccination and 30 placebo (saline) recipients, as well as 8 participants from a long-term vaccination follow-up study. Findings We detected increase in all antibodies against both protein antigens in vaccinated individuals. Critically, the assay reveals that anti-VSV-N IgA responses emerge earlier and more robustly than anti-GP responses in vaccinees, establishing anti-N IgA as a definitive, early, and durable biomarker of rVSVΔG-ZEBOV-GP vaccination with high sensitivity and specificity. Interpretation The assay enables rapid, accurate identification of vaccine recipients and supports real-time assessment of vaccine effectiveness during outbreaks. Furthermore, the assay's adaptability to the expanding pipeline of VSV-vectored vaccines positions it as a transformative diagnostic platform for accelerating preparedness for future infectious disease threats and outbreak response strategies. Funding Center for Biologics Evaluation and Research (CBER) at the U.S. Food and Drug Administration (FDA). The National Cancer Institute (NCI) and the Divisions of Intramural Research (DIR) and Clinical Research (DCR) of the National Institute of Allergy and Infectious Diseases (NIAID) at the U.S. National Institutes of Health (NIH).