As Nigeria becomes the fourth west African country fighting to contain Ebola, Professor Eleanor Riley from the London School of Hygiene and Tropical Medicine explains the latest medical science for preventing and treating the disease.

Why hasn't an Ebola vaccine been developed so far?

Ebola, which has occasional small, often easily contained, outbreaks, is never going to be a high priority for the World Health Organisation (WHO) or donors. They aren't going to vaccinate everybody at birth against the disease because of the amount of effort and resource you would have to put in to save a small number of lives.

Is that still true after this outbreak?

Yes and no. This is the biggest outbreak by far. We've known about Ebola since the 1970s and most outbreaks have been very well controlled. This one is unusual. Somebody took their eye off the ball and maybe that won't happen again, we don't know. But I think this has been a real learning point for public health.

It takes two or three weeks after vaccination for your immune response to get to a protective level. So it's not as if you can go into a village where there's been an outbreak, vaccinate everyone and they're protected. Once you've got an outbreak, vaccination isn't necessarily much help.

What about the 'secret serum' that was given to two American doctors? What is it and could it contain the outbreak?

This drug gives you antibodies directly, rather than waiting two to three weeks after vaccination for the antibodies to work. It could be used for treatment whereas a vaccine is more likely to be used for prevention.

Why wasn't more of the serum available?

It's a question of whether there is a market for this. Who is going to pay for it? How much do you actually need? It hasn't been through clinical trials yet, but if it's found to work it would be great to have a stockpile of this available to treat people who get Ebola. It's very expensive at the moment. But with most of these things, if you can make a lot of it then it tends to get cheaper. But the initial investment to develop and test in clinical trials is expensive.

By the sound of it there's not enough of it available to make a difference with this particular outbreak. But if they can give it to a few people and show that it really helps, they will presumably be able to raise more money to make more of it in the future.

What do you think about Canada's decision to send in doses of an untested vaccine?

It depends what they mean by untested. The vaccine that has been made in Canada has been tested on monkeys and seems to have some protective value. If I was about to go to west Africa to treat patients with Ebola I would probably take that vaccine. It might not work but it's unlikely to do me any harm. It might well save my life.

Is the untested vaccine dangerous?

We have no data about the hazard of the vaccine itself, but we have no evidence to say that it's dangerous. At this particular stage the cost benefit definitely lies with giving the vaccine. If we give to people and several of them start to develop unpleasant side effects, we would have to stop. But those side effects would need to be quite serious.

What is the effect of the outbreak on vaccine development?

In the end, we might be a lot further on with a vaccine than we would have been without one. Nobody wanted an Ebola outbreak in order to speed up development, but that might be the silver lining.

Read more stories like this:

• 15 ways to fund drugs for diseases of poverty

• Ebola: voices from the epicentre of the epidemic

• Preventing HIV: the latest on vaccine development and gene therapy

Join the community of global development professionals and experts. Become a GDPN member to get more stories like this direct to your inbox