Dr David Holtgrave, from Emory University's Center for Aids research, said that without the investment in HIV prevention the number people infected would now number the population size of a moderate or large US city.

He estimated that the better public awareness of the disease and health promotions had saved the US about �11 billion in medical costs.

Dr Holtgrave said that although it was impossible to chart the exact number of people saved from the disease, that he could plot a statistical model.

Saved

He said this showed roughly how many lives had been saved and the cost of savings to society.

"It is impossible to count exactly how many HIV infections would have occurred if efforts to prevent the spread of the virus had never been put in place.

"However, we can do the next best thing and explore through statistical modelling the course the US epidemic might have followed without HIV prevention programmes.

"Although limited by multiple sources of uncertainty, the findings indicate a strong likelihood that HIV prevention efforts to date in the United States have literally saved hundreds of thousands of Americans, and have resulted in significant cost savings to society.

"Another way of stating these results, is that each of the 204,000 to 1.5 million HIV infections that were prevented saved society $56,000 to $195,000 in averted medical costs - or a total of at least $11 billion saved.

Patterns

"These analyses do not include other real benefits of prevented HIV infections such as increased worker productivity and decreased pain and suffering."

Dr Holtgrave chartered the yearly patterns of new HIV cases and then calculated what the incidence would have been without the prevention.

The findings indicate a strong likelihood that HIV prevention efforts to date in the United States have literally saved hundreds of thousands of Americans

Dr David Holtgrave

If the number of infections prevented were close to the top of the range then it is estimated that the cost per infection prevented would be around $6,400, which is a small fraction of the cost of treating a person with Aids.

The research was published in AIDS: The Official Journal of the International Aids Society.