Scientists are no further forward in developing a vaccine against HIV after more than 20 years of research, a Nobel Prize-winning biologist has said.Professor David Baltimore, president of the American Association for the Advancement of Science (AAAS), believes the battle is too important to give up on, despite some saying a vaccine will never be found."This is a huge challenge because to control HIV immunologically the scientific community has to beat out nature, do something that nature, with its advantage of four billion years of evolution, has not been able to do," he said."Our lack of success may be understandable but it is not acceptable," he added.Speaking at the annual meeting of the AAAS in Boston, Prof Baltimore said HIV had evolved a way to protect itself from the human immune system."I believe that HIV has found ways to totally fool the immune system," he said. "So we have to do one better than nature."'One shot'Attempts to control the virus through antibodies or by boosting the body's immune system have ended in failure.This has left the vaccine community depressed because they can see no hopeful way of success, he said in his presidential address to the conference.Scientists are now turning to novel techniques, such as gene and stem cell therapy, although they are still in their infancy."In the human you really only have one shot which is to try to change genes in stem cells," said Prof Baltimore, one of the leading experts on the HIV virus."So we're trying to do that, to design vectors that can carry genes that will be of therapeutic advantage."Prof Baltimore won the Nobel Prize in Medicine in 1975 for the co-discovery of reverse transcriptase, an enzyme that was later found to be used by HIV to replicate in human cells.He now leads the Baltimore laboratory at Caltech, with support from the Gates Foundation, to look for ways to genetically boost the immune system against infectious agents, particularly HIV.Source: BBC