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HIV Prevalence Estimates and AIDS Case Projections for the United States : Report Based upon a Workshop -- Appendix B

HIV-Prevalence Estimates and AIDS Case Projections: Results

Estimates of HIV prevalence obtained from back-calculation

Analysts at the workshop estimated HIV cumulative incidence as of January 1986, July 1987, and early 1989 (Table B1) from reported AIDS cases adjusted for reporting delays. HIV prevalence was estimated by subtracting estimated numbers of deaths associated with reported AIDS cases and adjusting for incomplete ascertainment of life-threatening symptomatic HIV infection (Appendix A). The resulting ranges of estimates are contained in the footnotes to Table 1.

The estimate of HIV cumulative incidence for early 1986 depends partly on the series of AIDS cases modeled. Hay estimated approximately 500,000 infections from modeling all cases diagnosed through December 1989. Hyman also estimated 500,000 infections but modeled cases diagnosed through June 1989. Their estimates were substantially less than Brookmeyer's and Rosenberg's estimates of approximately 650,000 infections, estimates obtained from modeling cases diagnosed through June 1987 that are consistent with the pre-1987 definition. Hay's and Hyman's estimates are lower partly because they used AIDS cases diagnosed after mid-1987 and assumed that the distribution of the incubation period did not change with time. With this assumption, back-calculation accommodates the change in the trend in AIDS incidence in 1987 (Figure 1) by estimating that infections earlier in the epidemic, rather than recent infections, tend to be responsible for the AIDS cases diagnosed during June 1987-June 1989. After subtracting 12,000 deaths among diagnosed AIDS cases and adjusting for the estimate that reported AIDS cases represent approximately 75% of all persons with life-threatening symptomatic HIV infection (Appendix A, Section 4), the range of 500,000-650,000 cumulative infections yields an estimated HIV prevalence of 650,000-900,000 for early 1986.

There is greater variation in the estimates of HIV cumulative incidence for early 1989 than in the comparable estimates for early 1986. As shown by Brookmeyer's results, much of this variation is due to differences in the incubation period distributions used; back-calculation estimates are known to be sensitive to the form of this distribution (B1,B2). Hay's estimate is low partly because it is based on smaller adjustments to observed AIDS incidence for reporting delays. His estimate would be approximately 650,000 cumulative infections if it were based on the same adjusted AIDS incidence as the other estimates (Appendix A). After adjustments for deaths and incomplete ascertainment of life-threatening symptomatic HIV infection, this estimated range of 530,000-1,110,000 cumulative infections in early 1989 yields an estimated HIV prevalence of 630,000-1,380,000 in early 1989. If Hay's estimate of 530,000 infections were replaced by an estimate of 650,000, the lower bound for estimated HIV prevalence obtained from back-calculation would be 780,000.

These analyses demonstrate that results from back-calculation must be evaluated carefully. Analyses based on AIDS incidence for different periods gave markedly different estimates of cumulative HIV infections, depending on the time period modeled and the incubation period distribution used. Results also varied among analysts who used the same time period and similar incubation period distributions.

AIDS case projections: all cases

The number of AIDS cases diagnosed in the United States and reported to CDC was higher in 1989 than in 1988 (B3). These numbers are expected to continue to increase through 1993 (Table B2), with 52,000-83,000 cases diagnosed during 1993, before adjustments for underreporting. Projections adjusted for underreporting appear in Table B3. The lower bounds for these projections are from Hyman's and Hay's analyses, which gave nearly identical results. The upper bounds are from Gail's analyses, assuming 150,000 new infections per year.

The new projections are lower than the corresponding PHS projections made in 1988. For each year in the period 1990-1992, the midpoint of the plausible range is approximately 15% lower than the projection made in 1988. For example, the midpoint (71,500 cases) of the range for 1992 is 17% less than the 1988 prediction that 86,000 AIDS cases diagnosed in 1992 would be reported (Table 4; note that the original 1988 projections have been adjusted to agree with the current estimate that 85% of all diagnosed cases are reported). These decreases in the projections reflect the slowing of AIDS incidence since early 1987 (Figure 1).

