An experimental HIV vaccine is safe to use in South Africa, which paves the way for a larger trial, researchers say.

The investigational vaccine, known as RV 44, was shown to be 31 per cent effective in a trial in Thailand in 2009. Now preliminary evidence has looked at 100 healthy adults at three locations in South Africa.

HIV vaccine

A nurse holds vials of blood during tests of an AIDS vaccine in Bangkok. Immune responses among 100 South Africans were as good as what was seen Thailand five years ago. (Sukree Sukplang/Reuters)

"We showed that our immune responses were as good as what was seen Thailand and so this is a very important step," said Dr. Glenda Gray, the study’s co-principal investigator and president of the South African Medical Research Council.

The Thai results showed the vaccine waned over time. Now Gray's team will use a version modified to include the strain of HIV circulating in Africa. They’ll also add a booster shot after one year with the hopes of making it more potent and durable.

"The findings are encouraging as we move toward evaluating a modified and potentially improved version of the vaccine regimen in South Africa," Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, which sponsored the study, said in a release.

The next study will begin in January and will involve 200 people. If successful, a larger study on 7,000 people will follow within a year.

South Africa's regulator has indicated it would license the vaccine by 2019 if the larger trial can show even 50 per cent effectiveness.

With files from CBC's Pauline Dakin