Researchers have said for the first time an experimental vaccine has cut the risk of infection with the deadly Aids virus.
US military and Thai health officials told a news conference in Bangkok that the vaccine cut the risk of becoming infected with HIV by more than 31%.
The result came after the world's largest Aids vaccine trial of more than 16,000 volunteers in Thailand.
Recent failures led many scientists to think such a vaccine might never be possible.
Even though the benefit is modest, "it's the first evidence that we could have a safe and effective preventive vaccine", military spokesman Jerome Kim said in a telephone interview.
He helped lead the study for the US Army, which sponsored it with the National Institute of Allergy and Infectious Diseases.
The institute's director, Dr Anthony Fauci, warned that this is "not the end of the road" but said he was surprised and very pleased by the outcome. It gives me cautious optimism about the possibility of improving this result" and developing a more effective Aids vaccine, Dr Fauci said. "This is something that we can do."
Even a marginally helpful vaccine could have a big impact. Every day, 7,500 people worldwide are newly infected with HIV; 2 million died of Aids in 2007, the UN agency UNAids estimates.
The Thailand Ministry of Public Health conducted the study, which used strains of HIV common in Thailand. Whether such a vaccine would work against other strains in the US, Africa or elsewhere in the world is unknown, scientists stressed.
The study tested a two-vaccine combo in a "prime-boost" approach, where the first one primes the immune system to attack HIV and the second one strengthens the response. They are ALVAC, from Sanofi Pasteur, the vaccine division of French drugmaker Sanofi-Aventis; and AIDSVAX, originally developed by VaxGen, and now held by Global Solutions for Infectious Diseases, a non-profit founded by some former VaxGen employees.