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Current Trends Estimates of HIV Prevalence and Projected AIDS Cases: Summary of a Workshop, October 31-November 1, 1989
Currently about 1 million persons in the United States are infected with human immunodeficiency virus (HIV). The number of cases of acquired immunodeficiency syndrome (AIDS) will continue to increase over the next 4 years, with a projected 52,000-57,000 cases to be diagnosed in 1990. These estimates are based on AIDS case surveillance data, HIV seroprevalence data, and information provided by epidemiologists, statisticians, and mathematical modelers who attended a workshop on October 31-November 1, 1989, in Atlanta. More than 70 specialists from federal agencies, state and local health departments, academic centers, and voluntary organizations met to evaluate methods and data concerning HIV prevalence and incidence, AIDS case projections, the spectrum of HIV-related immunologic deficiency, and the impact of therapeutic interventions on AIDS incidence.* A summary of the conclusions from the workshop, together with current estimates of HIV prevalence and AIDS case projections, are summarized below. Prevalence and Incidence of HIV Infection
HIV Prevalence. Workshop participants assessed the 1986 Public Health Service (PHS) estimate of 1 million to 1.5 million HIV infections (1) and evaluated the range of current estimates derived from statistical models and from direct estimation based on HIV seroprevalence survey data. Based on analyses presented at the workshop, it is estimated that about 750,000 persons in the United States were infected with HIV at the beginning of 1986 (Table 1). This estimate is lower than the 1986 estimate, which was based on the more limited data available at that time. In 1989, an estimated 1 million living persons in the United States were infected with HIV (Table 1). Estimates of current HIV prevalence derived from statistical models ranged from 650,000 to 1.4 million, after adjustments for previous deaths, underreporting of AIDS cases, and nonascertainment of HIV disease outside the AIDS surveillance definition.*** Preliminary HIV seroprevalence survey data provided estimates most consistent with between 800,000 and 1.2 million HIV infections (3). Although based on independent data sources and subject to different biases, both methods provide estimates that overlap and center around the 1 million estimate.
Discussions at the workshop highlighted the importance of estimates obtained using back-calculation, a statistical method that estimates the number of prior HIV infections that would account for the AIDS cases that have subsequently occurred (4,5). Difficulties in the use of this method were also discussed. Current HIV prevalence estimates derived from back-calculation depend on the interpretation of the slowing in the rate of increase in reported AIDS cases that occurred in mid-1987, particularly among homosexual/bisexual men who were not users of intravenous (IV) drugs (6). Variations in the methods, assumptions, and data used by different statisticians make direct comparisons difficult and led to the wide range (650,000 to 1.4 million) in current HIV prevalence estimates derived from back-calculation.
HIV Incidence. The incidence of new HIV infections in the U.S. population is an indicator of the growth of the epidemic at a given time. Incidence can be either observed directly in groups that are repeatedly screened for HIV infection or estimated from serial prevalence measurements. Incidence estimates derived from HIV serosurveys based on blood specimens from newborn infants indicate that 1500-2000 HIV-infected infants (0.5 per 1000 births) were born in 1989 (Table 1). According to data from the U.S. Department of Defense, approximately 0.6-0.8 per 1000 active-duty personnel acquired HIV infection each year since 1986 (7,8). Extrapolation from the lower estimate (0.6 per 1000) suggests that at least 40,000 new HIV infections occurred in adults and adolescents in the United States during 1989, assuming that the risk of new infection is at least as high for young adult civilians as for military personnel (Table 1). This is a plausible assumption because the military actively discourages homosexual/bisexual men and IV-drug users (IVDUs) from applying for service and has policies against homosexual and drug-using behavior among military personnel. Spectrum of Immunologic Deficiency in HIV-Infected Persons
