A large clinical trial to assess the safety and efficacy of two experimental vaccines to prevent Ebola virus infection now is open to volunteers in Liberia. The trial is being led by a recently formed Liberia-U.S. clinical research partnership and is sponsored by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. The Partnership for Research on Ebola Vaccines in Liberia or PREVAIL, a phase II/III study, is designed to enroll approximately 27,000 healthy men and women aged 18 years and older.

One vaccine candidate, cAd3-EBOZ, uses a chimpanzee-derived cold virus to deliver Ebola virus genetic material from the Zaire strain of virus causing the outbreak in Liberia. Published interim results from a phase I trial of this vaccine, which was co-developed by NIAID scientists and GlaxoSmithKline, provided necessary safety information and showed that it prompted immune responses to the outer coat of Ebola virus.

The other candidate, VSV-ZEBOV, employs vesicular stomatitis virus, an animal virus that primarily affects cattle, to carry an Ebola virus gene segment. The VSV-ZEBOV vaccine was developed by the Public Health Agency of Canada and licensed to NewLink Genetics through its wholly owned subsidiary BioProtection Systems. Phase I trial results of this vaccine also provided safety information and showed that it prompted immune responses to the outer coat of Ebola virus. These results have not yet been published but were made available to the regulatory bodies reviewing the study.

“The scale of the current Ebola outbreak in West Africa is unprecedented, and specific medical countermeasures are needed for this and future outbreaks,” said Anthony S. Fauci, M.D., NIAID director. “It is imperative that any potential countermeasures, including vaccines, be tested in a manner that conforms to the highest ethical and safety standards in clinical trials designed to provide a clear answer to the question of whether a candidate vaccine is safe and can prevent infection. This trial is designed to provide such answers.”

In addition to healthy adults in the general population, the trial will seek volunteers from groups at particular risk of Ebola infection, including health care workers, communities with ongoing transmission, contact tracers and members of burial teams. Social mobilization and community engagement activities began in Montserrado County, where the Liberian capital Monrovia is located, before the trial started and will continue in order to successfully recruit thousands of participants.

Participants will be assigned at random to one of three equal-sized groups. Volunteers in one group will receive a placebo (saline) injection, while the others will receive a single injection of either the cAd3-EBOZ vaccine or the VSV-ZEBOV vaccine. In addition to including a placebo group, the trial will be double-blinded, meaning that neither volunteers nor staff will know whether a vaccine or placebo was administered. A randomized, double-blind, placebo-controlled trial is considered the gold standard in clinical research. All participants will be advised on how to minimize the risk of becoming infected with Ebola virus and will be contacted by study staff about one week after the injection and then monthly for the duration of the study, which is currently expected to last about twelve months.

Given the current decline in the number of new Ebola cases in Liberia, study investigators anticipate the need for flexibility in the conduct and design of the trial to address the changing nature of the outbreak.

The co-leaders of the trial are Stephen B. Kennedy, M.D., MPH, secretary-general of the Liberia College of Physicians and Surgeons; Fatorma Baloy, Ph.D., director, Liberian Institute for Biomedical Research; and H. Clifford Lane, M.D., NIAID’s deputy director for clinical research and special projects. The pharmaceutical company GlaxoSmithKline (Research Triangle Park, N.C.) will supply the cAd3-EBOZ investigational vaccine; Merck (Kenilworth, N.J.) and NewLink Genetics (Ames, Iowa) will supply the VSV-ZEBOV candidate.