A. Estimating the Spread of HIV/AIDS among MSM.
"Epidemiologists estimate that 30 percent of all 20-year-old homosexually-active men will be HIV positive or dead of AIDS by the time they are 30."
E. Goldman, “Psychological Factors Generate HIV Resurgence in Young Gay Men,” Clinical Psychiatry News, Oct. 1994.
Statistical models are tools.
Diagnosed infections are counted and sorted by relevant criteria. Incidence rates and prevalence ratios and baseline assumptions are fed into the process whereby epidemiologists assess the available evidence and offer recommendations to help society respond to an outbreak of disease. Projections help form the basis for contingencies at multiple levels -- from the individual person, to the local community, to the nation and, in the HIV/AIDS pandemic, right through to the international level.
The CDC's data is the basis for the 30% by 30 forecast. In 1994, epidemiologists at Columbia University reported that the CDC's data led them to forecast that
"lifetime individual risks of seroconversion of over 50 percent, with the highest annual hazard (4.4 percent per year) occurring before age 25 years."In 1994, W. Odets, a clinical psychiatrist in California, presented a paper to the international science community on the epidemiology of HIV/AIDS.
Odets said:
"Some simple epidemiology helps answer these questions: To date, more San Franciscans have died of AIDS - 90% of them gay men - than all the San Franciscans dead from the four wars of the 20th century, combined and quadrupled. Nationally, 30 percent of 20 year- old gay men will be infected with HIV or dead of AIDS by age 30. A majority of gay 20 year-olds will contract HIV during their lifetimes. In San Francisco, our current rates of seroconversion will much more than maintain a 50% prevalence of HIV infection indefinitely."Odets is the source for the article that Goldman wrote and which is quoted at the top of this blogpost.
He was not alone in making estimates in that range.
By 2001, researchers at the CDC reported that the HIV prevalence (i.e. those living with infection or with AIDS) in the 15-22 age group
"rose steadily with increasing age, to nearly 10% among 22-year-olds. [...] The emerging data presented here suggest that we may be headed toward a resurgence in HIV infections among MSM".In other words, the prevalence and incidence rates pointed toward fulfilling the 1994 projection quoted at the top of this blogpost.
B. Reviewing the history of the projections.
Since the outbreak of HIV/AIDS, the CDC has gathered statistics from across the country. In 2005, the CDC revised and refined its analysis of the available evidence. It released more accurate incidence rates and prevalence ratios.
By that time the indications were that the 30% by age 30 projection had been fulfilled, tragically, in 1995-2005.
In 2005, a leading HIV/AIDS researcher at the CDC, R. Stall, presented his findings at the International AIDS Conference.
He reviewed the historical data and the current trends. Stall calculated the lifelong risk for twenty-year-old homosexually-active men (i.e. men who have sex with men, aka MSM) of becoming infected with HIV. By age 30 -- some ten years hence -- about 25% would be infected; 40% by age 40; and 50% by age 50.
C. What the future may hold.
In 2008, at the Conference on Retroviruses and Opportunistic Infections, R. Stall presented more research and a new projection for coming decades.
Taking as his starting point the current incidence of HIV infection among 15-22 year-old men who have sex with men (MSM), professor Stall ran a mathematic model to see how this would translate into HIV prevalence as the 18-year-old group got older.
In a press conference he explained:
"The model that we constructed yielded an estimate that at about age 25, about 15% of the men would be HIV positive; by age 35, about a third; and by age 40, about 41%."He continued:
"We were kind of horrified that our model yielded prevalence estimates that high.
[...]
What we find is that the model actually fits exactly what's going on in terms of HIV prevalence among gay men, at least in America's largest urban centers. This model that we are extrapolating based on the incidence rates, which culminates in an HIV prevalence rate of 40% at age 40, is not a prediction of something that may happen one day. We are describing epidemiological phenomena that are occurring all around us, and will continue to occur among young American men, if we do not find ways to lower HIV incidence rates further."
The rest of this series of blogposts expands on each section (A to C) of this overview. References are included.
Return to Table of Contents.
