Written by Marsha Seidelman, M.D.
August 2, 2015
There’s new positive information on vaccine development, following the largest recorded outbreak of Ebola. Since last year, there have been 17,000 total cases (suspected, probable and confirmed) and 11,000 deaths, most of which were in Guinea, Sierra Leone and Liberia. Since Liberia was declared free of the virus in March (after 42 days, or two 21-day incubation periods), there was 1 case reported on June 28th according to the CDC website. However, new infections contiue to occur more commonly in Guinea and Sierra Leone.
An article published in the Lancet online on July 31st reports on a Phase III clinical trial for a vaccine using a different virus (VSV or vesicular stomatitis virus) in an altered form, with the addition of a surface protein of the Zaire Ebola virus. It’s referred to as rVSV-ZEBOV. It causes a transient infection after a single injection, and produces a rapid immune response against the Ebola virus surface protein. This latter reaction produces antibodies which should protect individuals when they are exposed to the actual Ebola virus.
In the past, this vaccine has gone through Phase I and Phase II trials, meaning that it is known to produce appropriate antibodies and felt to be safe and a dose was determined. In the current trial, the researchers looked at giving the vaccine to those already exposed to someone with Ebola in Guinea, to see if it could prevent these contacts from becoming ill. They gave the vaccine to contacts then contacts of those contacts in a ‘ring’ pattern, similar to how the smallpox vaccine was tested years ago.
Because the death rate from Ebola is so high, it was felt that a placebo arm in which subjects would get a fake vaccine, would pose too great a risk of death. So even before the vaccine was proven to work, it was felt to be better than no treatment at all. Therefore one group was given the vaccine immediately after exposure to Ebola and the other group after a 21 day delay from the time of exposure.
The trial is still ongoing, but the article in Lancet reports very favorable results from March through July 20th, 2015. There were 7,651 participants, 75 cases of Ebola, and 33 deaths (versus potentially thousands of deaths without the vaccine). Of great significance, all cases of Ebola occurred before vaccination or within 6 days of vaccination, suggesting that immunity is adequate about a week after the vaccine is given. Given these early results, the safety board decided that there should no longer be a group that has its vaccine delayed 21 days after exposure. This is very positive news on the Ebola front and much more hopeful than when we last reported on it.
In the words of the authors:
Implications of all the available evidence:
The results of this interim analysis suggest that rVSV-ZEBOV might be highly efficacious in preventing Ebola virus disease, and most likely effective at the population level when delivered during an outbreak using a ring vaccination strategy. These data can contribute to the ongoing assessment of this vaccine and help to inform policy and regulatory decisions with regard to the Ebola vaccination strategy.
REFERENCES:
http://lancet.com/pb/assets/raw/Lancet/pdfs/S0140673615611175.pdf
http://www.cdc.gov/vhf/ebola/outbreaks/2014-west-africa/case-counts.html