The West African outbreak accelerated the advancement of several of these vaccine candidates, ultimately resulting in the recent licensure of two Ebola virus vaccines: ERVEBO^® (Merck, Kenilworth, NJ, USA) in the U.S. and the European Union (EU), and the Zabdeno/Mvabea vaccine regimen (Janssen, Pharmaceutical Companies of Johnson & Johnson, New Brunswick, NJ, USA) in the EU. Although both of these vaccines entered clinical development during the West African outbreak, only ERVEBO^® was able to generate human immunogenicity and safety data in time to enter a 2015 phase III ring vaccination trial in Guinea, and ultimately obtain human effectiveness and efficacy data which permitted licensure via the FDA’s accelerated approval pathway [2]. As a result, the ERVEBO^® and Zabdeno/Mvabea vaccines followed different pathways to licensure, and their lessons learned are important to consider for the development of the Sudan and Marburg vaccines.

Both ERVEBO^® and Zabdeno/Mvabea vaccines are viral vectored vaccines that have demonstrated a high level of protection in the standardized NHP model following the intramuscular Ebola Zaire challenge [3,4,5]. ERVEBO^® is a live-attenuated, replication-competent, recombinant vesicular stomatitis virus-based vaccine expressing the full-length glycoprotein (GP) of a Zaire Ebolavirus (rVSV-ZEBOV), instead of its native VSV GP [6,7]. Results from the 2015 phase III ring vaccination study in Guinea indicated that ERVEBO^® was 100% effective, as no new EVD cases were identified 10 or more days following the immunization of vaccinated close contacts [2]. These results were confirmed when ERVEBO^® was deployed in the second largest Ebola outbreak ever recorded, between 2018 and 2020 in the Democratic Republic of the Congo; it was shown to be 97.5% effective at stopping Zaire ebolavirus Kikwit (EBOV) transmission among vaccinees [8].

In 2015, the Zabdeno/Mvabea vaccine was not yet positioned for evaluation in a phase III efficacy study and instead entered a safety and immunogenicity clinical trial in Sierra Leone. The Zabdeno/Mvabea vaccine utilizes Janssen/J&J’s AdVac platform and Bavarian Nordic’s MVA-BN technology in a heterologous prime boost strategy [5]. The Zabdeno (Ad26.ZEBOV-GP) component serves as the prime and is composed of adenovirus serotype 26 expressing the ZEBOV full-length GP in place of the replication-essential adenovirus early 1 region, rendering it replication incompetent. Mvabea (MVA-BN-filo) serves as the boost and consists of a modified Vaccinia Ankara virus (MVA) encoding GPs from EBOV, Sudan virus (SUDV), and Marburg virus (MARV), and Taï Forest virus (TAFV) nucleoproteins. As the number of cases started to decline in the West African outbreak, an evaluation of clinical efficacy was not possible for this vaccine. Instead, the company continued to develop Zavdeno/Mvabea through the conduct of multiple phase I/II/III clinical trials, demonstrating its safety and immunogenicity, along with additional efficacy studies in the NHP model, and pursued a “non-traditional pathway” to licensure [9,10,11,12]. In 2019, the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency (EMA) granted an accelerated assessment to Janssen for its Ebola vaccine, and Janssen submitted two Marketing Authorization Applications (MAAs) to the EMA for the approval of Ad26.ZEBOV and MVA-BN-Filo. The evaluation of the protective effect of the vaccine regimen was demonstrated through the bridging of clinical immunogenicity results to efficacy and immunogenicity data obtained in non-human primates (NHPs) [13]. In May 2020, the EMA’s CHMP recommended granting a marketing authorization for the combination of Ad26.ZEBOV (Zabdeno) and MVA-BN-Filo (Mvabea) vaccines.

Indeed, prior to the West African Ebola outbreak, it was assumed that the licensure of MCMs for filoviruses would not be feasible through a traditional clinical efficacy trial; therefore, regulatory agencies established guidelines for demonstrating a likelihood of clinical benefit that could facilitate a non-traditional path to licensure, such as the EMA’s approval under exceptional circumstances that was used for the Zabdeno/Mvabea vaccine as described above. In the U.S., the FDA established the Animal Rule (AR) regulatory pathway for the approval of drug and biological products for which human efficacy studies are not ethical or feasible (described in 21 CFR 314.600–650 for drugs and 21 CFR 601.90–95 for biologics; effective 1 July 2002) [14].

