By Michael Kahn

LONDON, Aug 27 (Reuters) - HIV has slashed life expectancy in Zimbabwe by up to 19 years for men and 22 years for women but births still outpace deaths, according to the first study to detail how the AIDS epidemic has impacted the country's wider population.

The study, led by Simon Gregson of Imperial College London, sought to gauge HIV's impact on Zimbabwe to see if researchers got it right in 1989, at the beginning of the epidemic, when they made initial predictions about how AIDS would affect populations in sub-Saharan Africa.

The most dire estimates by a group of epidemiologists at the World Health Organisation forecast population growth rates would turn negative, which the study found did not occur in Zimbabwe.

Even though the deadly virus has reduced population growth in the worst-affected areas of Zimbabwe by two thirds, the overall rate remains at about 1 percent a year, Gregson said in a telephone interview.

"The simple message is we can't pretend that it hasn't had a devastating effect on countries like Zimbabwe but in terms of demographic impact it has not had as much of an impact as some of the most pessimistic estimates," Gregson said.

"Our research shows that, in spite of countless people having lost their lives to the virus, more people are still being born than are dying," Gregson said.

But Gregson's team wrote in the Proceedings of the National Academy of Sciences the effects of HIV/AIDS in Zimbabwe were "substantial and still unfolding".

Life expectancy in rural areas in Zimbabwe has been cut by 19 years for males and 22 years for females, Gregson and colleagues wrote in their report.

More than 25 million people have died of AIDS since the incurable disease, which ravages the immune system, was first recognized in 1981.

Nearly 40 million people are now infected with the HIV virus that causes AIDS, most of them in sub-Saharan Africa.

"The prevalence of HIV has been coming down in the last few years and, as more people receive treatment, we hope the death rate will also soon start to go down," Gregson said.

The study, which focused on an area in eastern Zimbabwe over a period from 1998 to 2005, was likely representative of other places in Africa hard-hit by HIV, the researchers said.

The results were also in line with similar research looking at population growth in Uganda, Gregson added.

The study found that the most serve 1989 predictions missed the mark because researchers at the time did not realize behaviour contributing to the spread of the virus differed within populations and that things like transmission rates varied during the stages of infection, Gregson said.

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