There is new hope of finally defeating HIV - a vaccine to prevent the disease to be tested in South Africa in November.

South African scientists have been testing the newly modified vaccine for two years in this country: first to ensure its safety and then to see if it activated the immune system.

Yesterday, at the HIV Vaccine Trial Network conference in Washington DC, it was announced that the tests had been successful and a three-year trial involving 5700 people aged 18 to 35 will kick off in November. Results are expected by 2020.

The vaccine is so far advanced in testing that it is likely to be the first marketed. Glenda Gray, head of the Medical Research Council, who will lead the trial, said it was "great news". Current prevention regimens require strict daily adherence to treatment, which tends to lessen compliance .

HIV/Aids deaths among young people more than double in five years HIV/Aids deaths among young people have more than doubled in five years, but the health department says there is no cause for alarm.

"We need new tools or we won't have an Aids-free generation," Gray said.

The vaccine has been tested in Thailand and was shown to reduce the likelihood of contracting HIV by 60% in the first year, but its effectiveness fell to 31% after three years. The results were announced in 2009. Scientists involved in HIV research reportedly sobbed on hearing that 30 years of work on developing a vaccine had finally achieved some success.

The vaccine used in Thailand has been modified to fight the South African strain of the virus, and seven years has been devoted to strengthening its efficacy.

HIV researcher Linda Gail Bekker, who was involved in testing the efficacy of the vaccine in South Africa on 100 people, said the results suggested that the modified vaccine might be "more potent and longer lasting" than the vaccine tested in Thailand.

Anthony Fauci, director at the US National Institutes of Health, a co-funder of the trial, said the vaccine was "the product of years of study and experimentation".

"A safe and effective HIV vaccine could help bring about a durable end to the HIV/Aids pandemic and is particularly needed in Southern Africa, where HIV is more pervasive than anywhere else in the world."

The Medicines Control Council and scientists have discussed the level of protection that the vaccine must give if it is to be approved by regulators.

The vaccine is not expected to be 100% effective. Scientists want to use it in conjunction with male circumcision and other HIV-prevention methods, such as treating infected people, in an attempt to lower the rate of new infections. About 1000 South Africans are infected with HIV every day.

Mitchell Warren, director of NGO AVAC, said even a partially effective vaccine could significantly decrease the infection rate over decades.

Warren said that a vaccine that would reduce the risk of contracting HIV by 70% and lasted for five years "would reduce annual new infections by 44% over the first decade, by 65% in the first 25 years and by 78% to 122000 in 2070".

"We don't know if the vaccine being trialled in South Africa will be as effective as this but the modelling does confirm that a partially protective vaccine would have significant impact."

South Africa has about 6.4million people with HIV - the highest incidence in the world - and it has the world's biggest treatment programme with 3.2million people on antiretrovirals and there are plans to expand this by 300000 a year.

An extra R1-billion has been set aside to increase access to treatment.

Deputy director of Wits Reproductive Health and HIV Institute Francois Venter said: "Having something that doesn't have to be carried in your pocket or used during sex, and which is permanent, would be a huge benefit."