An experimental Ebola vaccine initially developed by the Public Health Agency of Canada and the US Army and eventually licensed to Merck, Sharp and Dohme, the international arm of US drugmaker Merck & Co., made headlines just before Christmas as a study declared it to be 100% effective against certain forms of the disease.

While the vaccine known as rVSV-ZEBO, developed more than ten years ago, has not yet been approved by a worldwide regulatory body, health authorities are said to have nevertheless built up a stockpile of a reported 300,000 doses to deal with any outbreaks that may flare up in the meantime.

As the licensee, Merck / MSD is now required to seek approval from the World Health Organization, which also requires licensing by a major regulatory agency such as the US Food and Drug Administration (FDA) or the European Medicines Agency (EMA).

In January 2016 the US drugmaker signed an agreement with Geneva, Switzerland-based GAVI (Global Alliance for Vaccines and Immunization), a public-private partnership and the world’s largest funder of vaccines for developing countries to submit the vaccine for regulatory approval by the end of 2017. 

As part of the race for prevention, if not a cure of the disease in response to the explosive 2014 outbreak that killed 11,000 people in West Africa –  primarily in Liberia, Sierra Leone and Guinea – other drugmakers presented possible Ebola treatments but this was the only one to meet the efficacy parameters of international health authorities.

The recombinant, replication-competent vesicular stomatitis virus-based candidate vaccine expresses a surface glycoprotein of Zaire Ebolavirus that infects cattle but usually does not harm humans. It works by prompting the immune system to produce antibodies.

Although the vaccine appears to be effective against only one of the two most common strains of the virus and may not give lasting protection, “the world can’t afford the confusion and human disaster that came with the last epidemic,” Marie-Paule Kieny, WHO assistant director general for health systems and innovation and the lead author of the clinical studies, said commenting on   the results.

Of the five known Ebola subtypes, Ebola-Zaire, which triggered the West African outbreak, is the most common, along with Ebola-Sudan.

In the trials backed by the World Health Organization, the Guinean Health Ministry, Medecins sans Frontieres, the Wellcome Trust, Norwegian Institute of Public Health and other institutions, nearly 12,000 residents of Guinea were tested in 2015. None of the 5,837 people receiving the vaccination immediately developed the disease within a 10-day incubation period. A number of those in a control group waiting three weeks for vaccination did develop Ebola.

Gary J. Nabel, chief scientific officer for global health research at French drugmaker Sanofi, who designed a different Ebola vaccine in the 1990s when working at the US National Institutes of Health, called the new vaccine “a step in the right direction but not the ultimate solution.”

As a candidate for a routine Ebola vaccine, Nabel pointed to one being developed by British drugmaker Glaxo Smithkline GSK. The first of the two GSK injections contains the Ebola surface protein attached to a chimpanzee adenovirus; the second uses a weakened pox virus similar to that used in smallpox vaccine.