The state health department has dramatically reduced its estimate of how many Marylanders are infected with the AIDS virus -- suggesting that the true number is one-quarter to one-half previous estimates.
Cautioning that the lower estimates should not lull people into complacency, top health officials said yesterday they believe that between 16,000 and 28,000 people across Maryland were infected at the close of 1990.
That compares with a previous estimate of 60,000, a projection that officials said was based, in part, on an outmoded formula and the belief that infected individuals were transmitting the virus at an unrealistically fast rate.
Dr. Audrey Rogers, the department's chief epidemiologist, said she and other officials were making the new estimates public to give people a more realistic picture of the epidemic -- and not to play down its severity.
She also cautioned that estimating the epidemic's size is "an inexact science" and that no one can place absolute faith on any number.
Over a year ago, the federal Centers for Disease Control brought down its own estimate of how many Americans carried the virus -- saying 1 million was probably more accurate than the previous estimate of 1.5 million. The new estimates should not obscure other studies showing that about half of all infected people unknowningly carry the virus -- and therefore may not be taking proper precautions against transmitting the virus to others, said Dr. Joseph Horman, assistant director for prevention for the state AIDS Administration.
The previous estimate that about 60,000 Marylanders were infected with the AIDS virus was based on a now-outdated suggestion by the federal Centers for Disease Control that 25 to 30 people carried the virus for every person who suffered from full-blown acquired immune deficiency syndrome, Dr. Rogers said.
But Maryland trends, she said, show that the ratio is more in the neighborhood of 9-to-1 -- that is, nine infections for every diagnosed AIDS case.
Using four different statistical methods, the state AIDS Administration came up with four different estimates of how many people are infected. But Dr. Rogers noted that they all clustered in a range of 16,000 to 28,000 -- and that not one came close to the old figure of 60,000.
The department's most conservative estimate was produced by Dr. Ivan Kramer, assistant professor of physics at the University of Maryland Baltimore County.
He devised a new formula that took several trends into account -- such as the number of known AIDS cases, the time it takes for infection to progress to AIDS and the rate at which infected individuals were exposing others.
He said scientists now estimate that it takes an average of 11 years for infection to progress to full-blown disease -- and not the six or seven years that was widely believed a few years ago. Also, he said it appears that infected individuals are exposing others through risky sex and drug use at a slower rate than once believed.
If Dr. Kramer's estimate is correct, it would mean that three of every 1,000 Marylanders are infected with the AIDS virus.
The highest estimate -- 28,000 -- was based on surveys of several different populations, including childbearing mothers, military recruits and blood donors. If true, it would suggest that almost six of every 1,000 Marylanders are infected.
Whatever the true number, AIDS Administration Director Kathleen Edwards said the nation still faces a daunting epidemic of a disease that has no cure, no vaccine and people who are putting themselves and their children at risk through unprotected sex and drug use.
Dr. Richard Chaisson, director of the AIDS service at Johns Hopkins Hospital, said the most conservative estimate sounded low, based on the high rates of infection among certain high-risk populations. Surveys, for instance, show that one-quarter of Baltimore's estimated 30,000 injecting drug addicts are infected, as well as 35 percent of the gay men living in the Baltimore-Washington corridor.