FAQs About the Efficacy of the Experimental Vaccine

39. What was the intensity of the antibody response observed on volunteers? 

At the HUG, all the volunteers who received 10 or 50 million vaccine particles of VSV-ZEBOV pro-duced antibodies able to bind to the envelope of the Ebola virus. This is an excellent result. Volun-teers who received the highest dose produced higher rates of antibodies potentially able to neutralize the Ebola virus. This observation was confirmed by the results obtained by the other teams: VSV-ZEBOV induces antibody responses even when used at a dose of 300,000 vaccine particles, but levels of neutralizing antibodies increase with dose and are at their optimal level with a dose of 20 million vaccine particles.

40. Do we already know how long after the injection the person is potentially immunized, and for how long?

Analyses performed on the first group of volunteers show that the immune response is already traceable 14 days after the injection. But the concentration of antibodies is higher after 28 days. We do not know yet if the antibody levels continue to increase, remain stable or decrease in the following months. This will be determined by monitoring the volunteers 6 and 12 months after vaccination.To date, we do not know the level of antibodies required in order to protect against the Ebola virus. This will be determined by the vaccine efficacy trials in West Africa.

41. On the basis of these results, what dose of the vaccine has been selected for the next clinical trials in Africa?

The observations made by the four teams (at the HUG and in Gabon, Kenya and Germany) on a total of 158 volunteers, as well by two US teams on 58 additional volunteers, led us to select a dose of 20 million vaccine particles for the phase III clinical trials. This high dose will increase the likelihood of this candidate vaccine being effective. The first phase III clinical trial on vaccine efficacy started in Guinea in March 2015, under the aegis of the World Health Organization. It will rely on several thousand volunteers, including some who have been in contact with patients infected by the Ebola virus.

42. Does this first clinical trial of a vaccine based on the vesicular stomatitis virus (VSV) open up other perspectives?

This is indeed the first time that this type of virus is tested as a "vaccine carrier" in a large-scale study. Teams working on cancer and HIV have also planned to use the vesicular stomatitis virus to develop vaccines, and some phase I clinical trials have begun, or are about to begin. Although each vaccine is different, the observations made by the teams studying VSV-ZEBOV will be useful for the development of other vaccines based on the VSV, in particular to better anticipate or even avoid potential side effects. In order to improve the tolerance profile of this type of vaccine, an even more attenuated strain of the VSV-ZEBOV is currently in a stage of preclinical development.

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Last update : 29/01/2019