By Will Dunham

WASHINGTON, Mar 13 (Reuters) - The use of a weakened strain of live HIV--the virus that causes AIDS--in mass vaccination campaigns may actually increase death rates from the disease in many countries, researchers said on Monday, describing a medical "Catch-22" situation.

A research team headed by a University of California at Los Angeles scientist created a sophisticated mathematical model to predict for the first time the outcome of future vaccination efforts using an AIDS vaccine made from an attenuated--or weakened--form of live HIV.

To their surprise, the researchers said such a vaccine would increase death rates in countries where the AIDS epidemic was either low or moderate, while the same vaccine would reduce greatly the death rates in nations where AIDS was rampant.

"The exact same vaccines could be beneficial in one country, but detrimental in another," Dr. Sally Blower, professor of biomathematics and a member of the UCLA AIDS Institute, said in a telephone interview.

Blower said an attenuated vaccine might prove worthwhile only in developing countries facing perilous transmission rates, such as those in sub-Saharan Africa. "If you use such a vaccine, for example, in the United States, you would definitely make things much, much worse," she said.

Such a vaccine could be instrumental in fighting the disease because it generates a strong immunity, but it could also cause AIDS in some vaccinated people because of its use of the live virus.

Blower, the study's principal investigator, described a "Catch-22" situation for public health officials around the world if researchers fashioned an AIDS vaccine using a live virus.

"The vaccines have the potential to do a great deal of good, but they also have the potential to do harm. That's the essence of the Catch-22 problem," Blower said.

"You can develop very effective vaccines. But they may well be the ones that are the least safe. There may be a trade-off that people will have to consider between efficacy and safety once these vaccines have been developed."

The development of any such vaccine is still years away. Live attenuated vaccines have been used in the past to control such diseases as polio, smallpox and measles.

ZIMBABWE AND THAILAND STUDIED

The researchers matched their mathematical model to the current AIDS death rates in Zimbabwe--where one in four people in the population of 11.3 million is infected with HIV--and Thailand, where 2% of the population of 61 million people is infected.

The study, published in the Proceedings of the National Academy of Sciences, found that a live-virus vaccine would wipe out the naturally occurring strains of HIV in both countries within 50 years.

But researchers said that in Thailand, such a vaccine would increase the death rate if the vaccine caused more than 5% of the population to develop AIDS over 25 years. In Zimbabwe, researchers said their model predicted that more people would die of AIDS with no vaccination campaign than with a public vaccination effort using the live HIV vaccine.

Public health officials face a difficult ethical dilemma.

"Should you kill some people for the greater good of the rest of the people? That is a huge, huge ethical debate," Blower said. She added that if those vaccines were developed, public health officials would know that some people who were not infected would develop AIDS and die because of the vaccine. But she added that not using such a vaccine could mean "letting the entire continent of Africa be totally destroyed."

Blower said she and her colleagues were not advocating that an attenuated vaccine be used, but merely wanted to provide a theoretical framework for predicting the outcome of using 1,000 different potential versions of such a vaccine.

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