Estimate of AIDS Cases In U.S. Rises

Rumors have circulated for weeks in newsletters and blogs that CDC, the federal government's principal epidemiology agency, was preparing a dramatic upward revision of HIV incidence. The Washington Blade, a gay-oriented newspaper, reported rumors of the new estimates two weeks ago.

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The CDC has reported the figure of 40,000 new infections each year for more than a decade, citing it as evidence that the epidemic in this country is stable. But while widely quoted, that number has never been adequately explained or justified, in the eyes of many epidemiologists.

"There was skepticism about the validity of how that estimate was reached," said Rochelle Walensky, an infectious diseases physician and mathematical modeler at Harvard Medical School.

Some activists also were skeptical about it.

"It just doesn't seem plausible to me that it would be the same year after year," said Mark Harrington, executive director of Treatment Action Group, an AIDS activist think tank in New York.

Few doubt, however, that accurately counting new HIV infections is unusually difficult. About one-quarter of people infected with the virus do not know they are. The infection is largely "silent" for a decade in most people, and a substantial number go for testing only as they develop the symptoms of AIDS, the late stage of the illness.

Only recently has CDC put intense pressure on state and city health departments to report by name everyone who tests positive for HIV. Previously, health departments had to report only the people who had progressed to AIDS.

Counting only AIDS cases was an acceptable substitute for counting new infections in the era when AIDS treatment did not significantly prolong life. But with the arrival of combinations of potent antiretroviral drugs in 1995, AIDS patients began living years longer, making the estimates increasingly less accurate.

The new system in which health departments record individuals who have just tested positive for the first time will eventually provide a much clearer picture of the epidemic. However, some people oppose it, arguing that it will keep the potentially infected from coming in to be tested.

"There are so many barriers to testing and reporting," Harrington said. "We are grasping in the dark, as far as I am concerned, about the real size and shape of the epidemic."

The 19 states and cities that contributed the data for the new estimate include New York City, New York, New Jersey, Connecticut, Texas, Florida, and several Southern and Midwestern states.

The new method of estimating HIV incidence makes use of the observation that a person who is recently infected with HIV and whose immune system has just begun to make antibodies against the virus shows a weaker reaction in the standard AIDS blood test than those whose immune systems have been making antibodies for years.

By altering the test-tube conditions, scientists can identify those who react weakly -- and with them, the percentage of a batch of HIV tests that come from people newly infected.

The method is called the STAHRS method, for serological testing algorithm for recent HIV seroconversion.