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Encouraging first results from trial of Merck’s Ebola vaccine

Published on 31/07/15 at 04:20pm
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The trial is sponsored by the World Health Organisation

The first trial of an experimental Ebola vaccine suggests it provides high protection against the disease as early as ten days after vaccination.

The study in 7,500 people in Guinea, one of the countries heavily affected by the Ebola outbreak, found the vaccine – Merck Sharp & Dohme’s VSV-ZEBOV – was safe and gave complete protection against Ebola virus disease ten days after vaccination in adults who have been exposed to the virus.

The study, which was sponsored by the World Health Organisation, used a ‘ring vaccination’ strategy. This technique has been used in the past to eradicate smallpox. It is intended to create a buffer of protection to prevent the spread of the disease, by vaccinating and monitoring the contacts, and the contact’s contacts (the ‘ring’), of each newly-diagnosed person with Ebola.

Researchers traced all individuals who had been in contact with each person diagnosed with Ebola virus in Basse-Guinea, the only region in the country with new Ebola cases between April 1 and July 20, 2015. All adults who had been in contact with people with Ebola aged over 18 (excluding those who were pregnant or breastfeeding) were offered either immediate vaccination with VSV-ZEBOV, or delayed vaccination and were given the jab after 21 days.

After 10 days a single injection of VSV-ZEBOV gave complete (100%) protection against the virus. No cases were recorded 10 days after the start of the study in the immediate group, compared to 16 cases in the delayed vaccination clusters. The trial suggests that ring vaccination also reduced the risk of contracting the virus for non-vaccinated individuals in the ring.

The overall efficacy of the vaccine in adults, including both those who consented to vaccination and those who did not, was 75% – meaning the risk of testing positive for Ebola virus disease was reduced by around 75%. In all members of the clusters, including non-vaccinated children and pregnant women, it was 75%.

The researchers say the vaccine was well-tolerated. One vaccinated patient experienced an episode of fever that was found to be related to the vaccine.  The results are from an interim analysis and as the study continues serious adverse events will be fully assessed.

The researchers conclude in The Lancet that the results “support the conclusion that the rVSV-ZEBOV vaccine is efficacious and effective.”

“Before the trial started, in most clusters there had been a series of Ebola cases over the weeks prior to randomisation. However, since the trial started, we have seen no new cases in vaccinated volunteers within 10 days of vaccination, regardless of whether vaccination was immediate or delayed,” explains co-author Dr Marie Paule Kieny, from the WHO in Geneva, Switzerland.

And study co-author Professor John-Arne Røttingen, from the University of Oslo, Norway, adds: “Our results are encouraging in that they suggest that ring vaccination could substantially reduce rates of Ebola virus disease in the community. Because the way that Ebola virus transmits has been shown to be consistent across countries and regions, we believe that these results are likely to be applicable to other regions of Guinea and to Sierra Leone and Liberia. 

"But whether this candidate vaccine could become a licensed vaccine for widespread use against Ebola outbreaks is still uncertain, and further evidence is needed to evaluate the safety and efficacy of the vaccine before it is used outside of a clinical trial setting.”

The vaccine is still not licensed or available in Africa, and more data on its efficacy will be needed before it can be widely deployed.

Lilian Anekwe

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