In fact, the US doctor, founder of the New York-based International Aids Vaccine Initiative (IAVI), says a vaccine is the only way to stop what he calls the "greatest plague since the 14th century."

"At the end of the day, prevention will not fully work to end the epidemic," Berkley, told AFP in an interview in New Delhi where he met officials to discuss the spread of AIDS in India which has 5.1 million HIV-infected people, the second most in the world after South Africa.

"We need a vaccine," said Berkley, a medical doctor specializing in infectious disease epidemiology, who launched IAVI in 1996.

The quest for a vaccine is particularly crucial in India where the World Bank has warned the disease could become the single largest cause of death in the nation of more than a billion people by 2033.

In India, human trials of a vaccine to focus on the sub-type C of the virus, the most common in the country, are set to begin next month at the National AIDS Research Institute in Pune, west of Bombay.

Berkley gets particularly riled by critics who say funds spent on researching a vaccine would be better spent on prevention.

In fact, he says it is now more urgent than ever to develop a vaccine because if "we don't get better tools to stop AIDS, the effects on society and on development will be just too horrible."

Now around five million people a year or about 14,000 daily get infected with HIV, the virus which causes AIDS, and three million die annually from the illness. Around 38 million worldwide are living with HIV, according to UNAIDS.

About 650 million dollars is spent annually by governments and corporations on AIDS vaccine research. Berkley would like to see that double to 1.1 billion dollars. But the sum is still small when set against the 70 billion dollars spent globally each year on total health product research and development.

"Behavioural strategies such as ABC -- Abstain, Be faithful, use Condoms -- have had an effect (but aren't) adequate to stop the spread" of AIDS globally, he said. "Look for example at all the monogamous women who have practised safe sex in developing countries and get infected by their partners."

An AIDS vaccine could protect women before the onset of infection, he said, noting in many nations where there is the greatest level of infection "women are generally not able to make decisions about their own reproductive health."

"In countries where heterosexual sex is the chief mode of HIV transmission, women's vulnerability to the virus often stems not from their own actions but from those of others," he said.

The search for a vaccine has been going on since the first case of AIDS was diagnosed in 1981. Researchers immediately started looking for a vaccine to immunise people against the illness as they had done for polio, smallpox and other diseases.

But the quest turned into what Berkley said was a "greater challenge than medical science imagined when pursuit of this goal began."

So what makes it so tough? The virus constantly mutates and nobody has devised yet a vaccine that will trigger the immune system to produce antibodies able to eliminate the AIDS virus before it infects the body, scientists say.

Besides the planned trials in India, others are underway against different virus strains in the US, Europe, Africa and South America.

Over 20 potential AIDS vaccines have been developed worldwide but the lone one to reach large-scale testing failed in clinical trials in 2003 in North America and Thailand. Scientists believe a vaccine is at least a decade away.

But even a partially effective vaccine could have a big effect in curbing HIV's spread, Berkley said, citing a study funded by the World Bank and European Commission suggesting a vaccine with 50 percent efficacy could cut new infections by up to 60 percent.

He said governments must boost contributions for developing a vaccine as drug companies and charitable groups cannot be expected to do all the research.

"Vaccines are not the most profitable types of products as it's not a drug people take daily so there's not such an incentive (for drug firms)," he said. "There's not the market drive" to "invest the large amount of capital needed."

Does he ever get discouraged? He says he cannot afford to given the need for "to protect those who are not yet (infected)."

"Science is extremely difficult. Without effort, well never break the problem. We've lots of promising information that suggests a vaccine is possible."

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