The annual number of new patients diagnosed with AIDS will level off over the next five years after a decade of steady increase, predicts a Johns Hopkins scientist who developed a new method of forecasting the course of the epidemic.
Dr. Ronald Brookmeyer, a professor of biostatistics, said two trends with roots in the mid-1980s will make this possible: People are spreading the virus at a slower pace than before, and drugs are delaying onset of the full-blown illness in people who are infected.
All this should bring about a slower growth in AIDS cases, upsetting earlier predictions by medical experts that doctors would continue to diagnose a larger number of AIDS cases each year through the middle of the decade.
But there's bad news, too.
"The important message is that although the infection rate has declined, there are still a lot of people who were infected through the mid-1980s, and now they are moving to more advanced stages of disease," Dr. Brookmeyer said in an interview.
"The future demands on the health care system remain enormous. We still have to treat large numbers of individuals," said Dr. Brookmeyer, who unveiled his projections in an article appearing in today's issue of the journal Science.
He also cautioned that his projections were based on infection rates and other data currently available and that it was possible that the epidemic would enter "a second wave" in years to come if new trends surface.
"There's always the possibility of a second wave, particularly among a new generation of adolescents. And particularly if we become lax in AIDS education and prevention programs," said Dr. Brookmeyer, a member of the university's School of Hygiene and Public Health.
"We still have concerns and need to be worried about future infection rates and whether they start up again," he said.
All told, about 180,000 Americans have been diagnosed with acquired immune deficiency syndrome since the epidemic surfaced in 1981.
The number of people diagnosed each year grew dramatically during the past decade: 330 new cases were reported in 1981, 11,000 in 1985, 42,000 in 1989 and about 50,000 in 1990.
"The plateau will be between 60,000 and 67,000 per year over the next five years," Dr. Brookmeyer said, adding that he couldn't pinpoint when the plateau would be reached or when the yearly total would begin to decline.
Unforeseen trends aside, here is his evidence for a plateau in new cases:
* The human immunodeficiency virus, which causes AIDS, spread most rapidly in the mid-1980s, when an estimated 160,000 people each year were infected. By 1989, the rate
probably declined to between 40,000 and 90,000 new infections each year.
* New infections among homosexual and bisexual men appear to have peaked in 1984, declining dramatically in subsequent years. Meanwhile, new infections among intravenous drug users, the second-largest risk group, peaked in 1986 and have tailed off since then.
* New infections among heterosexuals have remained constant for several years but haven't grown dramatically, as many experts had feared they would.
Dr. Brookmeyer based his projections on a statistical technique called "back calculation" that involves reconstructing the AIDS epidemic using what information is now known about it. He took into consideration yearly AIDS diagnoses, the infection rate and the time it takes for an infected person to develop the full-blown illness.
AIDS has an exceptionally long incubation period. Scientists now believe that half the people who contract the virus take 10 years to develop symptoms of AIDS; the other half take longer, but the epidemic is still too young to show the outer limits of lag.
Two new drugs, AZT and aerosol pentamidine, have contributed greatly to the "plateau." They have enabled many infected people to delay the onset of AIDS.