Nearly one year after the Ebola outbreak in west Africa’s official end, final results from one of the only vaccine trials for the virus are now in, and they look quite promising.

An international group of researchers associated with the World Health Organization reports the vaccine is 100 percent effective at preventing Ebola when given 10 or more days prior to exposure to the deadly virus.

Ira Longini, a professor in biostatistics at the University of Florida, says:

“The goal was to estimate the vaccine efficacy from a phase III randomized vaccine trial.”

Longini is a key figure in the design of the Ebola vaccine trial and the analysis of its statistical data.

rVSV-ZEBOV Vaccine

Writing in the journal The Lancet, the international team has shown the rVSV-ZEBOV vaccine produced by Merck can protect both individuals and a whole population through its trials in Guinea.

The 2014 Ebola virus outbreak was by far the largest and most lethal Ebola outbreak ever recorded. Transmission occurred primarily in three West African countries: Liberia, Sierra Leone, and Guinea.

According to the Centers for Disease Control and Prevention, the virus infected nearly 29,000 people in the region, with more than 11,000 deaths.

The trial was called “Ebola ça suffit!” and it was the first successful phase III clinical trial for an Ebola vaccine. Having successfully completed phase III indicates it has been tested on hundreds of subjects and proven both safe and effective.

Ring Vaccination Trial

Trial participants were organized into 117 clusters, of which 70 clusters received the vaccine immediately and 47 clusters received it 21 days later. Disease takes several days, even weeks, to develop following Ebola infection, yet there were zero cases among vaccines more than 10 days after any cluster received the vaccination.

Those in immediately vaccinated clusters who were not vaccinated still received protection, thanks to the trial design—known as a ring vaccination trial.

This type of trial creates clusters around contacts of people who have contracted the pathogen, as well as contacts of contacts, since these people are at a higher risk of contracting disease. Since the immediately vaccinated participants reduced the number of infections, those ineligible for vaccination within immediate clusters benefitted from herd immunity.

According to the report, vaccinating only 52.1 percent of the participants was still 70.1 percent effective in preventing the spread of Ebola.

No Vaccine Is Perfect

While the data suggests the vaccine is 100% effective, this is not likely to be the definitive number. Biology simply does not work like that. This is especially true when the numbers looked at are so small (23 cases in the control group).

The potential inaccuracy of this number likely reflects the challenges in conducting a vaccine trial such as this, for a disease like EVD in the middle of a very serious outbreak.

There is also the fact that no vaccine is perfect. Even one of our very best vaccines, the measles vaccine, is only about 95% effective.

Previous studies have even shown that rVSV-ZEBOV produces an immune response in only 94% of individuals. Being certain about the effectiveness is important because computer models suggest that ring vaccination for Ebola may only be successful when there aren’t that many cases.

Despite being carried out under some of the most challenging conditions, the trial reported here appears to be an exceptionally well-run study, comprehensive in its nature and with a very positive result. This surely cements this vaccine as one important tool in controlling outbreaks of Ebola in the future.

It also gives us a way to test new vaccines for other viruses that can crop up and take us by surprise, such as Lassa fever virus and Nipah viruses. We just have to hope that scientists have got more vaccines in the pipeline for us to test.

The vaccine is not available for sale. The WHO has collected a stockpile as a safeguard, just in case another Ebola outbreak occurs.

Henao-Restrepo, Ana Maria et al. Efficacy and effectiveness of an rVSV-vectored vaccine in preventing Ebola virus disease: final results from the Guinea ring vaccination, open-label, cluster-randomised trial (Ebola Ça Suffit!) The Lancet DOI:

Portions of this post are taken from an article by Connor Bamford, Post-doctoral Research Assistant, University of Glasgow, originally published on The Conversation.

Top Image: WHO/S. Hawkey

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