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Epidemiology of HIV/AIDS in the United States
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History of the AIDS Epidemic in the United States
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transparent imageBeginnings of the AIDS Epidemic
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transparent imageEarliest Evidence of HIV Infection in the United States
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transparent imageTracking the Epidemic by AIDS Diagnoses and by HIV Test Results
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transparent imageGrowth of the Epidemic in the United States from 1981 to 1996
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transparent imageChanges in Demographic Characteristics of Cases over Time
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transparent imageImpact of the Epidemic: AIDS Mortality
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Prevalence and Incidence of HIV Infection in the United States
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transparent imageEstimating HIV Prevalence
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transparent imageEstimating HIV Incidence
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transparent imageCurrent Estimates of Prevalence and Incidence
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transparent imageHIV-2 Infection in the United States
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Projections for the Future of the Epidemic in the United States
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References
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Tables
Table 1.Estimated Persons Living with AIDS, by Region of Residence and Year, 1993 Through 2001, United States
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Table 2.Distributions of Adult U.S. AIDS Cases by Transmission Exposure Group over Time
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Table 3.AIDS Cases and Deaths, by Year and Age Group, Through December 2001, United States
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Table 4.Estimated Adult/Adolescent AIDS Incidence, by Race/Ethnicity and Year of Diagnosis, 1996-2001, United States
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Figures
Figure 1.Estimated Incidence of AIDS and Deaths of Adults with AIDS, January 1985-June 2000, United States
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History of the AIDS Epidemic in the United States
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Beginnings of the AIDS Epidemic
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Acquired immunodeficiency syndrome (AIDS) was first recognized as a new disease in the United States when clinicians in New York, Los Angeles, and San Francisco began to see young, homosexual men with Pneumocystis carinii (now P jiroveci) pneumonia (PCP) and Kaposi's sarcoma (KS), unusual diseases for young adults not known to be immunosuppressed. The first report in the medical literature that alerted the world to this new immunodeficiency syndrome appeared in June of 1981 and described five young, homosexual men in Los Angeles with PCP.(1) That observation was followed a few weeks later by a report of 26 homosexual men, from both New York and San Francisco, with KS (four of whom also had PCP).(2) Other reports followed of a similar syndrome in injecting drug users.(3) All of these individuals shared a profound immunodeficiency, the hallmark of which was a depletion of CD4-positive, or T-helper, lymphocytes. In mid-1982, the Centers for Disease Control and Prevention (CDC) published a report of 34 cases of KS and opportunistic infections (OIs) in Haitians living in several different states in the United States, none of whom reported homosexual behavior.(4) One week later, the CDC reported on PCP among persons with hemophilia.(5) The first case in a transfusion recipient was reported from San Francisco in an infant in late 1982. For a short time, the new disease was called gay-related immunodeficiency syndrome (GRIDS), but by September of 1982, the CDC had published a case definition, using the current designation of acquired immune deficiency syndrome (AIDS) in print, and it was rapidly adopted by researchers.(6)

The prominence of homosexual men and injecting drug users in the early cases of AIDS suggested an agent that was both blood borne and sexually transmitted, although early speculation about the etiology of AIDS included the hypothesis that all the patients were immunosuppressed because they had a history of drug use or multiple sexually transmitted diseases or malnutrition (the "immune overload" hypothesis).(7) The majority of researchers thought that the likely agent was a sexually transmitted virus that would be found in the peripheral blood. HIV was first isolated in France in 1983 by Françoise Barré-Sinoussi in the laboratory of Luc Montaignier as lymphadenopathy-associated virus (LAV),(8) but strong evidence that it was the AIDS virus did not appear until 1984, when four papers were published in one issue of Science by Robert Gallo and colleagues, who designated their isolate HTLV-III.(9) The virus was also isolated in San Francisco in 1984 by Jay Levy, who published his findings a few months later in 1984 and named his isolate AIDS-associated retrovirus (ARV).(10) All three of these designations for the virus appear in the early literature. The International Committee on the Taxonomy of Viruses chose the designation human immunodeficiency virus (HIV) in 1986. With the discovery by Montaignier's group in late 1986 of the related HIV-2 virus in West Africa, the original virus became HIV-1.(11)

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Earliest Evidence of HIV Infection in the United States
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Although AIDS was not recognized as a new clinical syndrome until 1981, researchers examining the earlier medical literature identified cases appearing to fit the AIDS surveillance definition as early as the 1950s and 1960s.(12) Frozen tissue and serum samples were available for one of these possible early AIDS cases, a 15-year-old black male from St. Louis who was hospitalized in 1968 and died of an aggressive, disseminated KS.(13) His tissue and serum specimens were HIV-antibody positive on Western blot and antigen-positive on ELISA. This appears to be the first confirmed case of HIV infection in the United States. The patient had no history of travel out of the country, so it is likely that some other persons in the United States were infected with HIV as long ago as the 1960s, if not earlier.

