41.   There have been scientific and logistical obstacles to finding an effective vaccine against the scourge.

42.   There is not yet an effective vaccine, though several are being tested.

43.   To Levy, the discovery of the protective effect of the specialized cells could be the key to developing an effective vaccine.

44.   Until an effective vaccine is developed, experts say, the best approach is simply to keep mosquitoes from biting people.

45.   Until an effective vaccine is developed, there is no hope for eradicating the disease.

46.   Yet effective vaccines do not exist, in part because most of the victims live in poverty and cannot afford drugs.

47.   A genetically weakened virus that many hoped would be a safe and effective vaccine against AIDS instead appears to mutate into a strain that actually causes the disease.

48.   A new vaccine effective in children is being tested and could be available in two to three years, Chesney said.

49.   Cuba has developed an effective vaccine against the deadly disease.

50.   However, no significant breakthroughs in this area were reported at the meeting, and experts believe an even modestly effective AIDS vaccine is several years away.