Vaccine challenges
The first major issue is running the clinical trials at speed, nothing like this has been done before.
Ethical issues surround testing a vaccine during an outbreak, and researchers face a lack of public trust in some communities
When the trials move to the front line of the Ebola outbreak, then it will be healthcare staff and burial workers who will be immunised.
There are no plans for mass vaccination of the general population before June 2015, but the WHO has not ruled it out.
There are also practical issues to address as both vaccines contains live, but weakened, virus.
It means the shots will have to be kept below minus 80C in hot countries with limited access to electricity.
The drug companies also want indemnity in case something goes wrong when they are asked to rush through a vaccine.