Predictions after 1990 are especially uncertain because there is little information on the duration of the effect of therapeutic interventions (e.g., the use of zidovudine and P. carinii pneumonia prophylactic therapy by symptomatic persons before AIDS is diagnosed, the time(s) at which these treatments began to be used extensively, or the number of persons treated). In particular, the range of predictions includes the roughly linear trend in cases seen during the last 2 years (Figure 1), but that linear trend is in the lower half of the range for predicted annual incidence. This comparison suggests that the change in the trend of AIDS cases during 1987 might be temporary, with AIDS incidence increasing more rapidly within the next several years than it has during the last 2 years.

Survival of persons with AIDS

The number of deaths from AIDS is projected to increase through 1993 (Tables B2, B3). As of September 1990, 23,500 persons with AIDS were known to have died in 1989, compared with 19,000 known to have died in 1988 and 15,100 known to have died during 1987. On the basis of delays in reporting deaths during September 1987-September 1990, the reported number of deaths during 1989 is likely to reach at least 27,000. The number of deaths reported for persons with previously reported AIDS is projected to increase to 45,000-64,000 in 1993 (53,000-76,000 after adjustment for underreporting). The number of persons who have AIDS and are still living is also projected to increase through 1993 (Tables B2, B3).

National mortality data suggest that there were approximately 33% more deaths related to life-threatening symptomatic HIV infection than deaths among persons with reported AIDS in 1987 (Appendix A, Section 4). Applying this factor to predicted deaths reported for persons with AIDS yields estimates that deaths attributable to HIV infection will increase from 35,000-38,000 in 1989 to 60,000-86,000 deaths in 1993 (Table B3).

These mortality and prevalence projections include an allowance for improved survival among AIDS patients taking zidovudine (or using other therapy), but assume that therapy continues to reduce the risk of death indefinitely. If the effect of therapy is only temporary, these projections are likely to underestimate the number of deaths--especially for 1990-1993--and consequently to overestimate the numbers of living AIDS patients during those years.

AIDS case projections: subgroups

AIDS incidence has increased steadily in most groups exhibiting risk behavior during the past 2 years. Incidence has increased almost linearly among homosexual and bisexual men (among both IVDUs and non-IVDUs), heterosexual IVDUs, non-IVDU heterosexuals, and among children. AIDS incidence is projected to continue to increase through 1993 among each of these groups (Table 3). Incidence has remained roughly constant during the last 2 years among adult transfusion recipients, adults with hemophilia, and persons without known risk behavior who were born in countries with predominantly heterosexual transmission.

AIDS incidence is also projected to continue to increase through 1993 among each of the principal racial/ethnic groups, as well as among both men and women (Table B4). The proportions of AIDS cases among blacks, Hispanics, and women have increased since the first half of 1987: among blacks, from approximately 25% in early 1987 to 30% in 1989; among Hispanics, from 15% in early 1987 to 16%-17% in 1989; and among women, from 9% in early 1987 to 11% in 1989. A major reason for these changes is the increase in the number of AIDS cases among IVDUs reported after the 1987 revision of the AIDS case definition (B4).


B1. CDC. Human immunodeficiency virus in the United States: a review of current knowledge. MMWR 1987;36(Suppl S-6). B2. Gail MH, Brookmeyer R. Methods for projecting course of acquired immunodeficiency syndrome epidemic. J Natl Cancer Inst 1988;80:900-11. B3. CDC. Update: acquired immunodeficiency syndrome--United States, 1989. MMWR 1990;39:81-6. B4. Selik RM. Current epidemiologic trends of AIDS in the United States in the context of the 1987 revision of the case definition. In: P Volberding and MA Jacobson, eds. AIDS clinical review 1990. New York: Marcel Dekker, 1990:11-24.

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