The most recent and tragic documented outbreak of Ebola has affected over 20000 people and caused over 11000 deaths since March 2015. The magnitude of the outbreak has triggered an international response led by Medecins Sans Frontiers, who have placed 100 international and 1700 local staff in endemic areas and taken care of more than 5000 patients. However, Ebola is deadly and contagious, and there is no treatment or vaccine. Until now, that is: the first ever Ebola vaccine has been developed and proved successful in a clinical trial known as Ebola ca suffit (Ebola, that’s enough).
Ebola virus is endemic in equatorial Africa, the habitat of some bats and rodents which are thought to be its natural reservoirs. It is a small, elongated virus that contains 7 genes, which orchestrate both the inhibition of the immunological system and the entrance of the virus into the host cells. The virus is abundant in blood, organs and sexual fluids of the patients, and it is typically transmitted through contaminated needles and exposure to fluids of patients and corpses during care or burial rituals (aerial transmission has never been documented).
The virus first attacks the white blood cells, maiming the immunological system and rapidly spreading throughout the body. It accumulates in liver and in endothelial cells, which coat the blood vessels. As a result, the damaged liver stops producing clotting factors. This, added to the structural damage to the blood vessels, triggers fatal internal bleeding which typically kills the patient within 6-16 days since the start of the clinical symptoms. However, in case of less aggressive infections the body can mount an effective immune response, increasing the chances of survival of the patient.
The aim of developing a vaccine is to give the body the chance of getting that immune response ready beforehand. Vaccines that contain a safe, but live, version of the virus achieve the most powerful immunologic response. This is how the measles and rubella vaccines work. However, these attenuated vaccines have a tiny risk of virus reactivation, and in the case of Ebola, that risk is unacceptable. Other vaccines like Hepatitis B only contain only some pieces of the virus. These are generally safer, but they tend to trigger a less powerful response.
The Ebola vaccine was designed in 2003. It does contain an attenuated virus, the vesicular stomatitis virus (VSV), which causes a robust immunologic response but only results in a mild flu. This VSV has been engineered so it produces some of the proteins of the most aggressive strain of Ebola. In theory, exposure to this vaccine should cause the body to react robustly by generating antibodies against the Ebola proteins.
That is exactly what happens: it has proved to be both safe and extremely effective. Crucially, it is also effective in immunodepressed individuals, which is vital in West Africa, where the incidence of HIV reaches 10%. Early this year the vaccine was ready for phases II and III of the clinical trial. Prompted by the aggressiveness of the outbreak, the medical community designed an entirely new type of clinical trial aimed at testing the effects of the vaccine. (This clinical trial design will be the subject of the next blog post, so I won’t elaborate here.)
The results were really promising. No new infections developed amongst the 5000 recipients of the vaccine, even those who had had direct contact with patients. Experts predict that the overall efficiency will be 75-100% and that the protection will extend to other strains of Ebola virus. While these predictions need to be confirmed, it is certain that this Ebola vaccine will be one of the medical breakthroughs of 2015.