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Tracking the Epidemic by AIDS Diagnoses and by HIV Test Results
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Before HIV was identified as the virus that causes AIDS, tracking the course of the epidemic depended on reporting AIDS diagnoses to public health departments. All fifty states have required reporting of AIDS diagnoses since early in the epidemic. A number of states adopted legislation that also required reporting of positive HIV test results, but throughout the 1980s most of the states with the largest numbers of AIDS cases did not report HIV test results. This trend began to change in the 1990s and when the new multidrug regimens of antiretroviral therapy became widely available in 1996, the rapid drop in the number of reported AIDS diagnoses led to increased interest in attempting to monitor the epidemic by reporting HIV infection in addition to AIDS diagnoses. Most states have now adopted some version of reporting HIV test results, although they differ in whether the result is reported by name or by unique identifier and in whether anonymous HIV testing is offered as well as reportable HIV testing. In 2000, 35 states reported a positive confidential HIV test result by name. Monitoring the epidemic has become more difficult since the introduction of multidrug antiretroviral therapy. The interpretation of trends in both AIDS case reporting and HIV infection reporting has to take into account the effect of treatment on slowing disease progression and the effect of test-seeking behavior on the numbers and characteristics of persons being tested for HIV. When zidovudine monotherapy was the main treatment option, models of the epidemic adjusted for estimates of its effect on the incubation period from infection to AIDS, but the complexities and stronger effects of the multiple therapies now available have made treatment adjustment too uncertain for modeling. The history of the epidemic that follows below begins with the pattern of AIDS case reporting.

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Growth of the Epidemic in the United States from 1981 to 1996
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AIDS case surveillance reported the highest number of new cases by date of diagnosis in 1993, but this was in part an artifact of the change in the AIDS case definition in 1993 to include a CD4-lymphocyte count below 200 cells/mm3 or CD4 percentage below 14%. CDC estimated that the peak incidence based on the pre-1993 AIDS case definition would have occurred in 1995. Since the incubation period of AIDS prior to the use of effective antiretroviral therapy was approximately ten years, the epidemic of HIV infections must have peaked around the mid-1980s. The period from 1981 to 1995 was characterized by a very rapid increase in AIDS diagnoses. By the fall of 1981, more than 100 AIDS cases had been reported to the CDC. By February 1983, the first 1,000 cases had been reported. The second 1,000 were reported in the next 6 months and the third 1,000 before the end of the year. By the late 1980s, cases had been reported from every state. It took 8 years (until August 1989) for the first 100,000 cases to be reported; the second 100,000 were reported in just 2 years (by November 1991). The half million total was passed in October of 1995. The cumulative total of cases through December 2001 was 816,149 of whom 666,026 (81.6%) were men, 141,048 (17.3%) were women, and 9,074 (1.1%) were children under age 13.

The cumulative incidence of AIDS deaths has lagged behind new diagnoses by one to two years. The peak incidence of deaths was 50,877, reached in 1995 (Figure 1). Both projected incidence based on diagnoses other than a CD4-lymphocyte count/percentage and deaths first declined in 1996. The rapid decline of deaths in 1996 is the result both of the peak in AIDS incidence and the effectiveness of multidrug therapy, which became widely available in 1996. The latest surveillance totals of AIDS cases, AIDS deaths, and reported HIV infections, cross-tabulated by a number of categories, can be found on the CDC's Web site (http://www.cdc.gov/hiv/stats.htm) where individual tables may be viewed or a PDF file of the entire semiannual surveillance report may be downloaded. Earlier reports extending back to 1981 (http://www.cdc.gov/hiv/stats/hasrlink.htm) are also available. Users of the data should note the distinction between the number of AIDS cases by date of report and the number by date of diagnosis. Technical notes that accompany the reports should be consulted for changes in the AIDS case definition over time, adjustment for reporting delays, and other technical aspects of surveillance.

Declining AIDS mortality rates have increased the number of persons known to be living with AIDS (prevalent AIDS cases). Table 1 shows annual estimates from CDC of the number of prevalent cases since 1997.

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Changes in Demographic Characteristics of Cases over Time
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The characteristics of the first 50,352 cases, reported between 1981 and 1987 to CDC, differ significantly from the characteristics of the 69,151 cases reported a decade later in 1996. The epidemic has become increasingly an epidemic of non-white populations, of women, and of heterosexuals and injecting drug users. Only 8% of the earlier cases were female. That proportion rose to 21% of the 1996 cases. That trend has continued but with a slower increase in the proportion of female cases to 23% in 1999 and to 26% in 2001.