Assessments of immune status in a population infected with HIV help quantify morbidity, estimate the future burden of HIV disease, and estimate the potential need for antiretroviral and other therapies. Because the primary target of HIV is the T-helper lymphocyte (CD4+ cell), monitoring the CD4+ cell counts of persons with HIV infection provides a measure of HIV-related immune dysfunction. Workshop participants reviewed data from immunologic studies in active-duty military personnel with HIV infection (9; National Naval Medical Center, unpublished data) and in cohorts of homosexual/bisexual men (10,11). These studies suggest that by 1989 approximately 17%-19% of HIV-infected persons evaluated between 1985 and 1989 had less than 200 CD4+ cells/mm3. An additional 41%-45% had between 200 and 500 CD4+ cells/mm3. Thus, 58%-64% of persons with HIV infection may have CD4+ cell counts of less than 500/mm3. AIDS Case Projections
Participants concluded that AIDS cases in the United States will continue to increase through 1993 in each of the current principal transmission categories (i.e., homosexual/bisexual men, IVDUs, persons infected through heterosexual transmission, and children infected perinatally). An estimated 37,500 cases diagnosed from October 1988 through September 1989 eventually will be reported, a 14% increase over the corresponding count for October 1987 through September 1988 (6). Between 52,000 and 57,000 cases of AIDS will be diagnosed during 1990, and the annual count will increase to 61,000-98,000 cases diagnosed during 1993 (Table 2). These projections include an adjustment for the estimate that about 85% of diagnosed AIDS cases are eventually reported. Effects of Therapy on Disease Progression
Data presented at the workshop indicate that the use of zidovudine (formerly called AZT) initially reduces the risk for developing AIDS in HIV-infected persons who are asymptomatic or mildly symptomatic but who have CD4+ cell counts of less than 500/mm3. Current data indicate that, in a clinical trial setting, the risk in treated patients is one third to one half the risk in untreated patients (National Institute of Allergy and Infectious Diseases (NIAID), unpublished data). Although the use of zidovudine only temporarily delays onset of AIDS, the therapeutic benefit may be extended by new therapies currently being evaluated. Data available at the workshop were insufficient to estimate the relative contribution of therapeutic interventions, such as zidovudine or prophylaxis for Pneumocystis carinii pneumonia, to the slowing in the rate of increase in reported AIDS cases that occurred in the middle of 1987. Reported by: Div of HIV/AIDS, Center for Infectious Diseases, CDC.
Editorial Note: Estimates of the number of HIV-infected persons, the number with laboratory evidence of immune dysfunction, and the projected number of persons with AIDS are used to assess current and future health-care needs. Although these estimates cannot be made precisely, ongoing studies will provide additional data to improve the estimates and test the assumptions on which they are based.
Current HIV prevalence estimates and AIDS case projections are influenced by the slowing of the rapid upward trend in AIDS incidence that occurred in 1987. The number of AIDS cases diagnosed per month continued to increase in 1987, but the rate of increase declined in the middle of that year, particularly in non-IV-drug-using homosexual/bisexual men (6). Reasons for this change in trend include: 1) a decline in the incidence of new HIV infections in homosexual/bisexual men in the early 1980s, leading to a subsequent decline in AIDS case incidence (12); 2) use of antiretroviral and other therapies by mid-1987, leading to a lengthening of the incubation period from infection to AIDS; and 3) possible decreases in the completeness or timeliness of reporting. The accuracy of HIV prevalence estimates and AIDS case projections depends in part on the determination of the relative contribution of these or other factors.
After the workshop, additional data became available on zidovudine use in mid-1987, and estimates were made of the possible effect of medical therapy on the change in trend in AIDS incidence that occurred in that year. One study estimated that zidovudine treatment given during early 1987 to 5000-7000 homosexual/bisexual men with severe immunodeficiency but without AIDS could account for the change in the trend in AIDS incidence in that group in the last half of 1987 (13). More than 10,000 persons received zidovudine from the manufacturer under a limited drug distribution system during March-September 1987. Data from a 4% systematic sample of this group indicate that about 4000 homosexual/bisexual men who were infected with HIV and had low CD4+ counts but who had not yet developed AIDS received zidovudine during that time (14). While this suggests that medical therapy could have made a substantial contribution to the change in trend in AIDS incidence in this group since 1987, the relative contribution of this and the other factors noted above requires further study.
