HIV spread most by those with moderate virus level

LONDON, Oct 23 (Reuters) People with moderate levels of HIV in their blood are the most likely to infect others, researchers said in a study that provides a better understanding of how the deadly virus spreads.

Looking at several groups of HIV-positive people in Europe, the United States and Africa, the researchers found that people with high levels of the AIDS virus were the most infectious but had only limited time to pass it on to others.

Those with moderate virus levels -- using a measure called viral load -- had plenty of virus circulating in their blood as well as the opportunity to infect others, the researchers said yesterday.

This means that trying to halt the spread of AIDS by focusing on people with the highest viral loads may not be the most effective approach, said Christophe Fraser, an epidemiologist at Imperial College London who led the study.

''The surprise was that those people with high viral loads actually infected fewer people because they progressed to AIDS more quickly,'' Fraser said in a telephone interview.

The findings may also suggest that the virus has somehow adapted to find the optimal balance between infectiousness and virulence in order to maximise its chances of spreading -- something one would expect from evolution, the researchers wrote in the Proceedings of the National Academy of Sciences.

''We now want to see whether the virus has adapted in order to allow it to infect the most people, which seems plausible given the results of our study,'' Fraser said.

''While it is too early to sound the alarm, more research to prove or disprove this theory is urgently needed.'' More than 25 million people have died of AIDS since the incurable disease that ravages the body's immune system was first recognized in 1981. Almost 40 million people now live with HIV, the virus that causes AIDS, mostly in sub-Saharan Africa.

In their study, the researchers analysed previously published European and African studies linking viral load, infectiousness and survival.

The researchers focused on people with medium viral loads as individuals may show no symptoms and not progress to full-blown AIDS for about seven to eight years. Those with high loads were typically people who developed AIDS in a short period of time, usually two to three years, Fraser said.

People with a medium viral load also form the largest, most common group not to receive treatment, so these individuals are far more dangerous when it comes to contributing to the AIDS epidemic, the researchers said.

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