Researchers expect different types of vaccines to have different effects on the immune system. Because the vaccine being tested in Mali and Baltimore uses a weakened virus, it might be tolerated better but not last as long as vaccines that use live viruses, said Dr. Thomas Geisbert, a professor of microbiology and immunology at the University of Texas Medical Branch at Galveston who has studied Ebola for decades and helped develop the Profectus vaccine. The trials are also testing the performance of varying dosage levels, ranging from 25 billion to 50 billion modified virus particles.