 But since some of the Phase I and Phase II volunteers had become infected with HIV while taking the vaccine, showing that the vaccine was not 100% effective, and it was not proven that the vaccine itself hadn't caused these infections, the FDA and other members of the medical community hesitated and finally declined to approve Phase III testing "until more was learned about HIV immunity", despite the fact that early versions of successful vaccines have rarely been 100% effective, and even the 1955 Salk polio vaccine was only 70% effective, superseded as it was seven years later by the Sabin vaccine.With human lives at stake, however, the FDA could not risk condoning further trials until it knew what had caused the infections.Another problem with AIDSVAX was that it provoked an entirely humoral, antibody immune response in its subjects, unlike other HIV vaccines in development in Europe and elsewhere that were provoking balanced antibody and cellular defenses.