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Global health officials are preparing to launch a new clinical trial that could help tamp out the final embers of West Africa’s two-year Ebola outbreak, according to the World Health Organization.

Guinea’s health ministry, with the assistance of the WHO, intends to offer an experimental Ebola vaccine to people who are at high risk of contracting the disease from survivors who may still be secreting viruses.

Although there are currently no confirmed Ebola cases in the three countries ravaged by the outbreak that started in December 2013, there remain serious concerns that the outbreak could be reignited through sexual transmission involving a survivor, said Dr. Marie-Paule Kieny, the WHO assistant director general who has led efforts to speed development and testing of Ebola vaccines and therapies.


In fact, this phenomenon was probably responsible for the most recently discovered cases. While there has been no official report yet, it’s believed a cluster of three infections in a family in Liberia was initially set off by a sexual transmission.

It has been known for decades that in rare cases people have caught Ebola from having unprotected sex with a man who survived the infection. One such case involved a Liberian woman who contracted Ebola last March; the source of her infection was traced to a sex partner who had been infected six months earlier.


Earlier this fall, scientists from Sierra Leone’s health ministry, the WHO, and the Centers for Disease Control and Prevention reported that a study of 93 male Ebola survivors showed that nearly half were shedding virus in their semen four to six months after their recovery and a quarter were still emitting virus seven to nine months after they recuperated. It’s thought that it is harder for the immune system to root out the viruses in certain parts of the body, such as the eyes and testicles.

From March 2014 to November 2015, the suspected, probable and confirmed cases of the Ebola virus in Guinea, Liberia, and Sierra Leone topped 28,000. Alex Hogan/STAT

The WHO recommends that Ebola survivors forgo sex for the first three months after they recover, or to practice safe sex if abstinence is not an option. Semen from male survivors should be tested for Ebola after three months; if it is positive, abstinence or safe sex should be extended to six months, the global health agency says.

To date, the West Africa outbreak has led to at least 28,637 infections, 11,315 of them fatal. The outbreak, though, peaked in the late summer and early fall of 2014. At this point, it is thought the number of individuals still harboring Ebola viruses has diminished steadily.

In fact, researchers at the London School of Hygiene and Tropical Medicine published a modeling study earlier this month that estimated that by early January, as few as 73 men in the three countries — Guinea, Sierra Leone, and Liberia — may still be shedding Ebola viruses in their semen.

There remain serious concerns that the Ebola outbreak could be reignited through sexual transmission involving a survivor.

It’s only an estimate. But to lower the risk that new cases might emerge, Guinea and the WHO will offer an Ebola vaccine to people at high risk of contracting Ebola from a survivor. The vaccine, being developed by pharma giant Merck, was created by scientists at the Public Health Agency of Canada’s National Microbiology Laboratory in Winnipeg, Manitoba.

Although there are at least 13 experimental Ebola vaccines at various stages of development, the Merck vaccine is the only one supported by data showing that it cuts the risk of infection in people. The trial that came up with those data is still officially underway in Guinea, but will likely be wound down in late January if no additional cases crop up, Kieny told STAT.

The new trial, which will be led by Dr. Sakoba Keita, coordinator of Guinea’s Ebola response, will attempt to minimize the risk of survivor-linked cases. It should also answer a question scientists have about the Merck vaccine.

The virus is made with live vesicular stomatitis viruses, a livestock disease that does not make people sick. The VSV viruses have been engineered so that they carry a protein found on the exterior of Ebola viruses. The vaccine viruses essentially present the Ebola protein to the immune system, in effect teaching it to see Ebola as a threat. In the trial overseen by Kieny, the vaccine appeared to be highly effective.

But there are concerns that some people who have been in close contact with Ebola cases may have already developed low levels of antibodies to the virus. These antibodies may not be enough to protect them from infection, but may render the vaccine ineffective for them. The theory is their immune systems may kill the vaccine viruses before they have a chance to trigger a substantial antibody production.

Giving the vaccine to those who have been in contact with survivors should help illuminate whether the vaccine is useful in this kind of circumstance, Kieny said. “So in addition to being an operation to reduce the potential risk of transmission from survivors, there are also a number of scientific questions that could be addressed through such a trial,” she said.

In a separate effort, the organization Doctors Without Borders is working to ensure that the Merck Ebola vaccine can continue to be tested in West Africa and further afield in future outbreaks.

Because the vaccine is not yet licensed, it can only be used in the context of a clinical trial. Doctors Without Borders, also known by its French acronym MSF, is trying to have trial protocols for different countries approved and at the ready so that if a case is found, the people who are close contacts of the infected person and the health-care workers caring for the patient can be enrolled in a trial and vaccinated.

The vaccine goes by the name VSV ZEBOV.

For now, Doctors Without Borders is preparing for future Ebola outbreaks, Annick Antierens, coordinator of the organization’s investigational platform, said in an email. “And … having access to the VSV ZEBOV vaccine so as to be able to vaccinate frontline workers and do ring vaccination of contacts of confirmed cases if relevant is part of the preparedness,” she said.