Reports on HIV/AIDS: 1990

NOVEMBER 30, 1990, MMWR, Vol. 39, RR-16 [inclusive page numbers]
during early 1987 to 5-7 thousand homosexual or bisexual men with severe immunodeficiency, but without AIDS, could account for the change in the trend in AIDS
incidence among men in this category during the last half of 1987 (12). More than
10,000 persons received zidovudine from the Burroughs Wellcome Company under a
limited drug distribution system during March-September 1987. Data from a 4%
systematic sample of this group indicate that approximately 4,000 homosexual men
who were infected with HIV and had low T4 lymphocyte (CD4+ cell) counts, but who
had not yet developed AIDS, received zidovudine during that time (13). Although
these data suggest that medical therapy could have made a substantial contribution
to the change in trend in AIDS incidence among homosexual men during 1987, the
relative contribution of this and the other factors noted above can only be determined
after further study.
Incubation-time distribution
Backzcalculation estimates are known to be sensitive to the incubation-time
distribution used (11). The incubation-time distribution may have changed recently
as a result of improved medical care (delaying a diagnosis of AIDS and lengthening
the incubation time), the revision of the AIDS surveillance definition in 1987 (5)
(possibly resulting in the reporting of some patients at an earlier stage in their illness),
and increased outpatient diagnosis and treatment of persons with symptomatic HIV
infection (possibly resulting in the reporting of some patients later in their illness,
when they are hospitalized). Therefore, back-calculation may not provide accurate
estimates of HIV prevalence and future AIDS trends unless the model uses a
distribution for incubation time that incorporates the effect of these possible changes.
Because of the length of the incubation period-with few AIDS cases developing
within the first 2 years of infection-back-calculation also cannot detect recent
changes in HIV incidence. Estimates of HIV incidence during the last 2 or 3 years are
less likely to be accurate than estimates of HIV incidence from at least 4 years ago.
Results presented at this workshop confirmed that estimates of recent HIV incidence
obtained from back-calculation are subject to much uncertainty (Appendix B).
PREVALENCE AND INCIDENCE OF HIV INFECTION
Estimates of HIV prevalence
The estimated prevalence of HIV infection in the U.S. population is an important
measuire of the extent of the nation's HIV-related problem. The PHS has used a
working estimate of 1-1.5 million infections in the United States, but arguments have
been presented for both lower and higher estimates (1). Workshop participants
assessed the 1986 PHS estimate and evaluated the range of current estimates derived
from back-calculation and from direct estimation using data from HIV seroprevalence
surveys. On the basis of information provided at the workshop, CDC estimates that
approximately 1 million persons in the United States are currently infected with HIV.
Previous estimates. In 1986, a workshop of medical researchers, epidemiologists,
and statisticians in Coolfont, West Virginia, estimated that a total of 1-1.5 million HIV
infections existed in the United States (1). To arrive at this range, the workshop

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