Phase 1 Trials of rVSV Ebola Vaccine in Africa and Europe

Abstract

Background: The replication-competent recombinant vesicular stomatitis virus (rVSV)-based vaccine expressing a Zaire ebolavirus (ZEBOV) glycoprotein was selected for rapid safety and immunogenicity testing before its use in West Africa.

Methods: We performed three open-label, dose-escalation phase 1 trials and one randomized, double-blind, controlled phase 1 trial to assess the safety, side-effect profile, and immunogenicity of rVSV-ZEBOV at various doses in 158 healthy adults in Europe and Africa. All participants were injected with doses of vaccine ranging from 300,000 to 50 million plaque-forming units (PFU) or placebo.

Results: No serious vaccine-related adverse events were reported. Mild-to-moderate early-onset reactogenicity was frequent but transient (median, 1 day). Fever was observed in up to 30% of vaccinees. Vaccine viremia was detected within 3 days in 123 of the 130 participants (95%) receiving 3 million PFU or more; rVSV was not detected in saliva or urine. In the second week after injection, arthritis affecting one to four joints developed in 11 of 51 participants (22%) in Geneva, with pain lasting a median of 8 days (interquartile range, 4 to 87); 2 self-limited cases occurred in 60 participants (3%) in Hamburg, Germany, and Kilifi, Kenya. The virus was identified in one synovial-fluid aspirate and in skin vesicles of 2 other vaccinees, showing peripheral viral replication in the second week after immunization. ZEBOV-glycoprotein-specific antibody responses were detected in all the participants, with similar glycoprotein-binding antibody titers but significantly higher neutralizing antibody titers at higher doses. Glycoprotein-binding antibody titers were sustained through 180 days in all participants.

Conclusions: In these studies, rVSV-ZEBOV was reactogenic but immunogenic after a single dose and warrants further evaluation for safety and efficacy. (Funded by the Wellcome Trust and others; ClinicalTrials.gov numbers, NCT02283099, NCT02287480, and NCT02296983; Pan African Clinical Trials Registry number, PACTR201411000919191.).

Figure 1. Plasma Recombinant Vesicular Stomatitis Virus (rVSV) Viremia in the Four Study Cohorts.

The presence of rVSV viremia was assessed with the use of quantitative reverse-transcriptase–polymerase-chain-reaction (RT-PCR) assays (TaqMan) of total RNA derived from plasma. The viral load is expressed as log₁₀ rVSV RNA copies per milliliter. Each measurement included no-template and standard controls. The shaded area indicates values that are below the limit of quantification (≤100 copies per milliliter of RNA in Lambaréné, Kilifi, and Geneva and ≤200 copies per milliliter in Hamburg). Panels A and B show the plasma viral load through day 28 for the two Hamburg cohorts that received doses of 3 million or 20 million plaque-forming units (PFU), with daily sampling from day 1 to day 7. Panels C and D show individual viremia patterns in two cohorts that received doses of 300,000 or 3 million PFU, as monitored in Lambaréné. Plasma was analyzed between day 0 and day 2 and on day 7 in all participants. Panels E and F show the plasma viral load on days 0, 1, 3, and 7 among participants in Kilifi who received doses of 3 million or 20 million PFU. Panels G and H show rVSV RNA copy numbers on days 0, 1, 3, and 7 in participants in Geneva who received vaccine doses of 10 million or 50 million PFU.

Figure 2. Vaccine-Induced Maculopapular and Vesicular Dermatitis.

Panel A shows maculopapular lesions on the thigh of a participant (subpanel a). Histologic analysis of one papule revealed a dermal T-lymphocytic infiltrate (subpanel b, hematoxylin and eosin staining) characterized mainly by CD3+ T cells (subpanel c). Panel B shows vesicular lesions on the plantar side of the toes of a participant (subpanel d). Histologic analysis shows subepidermal vesicular dermatitis with vacuolar degeneration, keratinocyte necrosis, acute and lymphohistiocytic inflammation and fibrin (subpanel e, hematoxylin and eosin staining). Higher-power magnification of the same area shows abundant immunostaining of rVSV antigens associated with cellular debris and inflammatory infiltrate (subpanel f, immunoalkaline phosphatase technique with the use of a mouse antibody against VSV and Naphthol fast red substrate). Panel C shows the isolation of rVSV-ZEBOV, with a cytopathic effect induced on Vero E6 cells after culture with control medium (subpanel g) or a swabbed skin vesicle (subpanels h and i) observed by means of either phase-contrast light microscopy (subpanels g and h) or after immunostaining (subpanel i) for VSV matrix protein (green, rVSV-ZEBOV–infected cells, with Evans blue counterstaining; red, noninfected cells).

Figure 3. Glycoprotein Antibody Titers, According to Vaccine Dose, Study Site, and Assay.

Individual antibody titers were assessed at baseline and at 28 and 180 days after vaccination in 158 participants, according to study site and dose group. The vaccine dose (in PFU) is indicated for each site in Panels A through D; the shaded areas indicate negative titers. The numbers of samples tested are provided in Figure S4 in the Supplementary Appendix. Antibodies were measured on enzyme-linked immunosorbent assay (ELISA) against the homologous glycoprotein (GP) of the Zaire–Kikwit strain (ZEBOV, Panel A) or inactivated whole virions of the Zaire–Guéckédou strain (Panel B). Results are expressed as end-point titers (Panel A) or the geometric mean concentration of arbitrary ELISA units (AEU) per milliliter (Panel B). Neutralizing antibodies were detected with the use of rVSV pseudovirion neutralization assay assessing the 50% serum neutralization capacity (PsVNA50) complemented by homologous glycoprotein (Panel C) or with infectious ZEBOV isolate Mayinga (Panel D). Geometric mean titers and 95% confidence intervals are shown for each study site, dose group, and time point. The results of glycoprotein ELISA (Panel E) and pseudoneutralization (Panel F) at 28 days were expressed as the reciprocal of the highest dilution showing a positive result. The curves represent the distribution of individual antibody titers in each cohort. The dashed curves in Panel E indicate baseline titers.