Ebola vaccines- end of the epidemic?

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Ebola is one of the deadliest diseases in the world, with up to 90% of infected people dying from the virus. The devastating 2014 Ebola epidemic, which killed 11,000 people across three countries in Africa (Guinea, Sierra Leone and Liberia), has intensified the search for a vaccine. A trial for a vaccine called VSV-EBOV started in April 2015 in Guinea. It was developed in Canada and is licensed by Merck and NewLink. Last week the first results of the trial have been announced and they are very promising. Effectiveness of the vaccine has been reported to be 100% by researchers and media.

You may wonder however, how effectiveness can be accurately measured. The obvious, but impossible way to test a vaccine is to vaccinate people and then expose them to the pathogen that causes the disease. So how did this, so called ring-trial work?

The vaccine was not given to healthy people, but people at risk. To be exact the vaccine was given to over 4000 people that had close contact to Ebola patients, such as family members and neighbours. This approach is called ring-vaccination as you build a protective ring around the newly diagnosed patient to prevent the virus from spreading further.

Half of the participants received the vaccine right after contact with the Ebola patient, the other half three weeks past diagnosis of the patient. This grouping allows the scientist to evaluate the efficiency of the vaccine. In most clinical trials patients will be randomly assigned to one of two groups: drug group (these people get the working compound) or placebo group (these people will get a non-active compound with no effect on the disease). Scientists then evaluate the difference in disease progression between the groups. Use of the placebo treated group allows to account for any psychosomatic effects that the act of given a medication can provide. Ebola is such a devastating disease however, that there was no placebo group but a delayed vaccination group. Ten days after the vaccination of the people, the scientists evaluated how many of the vaccinate people in both groups developed Ebola symptoms. None of the immediately vaccinated got ill, while 16 people in the delayed-vaccinated people got ill. This finding leads to the statement that the vaccine is 100% effective.

What do those 100% mean?

It means that everyone immediately receiving the vaccine was protected from the disease. The trial was structured in a way that clusters (people that were in contact with the same Ebola patient) received the same treatment – immediate or delayed. Could this induce bias by chance? This was not the case, as people that chose not to be vaccinated in an otherwise immediate vaccination clusters did get Ebola as well, thus proving that the incidence rate of Ebola in all clusters was similar.

16 people may seem like a small number to draw conclusions from. This is true, but as the number of Ebola-infected people is decreasing, it will get harder and harder to get data. Also for the other vaccines that are being tested. Nonetheless, more research is needed to establish how long the vaccine lasts and how soon it starts working. Furthermore, also the side effects of the vaccine need to be further evaluated; as so far 43 cases of adverse effects have been reported.

Even so, this trial is the best we have seen for an Ebola vaccine yet. Given the promising results, randomization has been stopped last weekend. So everyone taking part in the study now receives the vaccine right away. The study is also extended to children at the age of 13-17. However, the vaccine is not a cure for infected people, but it can help prevent spreading of the disease and if the vaccine proves to be safe and effective over longer periods, preventive vaccination are also possible.

Any questions regarding Ebola or vaccinations? Let us know.


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