The Ebola epidemic in eastern DRC, which has been declared a international health emergency, is the first time vaccines have been deployed against the killer virus discovered in 1976.

A vaccine by drugmaker Merck is being widely used in the affected provinces of Ituri and North Kivu.

But WHO has recommended the introduction of a second product made by healthcare company Johnson&Johnson, which could be used for populations removed from the epicentre, while saving limited stocks of the Merck vaccine for the frontlines.

The DRC health minister Oly Ilunga, who was in charge of the Ebola response, resigned this week citing the "strong pressure" he faced to greenlight the new product.

Without naming any individual or agency, he said "actors who have demonstrated a clear lack of ethics," were pushing the second vaccine.

WHO's emergency chief Mike Ryan told reporters in Geneva that "no pressure was ever applied to the minister (regarding a second vaccine) by WHO," while praising Ilunga's "front-line" leadership since the outbreak began a year ago.

But, Ryan added, WHO "will continue to work with the authorities in Congo... based on the recommendations of the (Strategic Advisory Group on Immunization) committee to introduce a second vaccine."

Kinshasa's new Ebola pointman Jean-Jacques Muyambe has "committed to bringing together a scientific group at the national level to look at all potential second vaccines," Ryan said.

Muyambe was a member of the team that investigated the first known Ebola outbreak in then-Zaire.

Ryan stressed that the final decision on the use of any medical product lies with Kinshasa, but said "we need to have all of the available tools," given the complexity of the outbreak.

The epidemic has killed more than 1,700 people and containment efforts have been hindered by conflict in the region and lacking trust for Ebola fighters within affected communities.

- Enough doses? -

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The Merck vaccine is tailored for emergency response as it requires one dose and builds immunity within 10 days.

Using the J&J product in eastern DRC is complicated because it requires two doses given nearly two months apart and access to patients can be challenging in the region.

Doctors Without Borders has called for immediate use of the second vaccine to "build a wall" of vaccinated people around the outbreak's epicentre.

The medical charity also complained about lacking transparency regarding available stocks of the Merck product.

Ryan said that based on the current situation "there are adequate stocks" of the Merck vaccine.

But, if the outbreak takes a turn for the worse, "there are scenarios... in which we would be very stretched on supply," he added.