PRETORIA -- Estimates of the number of people with the AIDS virus have been dramatically overstated in many countries because of errors in statistical models and a possible undetected decline in the pandemic, according to new data and specialists on the disease.
In many nations, analysts are cutting the estimates of HIV prevalence by half or more.
Rwanda, for instance, a new United Nations estimate due out next month will put HIV prevalence at about 5 percent, according to Rwandan officials, down from more than 11 percent four years ago. In Haiti, a recent unpublished study by the Centers for Disease Control and Prevention has found HIV prevalence was less than 3 percent, compared with the UN's most recent estimate of 6 percent. And the numbers in India are coming under increasing scrutiny because surveys in AIDS hot spots are indicating a prevalence rate that is much lower than the national average.
Even with lower estimates, health specialists agree that AIDS remains the most dangerous pandemic in the developing world. In particular, it threatens to ravage societies in southern Africa, and throughout the continent the disease has killed millions in the prime of their lives.
Several AIDS specialists said they think the current estimate of 40 million people living with the AIDS virus worldwide is inflated by 25 percent to 50 percent, based on a wide spectrum of household surveys in nearly a dozen countries. That would go against the grain of years of assertions by UNAIDS that the disease is relentlessly on the rise.
A significant downward revision in AIDS and HIV numbers calls into question many of the lessons on fighting AIDS that are based on prior estimates. It also is likely to affect future budgets and cause many countries to consider revising strategies on how to prevent and treat the disease.
"It is fundamental that we have accurate information of what we're up against," said Robert R. Redfield, cofounder of the University of Maryland's Institute of Human Virology and a leading AIDS specialist. "If you are overestimating the epidemic, you may attribute positive impacts to things that have nothing to do with it."
UN epidemiologists and statisticians responsible for the current estimates acknowledged in interviews that some country figures will be sharply cut in a forthcoming report, but they played down any possibility now of a sizable reduction in the overall number of infected people. They declined to disclose their most recent estimates for 2003, set to be released next month before the 15th International AIDS Conference in Bangkok.
The major error in the estimation of HIV and AIDS numbers has occurred primarily because epidemiologists relied too heavily on HIV rates in urban areas and failed to factor in much lower prevalence in rural areas, where surveys are rare, AIDS specialists said.
Suggestions that prevalence may be overstated may not be embraced by some who work to fight HIV or in countries hard hit by the epidemic. Billions of dollars in aid is at stake in the coming years. If HIV seems less menacing, more dollars may go to fighting other diseases or may be redirected away from global health initiatives entirely.
Already, earlier this year, US officials told Rwandan AIDS administrators that if HIV prevalence estimates were to drop to 5 percent, the country's AIDS funding may be cut, according to both US and Rwandan officials, speaking on condition of anonymity.
Another reason behind the plunging rates, some AIDS specialists say, is that the pandemic may be slowing.
The upcoming UNAIDS report is expected to show that HIV prevalence is declining in eastern Africa and leveling off or slightly slowing in West Africa, but still maintaining a high rate in southern Africa.
Statisticians traditionally have had a difficult time estimating the size of the pandemic. In 1986, Jim Chin, then a state epidemiologist in California who later developed models for the World Health Organization to calculate HIV prevalence, and several other US officials met in a West Virginia hotel room to figure out how many Americans had HIV.
Chin recollected that the group arrived at a range of 1 million to 1.5 million people; 18 years later, the number is at about 1 million Americans. "A lot of it was guesswork, based on limited studies," Chin said. "It was the best we could do."
The tools today are much more refined but still based on a long list of assumptions.
More than a decade ago, AIDS researchers in sub-Saharan Africa found that HIV tests on blood samples from pregnant women at prenatal clinics provided a good indicator of HIV prevalence among adults aged 15 to 49 in countries with high rates; early household surveys confirmed the finding.
But the surveys were limited at first to a few sites in countries. "We were talking about four or five urban sites and one or two rural sites, and extrapolating that to the whole country. You can see what potential inaccuracies there can be with this crude methodology," said Chin, who now is an independent AIDS analyst and criticizes UN estimates as overstated.
Other unknowns contribute to potential errors. One is estimating a country's population; the estimates for Nigeria, for instance, range from 120 million to 160 million people, but a census of the country has not been completed in more than half a century. Another is that most countries do not collect data on deaths.
Once officials from UNAIDS and the WHO arrive at an HIV prevalence estimate for a country, they use that estimate to help determine AIDS deaths, AIDS orphans, numbers of people needing antiretroviral treatment, and the average life expectancy for people in countries. As HIV prevalence numbers are adjusted downward, the numbers for the other AIDS-related categories also will be readjusted similarly.
To estimate the number of people dying from AIDS, for instance, epidemiologists assume that on average a person will live for eight or nine years after infection; they then plot the progression of a country's epidemic, determining how many people were infected in each year. If HIV prevalence estimates are significantly wrong in any year, estimates for AIDS deaths will be correspondingly wrong eight or nine years later.
Some specialists raised questions about the estimates in mid-January, after a report on a household survey in Kenya that estimated a 6.7 percent national HIV prevalence rate, compared with the UN's 15 percent estimate in 2002.
Two weeks later in Addis Ababa, Ethiopia, at a conference on estimating HIV and AIDS prevalence, epidemiologist Peter D. Ghys raised the question of potentially inflated AIDS rates before several dozen epidemiologists. Ghys estimates HIV and AIDS figures for UNAIDS. On the last slide of his presentation, Ghys wrote, "Should UNAIDS/ WHO estimates be lowered by 25 percent?"
Asked recently about his question, Ghys said he raised it in light of the new Kenyan data: "We already had lowered estimates quite a bit in the previous year, but since then, we've had additional information that came in somewhat lower than our estimates. Maybe it will change again, but it's difficult for us to say, yes, it should be 25 percent or something."
In 2003, the UN revised HIV prevalence in sub-Saharan Africa to 26.6 million people, from 29.4 million the year before.
Ties Boerma, who is in charge of the WHO's estimates on the HIV virus, said the UN was preparing to reduce prevalence estimates again this year, although not by 25 percent. Yet Chin and several other specialists say new data from several countries and from high-risk groups in countries suggest that the UN should do so.
Chin said he thinks the global rate is inflated by 25 percent to 40 percent, while two US health officials working on AIDS said they think the global numbers may be 50 percent inflated. The two spoke on condition of anonymity.
Among their reasons:
Earlier this year, the US government announced its first substantial grants in President Bush's multibillion-dollar plan to fight AIDS. The news release cited a 15 percent HIV prevalence rate in Kenya and a 6 percent rate in Haiti, even though US-funded surveys in both countries had recently concluded that the rate was at least half those figures.