The success of the phase I clinical trial for the vaccine paves the way for field-testing it in the Ebola-stricken West African nations of Liberia and Sierra Leone as early as January. The current vaccine, developed by the Institute of Allergy and Infectious Diseases and GlaxoSmithKline, is based on a virus called chimpanzee-derived adenovirus, which causes a common cold in chimps but has no effect on humans. Researchers have spliced portions of Ebola's genetic material into the chimp virus, to trick the immune system into creating Ebola-attacking antibodies without exposing the body to the virus itself. The second vaccine, also genetically engineered, is based on a virus common to cattle and horses called vesicular stomatitis virus (VSV). This is in the same family as the rabies virus but causes only flu-like symptoms in infected humans. Thomas Geisbert, an Ebola expert and a professor of microbiology and immunology at the University of Texas Medical Branch, believes the VSV-based Ebola vaccine may be the way to go.