The challenge dose of 1,000 pfu used corresponds to the amount of EBOV contained in a relatively small quantity of fluid (on the order of 1 ll) from an infected individual given the high titers of virus typically found in such individuals (on the order of 10 6 pfu/ml of blood, for example). Therefore, the challenge dose was not unreasonable in terms of a natural exposure to virus. The negative results with passive antibody contrast strongly with recent successes in preventing EBOV infection in macaques through vaccination [26, 27] . Does this mean that neutralizing antibody is unimportant in vaccine protection?