Despite the apparent change in reported AIDS incidence in 1987, needs for current and future health-care services are expected to increase. AIDS has been diagnosed in no more than 10% of the approximately 1 million persons currently infected with HIV. Recent studies indicate that early treatment with zidovudine can slow disease progression in asymptomatic persons with CD4+ counts less than 500/mm3 (NIAID, unpublished data). As discussed in the report, about 60% of the estimated 1 million HIV-infected persons in the United States--including about 500,000 persons without AIDS--may have CD4+ counts less than 500/mm3 and may benefit from such therapy.
In addition to the suffering and health-care burden involving those already infected, a major concern is the number of new infections that continue to occur. Currently an estimated 1500-2000 new infections occur each year in newborns as a result of perinatal transmission, and a minimum of 40,000 new infections occur each year in adults and adolescents. Comparing the estimate of about 750,000 HIV-infected persons alive at the beginning of 1986 with the current estimate of about 1 million alive in mid-1989 suggests that an average of more than 80,000 new infections have occurred yearly since 1986.
These incidence estimates must be refined to measure the growth of the epidemic and the effectiveness of current and future prevention efforts. Nonetheless, AIDS case projections and HIV-prevalence estimates indicate that the annual toll of AIDS cases and the nationwide burden of HIV-related disease will continue to grow, requiring further prevention efforts and increased medical and social services for the next several years for persons with HIV infection.
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3. Dondero TJ, St. Louis M, Anderson J, Petersen L, Pappaioanou M. Evaluation of the estimated number of HIV infections using a spreadsheet model and empirical data (Abstract). V International Conference on AIDS. Montreal, June 4-9, 1989:45.
4. Brookmeyer R, Damiano A. Statistical methods for short-term projections of AIDS incidence. Stat Med 1989;8:23-34.
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6. CDC. Update: acquired immunodeficiency syndrome--United States, 1989. MMWR 1990;39:81-6.
7. Garland FC, Mayers DL, Hickey TM, et al. Incidence of human immunodeficiency virus seroconversion in U.S. Navy and Marine Corps personnel, 1986 through 1988. JAMA 1989;262:3161-5.
8. McNeil JG, Brundage JF, Wann ZF, Burke DS, Miller RN, and the Walter Reed Retrovirus Research Group. Direct measurement of human immunodeficiency virus seroconversions in a serially tested population of young adults in the United States Army, October 1985 to October 1987. N Engl J Med 1989;320:1581-5.
9. Brundage JF, McNeil JG, Miller RN, et al. The current distribution of CD4+ T-lymphocyte counts among adults in the United States with human immunodeficiency virus infection: estimates based on the experience of the U.S. Army. J AIDS 1990;3:92-4. 10. Munoz A, Carey V, Saah A, et al. Predictors in the decline in CD4 lymphocytes in a cohort of homosexual partners infected with the human immunodeficiency virus. J AIDS 1988;1:396-404. 11. DeGruttola V, Lange N, Dafni U. Modeling the progression of HIV infection. Presented at the 47th session of the International Statistics Institute, August 1989. 12. CDC. Human immunodeficiency virus infection in the United States: a review of current knowledge. MMWR 1987;36(no. S-6). 13. Gail MH, Rosenberg PS, Goedert JJ. Therapy may explain recent deficits in AIDS incidence. J AIDS (in press). 14. Andrews EB, Creagh-Kirk T, Pattishall K, Tilson H. Number of patients treated with zidovudine in the limited distribution system, March-September 1987 (Letter). J AIDS (in press). *Estimates were developed from workshop reports and may not be endorsed by all participants. **Single copies of this document will be available until February 23, 1991, from the National AIDS Information Clearinghouse, P.O. Box 6003, Rockville, MD 20850; telephone (800) 458-5231. The full report and recommendations from the workshop will be published in a future issue of MMWR Recommendations and Reports. ***CDC estimates that 70%-90% of all HIV-related deaths in young adult men are reported through AIDS surveillance (2) and that 85% of all diagnosed AIDS cases are reported.
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