The vaccine, called rVSV-ZEBOV, was studied in a trial involving 11,841 people in Guinea in 2015. The results were published Thursday in the journal The Lancet.
Of the 5,837 participants that received the vaccine, no Ebola cases were recorded 10 days or more after vaccination, while 23 infections were observed in that timespan in the group that did not receive the vaccine.
“While these compelling results come too late for those who lost their lives during West Africa’s Ebola epidemic, they show that when the next Ebola outbreak hits, we will not be defenseless,” said Marie-Paule Kieny, the study’s lead author and assistant direct general for the World Health Organization’s health systems and innovation program.
The vaccine is expected to help in future Ebola outbreaks to avoid widespread local transmission, which West Africa experienced in a 2013 outbreak that killed more than 11,300 people.
The field test took place in the coastal region of Basse-Guinee, the area of Guinea that was still experiencing new Ebola when the trial began. The team used a so-called “ring vaccination” approach – a method previously used to eradicate smallpox.
Once a new Ebola case was diagnosed, the team traced all people who may have been in contact with an infected individual within the previous three weeks, including those who were visited by the patient, were in close contact with them, or who lived in the same household. They identified 117 “rings,” made up of an average of 80 people.
The researchers randomized which rings received the vaccine – either immediately or after a three-week delay – and only adults over 18 were offered the vaccine initially. Following early results that showed the vaccine’s efficacy, all rings were offered the vaccine immediately and the trial was also opened to children older than 6.
“Ebola left a devastating legacy in our country,” said Keita Sakoba, director of the National Agency for Health Security in Guinea. “We are proud that we have been able to contribute to developing a vaccine that will prevent other nations from enduring what we endured.”
To assess safety, the team observed people who received the vaccine for 30 minutes after vaccination, and performed repeated home visits up to 12 weeks later.
About half of the study participants reported mild symptoms soon after vaccination, including fatigue, headaches and muscle pain, which ended a few days later without long-term effects. One serious adverse event was judged to be possibly related to an influenza-like illness rather than a reaction to a vaccination.
However, two serious cases were determined to be related to vaccination: a febrile reaction that can result in seizures, and a case of anaphylaxis – a severe, potentially life-threatening allergic reaction.
While it wasn’t possible to collect biological samples from people who received the vaccine in order to analyze their immune response, other studies are examining the immune response to the vaccine – one of which was conducted in parallel to the ring trial, focusing on frontline Ebola workers in Guinea.
“This both historical and innovative trial was made possible thanks to exemplary international collaboration and coordination, the contribution of many experts worldwide, and strong local involvement,” said John-Arne Rottingen, specialist director at the Norwegian Institute of Public Health.
An agreement between the vaccine’s manufacturer, Merck, Sharpe & Dohme, and the Global Alliance for Vaccines and Immunization provides $5 million for future production once the vaccine is approved, prequalified and recommended by the WHO.
Merck also committed to ensure that 300,000 doses of the vaccine are available for emergency use in the meantime, and to submit the vaccine for licensing by the end of 2017. The company submitted the vaccine to WHO’s Emergency Use and Assessment Listing procedure, which provides experimental vaccines, medicines and diagnostics before formal licensing.
A separate study also published in The Lancet details a phase-2 trial of another Ebola vaccine candidate, the recombinant adenovirus type-5 Ebola vaccine. Researchers from the Beijing Institute of Biotechnology conducted a trial in Sierra Leone in 2015, involving 500 healthy participants.
Half of the participants were given a high-dose vaccine, while 125 people were given a low-dose vaccine. The rest served as a placebo.
The study found that the vaccine was safe, creating an immune response that peaked at 28 days, but decreased during the six months after injection. Further research on the vaccine is needed to determine its efficacy, according to the findings.