An AIDS vaccine may be one step closer after scientists found antibodies in the blood of a patient that kill more than 90 percent of HIV strains, according to a study in the online version of the journal Science.

The two proteins, which block HIV by attaching to an area of the virus it uses to enter healthy cells, are the most powerful and broadly effective yet against the myriad viruses that make HIV the world’s most deadly infectious disease, researchers led by John Mascola at the U.S. National Institutes of Health said in the study. Their findings improved upon those published in September by a rival group that found antibodies that neutralized about three-quarters of known strains.

HIV, the virus that causes AIDS, constantly changes its surface proteins to evade the immune system and thwart scientists who have so far failed to develop a vaccine against it. The antibodies Mascola and colleagues found target a part of the virus that rarely changes. While they can’t be used in a vaccine as is, the group has started work on a shot that would teach the immune system to make similar antibodies on its own.

“The discovery of these exceptionally broadly neutralizing antibodies to HIV and the structural analysis that explains how they work are exciting advances that will accelerate our efforts to find a preventive HIV vaccine for global use,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a statement.

HIV infects about 7,400 new people every day worldwide, and led to 2 million deaths in 2007, according to UNAIDS. While there are treatments that can suppress the virus for those who can afford them, there is no cure.

Scientists have failed to produce an effective vaccine to prevent infections after 25 years and billions of dollars spent on research since the virus was first identified. In October, the first vaccine to show promise in a large clinical trial reduced infections by 31 percent, not enough for a functional vaccine.