Scientists should first develop a so-called therapeutic vaccine to treat people already infected with HIV before moving on to a preventative one, the co-discoverer of the AIDS virus said on Friday.

Professor Luc Montagnier, president of the World Foundation for AIDS Research and Prevention, said there were immense technical obstacles in making and testing a prophylactic vaccine and a stepwise approach was the only realistic option.

Initially, researchers should test several formulations of therapeutic vaccines, whose effectiveness is easy to measure, and then derive a preventative one from the best formulation, he told a meeting of the European Medicines Agency.

“A therapeutic vaccine may take a few years,” he said.

“This is my prediction - we will have a therapeutic vaccine which will be a better substitute for antiretroviral therapy. This will be very important, especially for patients in the developing world because they won’t be able to afford (drug) treatment.”

An effective preventative vaccine is seen as the best long-term hope for ending the HIV/AIDS pandemic, which has infected around 40 million people, mainly in sub-Saharan Africa.

Some 30 different vaccines are being tested in small-scale trials around the world. But even the most advanced, assuming they work, are not expected to reach the market for many years, in part reflecting the difficulty of conducting clinical trials.

Microsoft Corp founder Bill Gates, a major funder of vaccine research, predicted earlier this month it would take more than a decade to develop an effective AIDS vaccine.

HIV is a notoriously difficult virus to fight because it constantly mutates and it attacks people’s immune systems, which means it does not trigger a normal immune response.