If there’s an Ebola vaccine that works on monkeys, then what about humans?

Tuesday’s death of Sheik Umar Khan, the doctor at the front lines of Sierra Leone’s battle against the worst Ebola outbreak in history, marks the latest person killed by the contagious, incurable disease that has devastated communities in Guinea, Liberia and Sierra Leone since February. At last official count, released July 23, 672 people have died from the virus.

Outbreaks like this one beg for a vaccine, especially given the fatality rate of the disease, which can be up to 90 percent, according to the World Health Organization. The virus has popped up sporadically since 1976, when the Zaire species of Ebola virus was first recognized in Africa.

According to a Scientific American Q and A of leading virologist Thomas Geisbert, progress on a vaccine is stalled. But he says there are three to five preventative vaccines that have proven to protect nonhuman primates against the fatal virus.

The University of Texas researcher says scientists are stuck in phase I trials in humans. He estimates we are anywhere from two to six years from being able to break ground on an effective vaccine:

“I hate to say this, but it really depends on financial support for the small companies that develop these vaccines. Human studies are expensive and require a lot of government dollars. With Ebola, there’s a small global market — there’s not a big incentive for a large pharmaceutical company to make an Ebola vaccine, so it’s going to require government funding.

In terms of potential, he gives the example of the VSV vaccine, which functions similarly to the rabies vaccine. But Geisbert explains that vaccines like this one require a booster. The catch in Africa, he notes, is that “you’re lucky to get someone into a clinic to be vaccinated once. It’s a trade-off — efficacy versus safety. That’s one of the biggest challenges.”

As shown by the CDC map above, Ebola treatment centers, laboratories and clinics are not necessarily easily accessible by all communities.