An Aids vaccine might never be found, claims one of the world's leading experts on the disease. David Baltimore, a Nobel laureate, said the complexity of the disease means scientists are no closer to a vaccine today than when they discovered the link to HIV more than 25 years ago.

Baltimore, a biologist at the California Institute of Technology, was awarded the Nobel prize for medicine in 1975 for discovering an enzyme that was later discovered to be the key reproductive mechanism used by the virus known as HIV.

As president of the American Association for the Advancement of Science, Baltimore used the opening address of the organisation's annual meeting in Boston, in the US, yesterday to highlight the "sad topic" of the search for the Aids vaccine.

When HIV was linked to Aids in the early 1980s scientists were convinced a vaccine would be around the corner. Baltimore said: "We've been working on that vaccine since then and we are no closer to a vaccine now than we were then."

He led a panel of experts in 1986 which concluded that, given the complexity of the problem, an Aids vaccine was at least 10 years away. "I still think an Aids vaccine is 10 years away," he said.

He added: "You are quite within bounds to ask, if it's been 10 years away for 20 years, does that mean it's really never going to happen? There are people saying it will never happen."

The latest disappointment came last year, after a trial of a promising vaccine by the pharmaceutical company Merck was halted when some recipients were left more prone to HIV.

Baltimore's own work involves using a combination of gene therapy, stem cells and immunology. "HIV has found ways to totally fool the immune system so we've got to do one better than nature because nature just doesn't work in this circumstance." But he cautioned that there was still a lot to do. "I don't want to pretend that we have found the route to a new vaccine."

In his address Baltimore also attacked George Bush's record on science. "There has been an attempt to suppress government scientists from speaking out, there has been control of what scientists can go to scientific meetings and give talks - something totally unknown to us in previous years. That kind of attempt to control scientific information will, I hope, end with the Bush administration."