Longitudinal Bundibugyo Virus Glycoprotein Seroreactivity Following rVSVΔG-ZEBOV-GP Vaccination in Outbreak-Affected Populations of the Democratic Republic of the Congo

Background: There are currently no vaccines approved for the prevention or treatment of Orthoebolavirus bundibugyoense (Bundibugyo virus; BDBV). The recombinant vesicular stomatitis virus- Zaire ebolavirus glycoprotein vaccine (rVSV-ZEBOV-GP; ERVEBO) has been widely deployed during Ebola virus disease (EVD) outbreaks caused by Orthoebolavirus zairense (Ebola virus; EBOV). Given the lack of vaccines and medical countermeasures we evaluated development of antibodies to Bundibugyo glycoprotein (GP) following rVSV-ZEBOV-GP vaccination in two EVD outbreak-affected populations in the Democratic Republic of the Congo (DRC). Methods: Between 2018 and 2023, serum samples were collected from vaccine recipients in Mbandaka, Equateur Province (n=482 at baseline), and Beni, North Kivu Province (n=599 at baseline). Antibody reactivity was assessed using a multiplex pan-filovirus immunoassay. We evaluated longitudinal trends in BDBV GP seroreactivity across follow-up visits extending to approximately five years after vaccination. Findings: We collected 2552 samples from 482 participants in Mbandaka and 3297 samples from 599 participants in Beni. BDBV GP responses diverged by location. Baseline BDBV GP seroreactivity differed between sites, with 3.3% of participants reactive in Mbandaka and 10.4% in Beni. In Mbandaka, BDBV GP titers remained unchanged through 6 months post-vaccination but increased markedly between 2.5 and 3.5 years (mean MFI 1,238 to 4,845; p<0.0001), accompanied by a rise in seroreactivity to 35.3%, followed by waning at later visits. In Beni, BDBV GP titers increased rapidly after vaccination, reaching peak seroreactivity at 21 days (52.3%) and 6 months (53.9%), with mean fold increases of 21.9 and 20.1 among baseline-naive participants. Although antibody levels declined after 6 months, EBOV GP and BDBV GP titers remained above baseline, and significant increases in EBOV GP, BDBV GP, and SUDV GP titers were observed between 2.5 and 5 years. Interpretation: We observed detectable BDBV GP seroreactivity in two geographically distinct populations in the DRC. However, there was no consistent evidence showing that rVSV-ZEBOV-GP vaccination was associated with increased seroreactivity to BDBV GP across study populations. Baseline BDBV GP seroreactivity observed prior to vaccination warrants further investigation into the origins and epidemiological significance of pre-existing filovirus-reactive antibodies, including the possibility of prior exposure to related filoviruses, cross-reactive immune responses, high non-specific background reactivity unrelated to filoviruses, or other unrecognized sources of earlier antigenic stimulation. In the absence of licensed BDBV-specific vaccines, these data may inform preparedness planning, evaluation of existing countermeasures, and future vaccine development efforts during current and future BDBV outbreaks. It may further contribute to the study of natural history of filovirus immunity with a direct implication to understanding the possibility of cross-reactive and possible cross-protective responses in humans.