The experts have high hopes for a treatment that will be given at an early stage of infection -- most likely a cocktail that includes an immunity booster and a virus killer.
But they said people with a long-running, untreated infection and a compromised immune system may never benefit from an envisioned "functional cure" -- which means a person retains traces of the virus but no symptoms.
"We have had some very interesting little lights at the end of the tunnel in individual studies," Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, said on the sidelines of a Paris conference to mark the 30th anniversary of the discovery of HIV, the virus that causes AIDS.
Proof of vaccine feasibility lay with a Thai study dubbed RV144, which in 2009 demonstrated protection for 31 per cent of some 16,000 people given an experimental vaccine, said Fauci.
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"I think we will likely have a (vaccine that works at) better than 31 per cent, but there's certainly the possibility that we won't have a 90 per cent," Fauci told reporters.
"And I think there is even a greater possibility that we won't have a pristine cure that would essentially cure everybody who is HIV infected.
"I think it's not only possible that that won't happen -- I think it is likely that that won't happen."
Antiretroviral drugs slow down virus reproduction, allowing people to live symptom-free lives and slowing transmission to others, but much of the virus hides away in "reservoir" cells only to reemerge and start spreading again once treatment stops.
A team of scientists at Monash University in Melbourne, Australia, is experimenting with an anti-cancer drug to flush the virus out of its hiding place, then to be killed.
"Ultimately, we want a cure that is available to a large number of people," a member of the team, Sharon Lewin from Monash University, told AFP at the conference.