Under the FDA’s AR, efficacy is established based on studies in animal models of human disease or conditions of interest using a relatively small number of animal subjects compared to the large-scale phase II or III efficacy trials in humans required to support the licensure of most new drugs and vaccine candidates. As the FDA relies on these studies to demonstrate the primary evidence of product effectiveness, studies conducted must be adequately designed and well-controlled to ensure the rigor of study control, data reliability and data integrity. As such, the animal model used for efficacy studies should be well-characterized and share in-common measurable disease endpoints with those found in humans. Typically, natural history studies that define the animal model are conducted at each testing facility, and when the animal model is shown to be suitable, the data packages are submitted to the FDA in support of the animal model use under the AR regulatory pathway. However, adequate and well-controlled natural history studies for Zaire ebolavirus were limited at the Animal Biosafety Level 4 (ABSL4) sites.

To coordinate the development of biodefense MCMs, a USG interagency group, the Filovirus Animal Non-Clinical Group (FANG), was established in 2011 to support the development of vaccines and therapeutics to be licensed using non-traditional pathways such as the FDA’s AR [15]. The FANG was instrumental in establishing standardized tools, such as animal models, associated viral and immunogenicity assays, and reagents to support product development prior to the West African outbreak. As these tools were already in place, product developers were able to utilize these tools and capabilities to facilitate the evaluation of candidate products and more rapidly provide data for regulatory submissions to the EMA and FDA for the licensure of products.

The FANG Animal Models Subteam, which included USG sponsors, as well as academic, private, and industry partners, collaborated to develop NHP models suitable for the evaluation of vaccine candidates and therapeutics under development using the FDA AR regulatory pathway. Through this public–private partnership, the National Institute of Allergy and Infectious Diseases (NIAID), the Department of Defense (DOD), and the Biomedical Advanced Research and Development Authority (BARDA) have sponsored numerous studies to develop and refine animal models, as well as screen and evaluate vaccine candidates. The NHP efficacy study parameters, challenge materials and dosing, disease endpoints and measures, and the associated viral and immunological assays used in the NHP studies were harmonized to the extent possible through the FANG government stakeholders in collaboration with the ABSL4 testing facilities and product developers.

The cynomolgus macaque (CM) model has been a commonly used model for the evaluation of Ebola vaccine candidates, as CMs are susceptible to infection and disease caused by the wild-type ebolavirus [16,17,18], and subsequently it has emerged as a lead NHP model to support vaccine development under the FDA’s AR regulatory pathway. Upon challenge, Ebola virus causes a disease in the CM which closely aligns with the human disease pathogenesis, including clinical signs, clinical chemistry and hematology values, and ultimately mortality. However, the CM model is considered very stringent as it frequently requires a very high dose (100–1000 PFU) of EBOV for intramuscular challenge, and the progression of the disease course is shortened in CMs as compared to human disease. The CM model is also typically uniformly lethal, whereas in humans, the case fatality rate varies from 25% to 90% [19]. Access to the CM model is limited, as it requires the work be performed in a high containment lab (ABSL4), which is a bottleneck for multiple reasons, including the high cost of performing studies in the ABSL4, the required technical and scientific expertise, the limited number of sites able to conduct this work, limited capacity at each site, and the high demand or competition for this space. Large licensure-enabling vaccine efficacy studies at a single testing facility are often logistically challenging, and thus, multiple testing facilities would be required to provide an adequate dataset for immunobridging NHP protection to human immunogenicity data. Therefore, to support licensure through the AR regulatory pathway, natural history studies are required at each testing facility to demonstrate the consistency of the animal model across facilities.

To accumulate an adequately powered natural history dataset to support the CM model for use in evaluating the efficacy of vaccine candidates, the USG sponsors leveraged datasets from challenged, untreated animals (control animals) from multiple immunogenicity and efficacy studies that were conducted at three ABSL4 testing facilities. These studies were performed under quality systems which were well-controlled, with the intent of potential use in regulatory submissions under the AR. USG sponsors with access to datasets from multiple studies conducted a meta-analysis of the combined datasets to describe the course of Ebolavirus disease in the CM model (natural history). Details of the CM animal model control meta-analysis are described in Niemuth, et al., 2021 [20], and the project was funded through the NIAID Preclinical Services Contract. A government interagency working group, led by NIAID Program scientists, was convened to refine the approach and statistical analysis plan, and to review the final report. Coordination and collaboration among government partners (NIAID, DOD, BARDA, FDA), ABSL4 testing facilities, and product sponsors was critical to the success of the project.