Ervebo (rVSV-ZEBOV-GP)

Overview

Ervebo was originally engineered at the Public Health Agency of Canada (PHAC), Winnipeg, by Gary Kobinger, Heinz Feldmann, and colleagues, who published the foundational preclinical data in Nature Medicine in 2005. PHAC licensed it to NewLink Genetics, which sublicensed it to Merck Sharp & Dohme in 2014 amid the West Africa Ebola emergency. Merck completed development, ran the pivotal Guinea ring trial ("Ebola ça Suffit!"), and manufactures the licensed product.

The molecular design is elegant: the entire VSV glycoprotein gene (VSV-G — the viral attachment/fusion protein and sole surface antigen) is completely replaced by the Zaire EBOV glycoprotein gene (GP1,2). The chimeric virus retains VSV's replication machinery (N, P, M, L proteins) and can replicate in a wide range of host cells, but presents exclusively EBOV GP on its surface. This simultaneously renders EBOV GP the sole immunogen and attenuates the virus relative to wild-type VSV, because EBOV GP is a less efficient fusion protein for most non-Ebola-susceptible cells and because deletion of VSV-G removes VSV's principal neurovirulence determinant.

Ervebo's real-world deployment has been almost exclusively in outbreak response via ring vaccination — vaccinating contacts and contacts-of-contacts of confirmed cases, adapted from the smallpox eradication playbook. The 2018–2020 DRC North Kivu/Ituri outbreak (3,481 cases, 2,299 deaths — second-largest in history) saw >325,000 doses administered under expanded access, with estimated field effectiveness of 97.5%. The vaccine is not currently used in routine childhood immunization; Ebola's episodic zoonotic spillover pattern makes outbreak-response deployment the rational strategy.

Platform & Antigen Design

rVSV-ZEBOV-GP Vector Architecture

  Wild-type VSV genome: N — P — M — G — L
                                    |
                                    └─ VSV-G REPLACED by EBOV GP1,2

  rVSV-ZEBOV-GP:         N — P — M — [EBOV GP] — L
    - Replication-competent (N, P, M, L intact)
    - EBOV GP is the sole surface protein and sole immunogen
    - Entry via macropinocytosis + late-endosomal NPC1 receptor (EBOV tropism)
    - Attenuated vs. wt-VSV: EBOV GP less efficient fusion protein

  IM injection (0.5 mL, 10⁸ PFU)
        │
        ├─ Local replication (1–4 days) in macrophages, DCs, fibroblasts
        │    dsRNA intermediates + 5'-ppp RNA → RIG-I / MDA5 → MAVS
        │    → IRF3/IRF7 → type I IFN (IFN-α/β) burst
        │    → TLR3/7/8 → NF-κB → IL-6, TNF-α, IL-12
        │    (strong innate adjuvanting; no exogenous adjuvant needed)
        │
        ├─ Transient low-level viremia (days 1–7)
        │    → systemic lymphoid tissue engagement (spleen, etc.)
        │
        ├─ DCs migrate to draining LN → EBOV GP on MHC II → CD4⁺ Th1/Th2
        │                                EBOV GP on MHC I  → CD8⁺ CTL
        │
        └─ B cells → EBOV GP-specific IgG (IgG1/IgG3)
             Detectable day 7–14; peaks day 28
             Correlate of protection: EBOV GP-specific neutralizing IgG

1. VSV-G → EBOV GP swap: Complete glycoprotein gene replacement makes EBOV GP the sole antigen, ensuring focused, monovalent immune responses without any anti-VSV immunity diversion.
2. Self-adjuvanting replication: VSV's dsRNA intermediates and 5′-ppp RNA activate RIG-I and MDA5 — producing a type I interferon burst that acts as an endogenous adjuvant, driving DC maturation without formulated adjuvant.
3. Rapid IgG kinetics: EBOV GP-specific IgG detectable within 7–14 days — critically matching the protection timeline in ring vaccination where early post-exposure protection is required.
4. CD8+ CTL via direct infection: VSV infects dendritic cells directly, loading EBOV GP peptides onto MHC I and priming CD8+ cytotoxic T lymphocytes — a cellular immune arm in addition to the dominant antibody response.
5. Synovial tropism: VSV-ZEBOV-GP replicates in synovial fibroblasts and macrophages at joint spaces, causing the characteristic ~25% arthritis/arthralgia adverse event and vesicular skin lesions (~10%).

Immunogenicity

Clinical Efficacy

Safety Profile

• Common Injection-site pain/erythema — ~60–70%; onset day 1–2; resolves 2–5 days. Fatigue, myalgia: ~35–45%; headache: ~30–40%; chills: ~15–25%.
• Common Fever (≥38°C) — ~25–30%; onset day 1–3; resolves 1–3 days.
• Common / Important Arthritis / arthralgia — ~25% of vaccinees. Onset day 7–21 (delayed relative to injection reactions). Mechanism: VSV-ZEBOV-GP replicates in synovial fibroblasts and macrophages. Polyarticular pattern (hands, wrists, ankles, knees). Duration: most resolve weeks–months; <3% persistent >6 months (chronic arthralgia). Management: NSAIDs; joint aspiration if large effusion. Skin vesicles (~10%) accompany arthritis in many cases.
• Uncommon Vesicular skin lesions — ~10%; onset day 7–21; small fluid-filled blisters on trunk/hands/oral mucosa; contain live rVSV-ZEBOV-GP; self-limiting; vaccinees advised to cover until healed.
• Uncommon Transient viremia — detectable in a subset of vaccinees days 1–7; potential but undocumented transmission risk; vaccine not recommended for close contacts of immunocompromised individuals during this window.
• Contraindication Severe immunocompromise — HIV with low CD4+, primary immunodeficiency, high-dose corticosteroids: risk of uncontrolled VSV replication; generally contraindicated except when risk of actual EBOV exposure clearly outweighs risk.
• Contraindication Pregnancy — not formally approved; WHO guidance allows use in pregnant women at high Ebola exposure risk (healthcare workers, direct contacts) under expanded access; no safety signal detected in DRC post-outbreak analyses.

Administration

Connections

References

• Henao-Restrepo AM, Longini IM, Egger M, et al. Efficacy and effectiveness of an rVSV-vectored vaccine expressing Ebola surface glycoprotein: interim results from the Guinea ring vaccination cluster-randomised trial. Lancet. 2015;386(9996):857-866. doi:10.1016/S0140-6736(15)61117-5 · PMID 26248676
• Henao-Restrepo AM, Camacho A, Longini IM, et al. Efficacy and effectiveness of an rVSV-vectored vaccine in preventing Ebola virus disease: final results from the Guinea ring vaccination trial (Ebola ça Suffit!). Lancet. 2017;389(10068):505-518. doi:10.1016/S0140-6736(16)32621-6 · PMID 28017403
• Jones SM, Feldmann H, Ströher U, et al. Live attenuated recombinant vaccine protects nonhuman primates against Ebola and Marburg viruses. Nat Med. 2005;11(7):786-790. doi:10.1038/nm1258 · PMID 15937495
• U.S. Food and Drug Administration. ERVEBO (Ebola Zaire Vaccine, Live) — Prescribing Information. FDA BLA 125630. December 2019. fda.gov/vaccines/ervebo