Updated at 7:30 p.m. on January 29, 2015

Only a day after the World Health Organization announced that an end was in sight for the Ebola epidemic in West Africa, scientists had less uplifting news: The virus may be mutating.

“The response to the EVD (Ebola virus disease) epidemic has now moved to a second phase, as the focus shifts from slowing transmission to ending the epidemic,” the WHO said in its latest report on the disease yesterday. There were 99 confirmed new cases last week, the lowest number since June.

But researchers at the Institut Pasteur, the French medical-research organization that first identified the outbreak in Guinea last March, said that they’re trying to determine whether the Ebola virus is becoming more contagious.

"We know the virus is changing quite a lot,” Anavaj Sakuntabhai, the head of the Laboratory for Genetics of Human Response to Infection at the Institut Pasteur, told the BBC.  Specifically, he said, there have been a number of asymptomatic cases, meaning that infected people may unknowingly be spreading the disease: “A virus can change itself to less deadly but more contagious, and that's something we are afraid of.”

Sakuntabhai and his colleagues are currently analyzing hundreds of blood samples from Guinean patients to monitor the virus’s evolution. "That's important for diagnosing and for treatment," he said. “We need to know how the virus [is changing] to keep up with our enemy."

Ebola, like measles, HIV, and influenza, is an RNA virus, meaning it can mutate quickly and often. In August, a paper published in the journal Nature found that the virus had already evolved several times over in the first month of the outbreak in Sierra Leone (five of the paper’s authors died of Ebola before their work was published), though there is still nothing to suggest that the virus has become airborne, a common fear over the course of the epidemic, or that it has evolved out of reach of existing treatments. "The mutations do not seem to be affecting the efficacy of experimental drugs and vaccines," Nature reported of the Sierra Leone paper in August.

"It isn't surprising at all that the virus is mutating," said Anthony Fauci, the head of the National Institute for Allergy and Infectious Diseases, but "I haven't seen any compelling data yet that the mutations are associated with a change in the function of the virus."

According to the most recent numbers from the WHO, the current Ebola outbreak has killed 8,461 people and infected around 22,000, nearly all in Guinea, Sierra Leone, and Liberia.

Meanwhile, scientists reported the results from the first human trials of an Ebola vaccine, published yesterday in the New England Journal of Medicine, with cautious optimism. “The safety profile is pretty much as we'd hoped and the immune responses are okay, but not great,” Adrian Hill, the lead researcher for the trials, told Reuters, and researchers believe a booster will probably be needed for full protection. Last week, GlaxoSmithKline, the company that developed the vaccine, shipped 9,000 doses to Monrovia, Liberia for a phase III clinical trial. The Institut Pasteur is also working on two vaccines, aiming for human trials by the end of this year.

“The best type of resource we can think of … is to have vaccination of global populations,” James Di Santo, the head of Pasteur’s Innate Immunology Unit, told the BBC. “This particular outbreak may wane and go away, but we're going to have another infectious outbreak at some point, because the places where the virus hides in nature—for example, in small animals—is still a threat for humans in the future.”