Leading article: The fruits of scientific persistence

When scientists identified HIV as the cause of Aids more than 20 years ago, they predicted that a vaccine to prevent the infection would be developed long before drugs to treat the symptoms. The opposite happened. Today, millions of people around the world, especially the wealthy but increasingly including the poorest in sub-Saharan Africa, depend on a cocktail of drugs to hold the disease in check.

Despite the expenditure of years of effort and billions of dollars, the search for a vaccine to prevent the infection proved fruitless. Four trials of separate vaccines failed to show a protective effect and there was even a suggestion they made certain people more vulnerable. The positive finding from the world's largest vaccine trial in Thailand, announced yesterday, is therefore significant on two counts.

Firstly, it provides proof of concept that a vaccine can be developed. Researchers have a handle on the virus that they did not have before which should mean we can now look forward to further progress, though there is still a long way to go before scientists will be able to say when, or whether, a viable vaccine against HIV may be available. Secondly, it is a tribute to those scientists who persisted with the trial, in the face of sometimes bitter opposition from colleagues, who accused them of wasting funds and even endangering lives.

HIV is a diabolically clever virus that disables the very immune responses a vaccine needs to trigger in order to work. After the failure of the four trials, pessimism within the scientific community grew and some scientists predicted a vaccine would never be found. Meanwhile, dissenting voices within the Aids community demanded the resources devoted to vaccine research be diverted into safer sex, condom distribution and other preventive measures.

Drugs to treat HIV have been successful in converting a disease that was once a death sentence into a chronic condition. But no one has ever been cured of HIV. Preventing the infection remains the best strategy. Yesterday's announcement brings hope that a successful vaccine, though still a distant prospect, will one day end the scourge of Aids.