Image: courtesy of the Centers for Disease Control

With its nightmarish symptoms and ferocious fatality rate, Ebola ranks among the most feared viruses. To make things worse, no antiviral drugs currently available have proven effective against it, and because scientists do not know where the virus resides between outbreaks, environmental control is impossible. Researchers have thus devoted considerable energy to developing an Ebola vaccine. Now it appears that they have made an important breakthrough. According to a report published today in the journal Nature, a team led by scientists from the National Institutes of Health (NIH) has developed a vaccine that protects monkeys from lethal doses of the virus.

Previously developed vaccines had been shown to protect mice and guinea pigs from Ebola-like viruses. The new vaccine, however, works against an Ebola strain that infects humans. Immunization in this case takes the form of a one-two punch. Researchers first injected monkeys with chains of DNA coding for proteins from three strains of Ebola in order to develop immunity. Then, after a rest period, the monkeys received an injection containing a common-cold virus engineered to express Ebola proteins. The hope was that this second injection would boost their immunity by stimulating antibody production. In fact, the combination worked: of the four monkeys that received the vaccination prior to being infected with the Zaire strain of Ebola, none expressed any symptoms. Furthermore, three of them completely cleared the infection within two weeks. More than six months later all four were still symptom-free, with no detectable virus in their blood. In contrast, all four of the monkeys that were infected but not vaccinated developed symptoms, and three died in less than a week (the fourth was euthanized).

Whether the vaccine works against all three strains of Ebola remains to be seen. And in a commentary accompanying the Nature report Dennis R. Burton and Paul W.H.I. Parren of the Scripps Research Institute note that although the monkeys received lethal Ebola doses, those doses were still relatively small. "Previous studies have shown that antibody preparations that protect against low doses of virus may be ineffective against higher doses," they warn. "It will be crucial to know whether the vaccine strategy can protect against more substantial challenge."