The drug, called Ervebo and manufactured by MERCK pharmaceuticals, was being used in an experimental phase in the Democratic Republic of Congo.

But now the WHO says there is sufficient evidence that shows that the vaccine works.

It means the world now has the first ever drug to prevent Ebola.

A WHO prequalification is granted only after a drug passes important safety and efficacy tests.

But this vaccine has already been in use since last year to contain two outbreaks in the Democratic Republic of Congo.

The government invoked "compassionate protocols" to allow its use while research was still under way.

The evidence collected during that time has convinced regulators that it was safe and effective against Ebola.

The vaccine will be available in the market from around mid-next year.