The distribution of new cases by race/ethnicity has also changed greatly in the first twenty years of the epidemic. The proportion of new cases in whites (not Hispanic) dropped from 60% in 1981 to 43% in 1996 and to 28% in 2001. The proportion of new cases in African Americans rose from 25% in 1981 to 50% in 2001, and the proportion of Hispanics rose from 14% to 20%. There were two-and-a-half times as many new diagnoses among African Americans and Hispanics than among non-Hispanic whites in 2001. African Americans are represented among AIDS cases at 4 times their proportion in the U.S. population, and Hispanics at nearly twice their population percentage.

Although the proportion of cases who were "men who have sex with men" (MSM) declined from 71% in 1983 to 44% in 1996 (of whom 4% also reported injecting drug use) that proportion has held steady in recent years, contrary to some predictions in the early 1990s that MSM cases would decline to about 25% of new diagnoses.(14) Users of CDC data on reported cases should note, however, that the risk groups in which cases are initially reported are skewed toward a high proportion in the "risk not reported/identified" category, the majority of which are eventually reclassified as MSM cases. Thirty-one percent of cases reported in 2001 were in this category but nearly all of them will be ultimately reclassified into the other categories. To account for this reclassification, CDC publishes tables estimating the numbers adjusted to reporting delays and reclassification. Those estimates projected MSM as 44% of new cases in 2001 (of whom 4.5% also report injecting drugs).

Also contrary to predictions a decade ago that injecting drug users (IDU) would account for 50% of new cases, the proportion of cases among IDU only increased from 17% in 1983 to 25% by 1992 and did not increase in the subsequent decade (26% in 2001).(14) The largest proportional increase has occurred in cases attributed to heterosexual transmission--from 5% in 1983 to 28% in 2001. No cases associated with the receipt of blood or blood products were diagnosed in 2001.

Table 2 shows the distribution of AIDS cases by transmission category in 1983, 1992, and 2001. The numbers for 2001 are estimates based on adjustment of reporting delay and reclassification of those reported without an identified risk. Past investigations of reports without an identifiable risk suggest that unidentified heterosexual transmission cases are not a significant fraction of male cases in this category, but are a large fraction of female cases reported in the category. This difference reflects the overall difference in the importance of heterosexual transmission for male and female cases. In the cumulative totals of AIDS cases, it is the largest category of female cases (over 40%) but only 5% of male cases.

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Impact of the Epidemic: AIDS Mortality
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By 1993, AIDS had become the leading cause of death among persons 25 to 44 years old and eighth overall among all causes of death, accounting for 2% of all deaths. These estimates were made by CDC from reporting of the underlying cause of death on a 10% sample of death certificates for U.S. residents filed in the 50 states and the District of Columbia. Over 40,000 U.S. residents died of AIDS in 1994 and 50,000 in 1995. In 1994, AIDS accounted for 23% of all deaths among men and 32% of all deaths among African American men. It was third overall among causes of death for women 25 to 44 years of age (11% of deaths), but first among African American women (22% of deaths). These data are probably underestimates of the impact of AIDS on mortality in young adults, because studies have found that using HIV as the underlying cause of death on the death certificate captures only two-thirds to three-fourths of deaths attributable to HIV infection. Since the peak number of deaths in 1995, there has been a steep decline in AIDS mortality (to 38,780 in 1996 and to 14,499 in 2000) (Table 3). The majority of this decline is probably attributable to the effectiveness of the multidrug treatment regimens that became widespread beginning in 1996, although it is also due in part to the peaking of infection incidence rates in the mid-1980s.

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Prevalence and Incidence of HIV Infection in the United States
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Trends in the characteristics of AIDS cases provide important information about how the epidemic is changing over time. In considering these trends, it should be remembered that the median time from HIV infection to AIDS is about 10 years in adults, even without effective treatments. In viewing trends in AIDS cases, we are observing trends in infection patterns mostly from a decade earlier. The most useful data to determine how well the HIV/AIDS epidemic is being controlled are good estimates of the incidence rates of HIV infection in different populations across the country. These data are scarce and are derived for the most part from small local studies. Methods for measuring HIV incidence and current national estimates are discussed below.

The prevalence of HIV infection has been somewhat easier to obtain than incidence data, but there is no representative national surveillance system for prevalent infections either, so estimates are based on a reporting of new HIV infections from states with reporting laws, on a variety of serological surveys, and on mathematical models that make use of reported AIDS cases. Prevalence data are useful for estimating the current burden of disease in the country and planning for demands on systems providing medical care and social services. The usefulness of prevalence obtained from the reporting system is limited by the proportion and characteristics of infected persons who seek HIV testing prior to an AIDS diagnosis. The total number of persons reported living with AIDS plus the number reported living with HIV infection (but not AIDS) provides a minimum number of prevalent infections in the U.S.

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Estimating HIV Prevalence
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Prevalence of HIV infection in the United States overall has been estimated with two different methods. The first method is to gather results from serosurveys in different populations and different geographic regions, put them together with estimates of the size of the populations at risk, and develop an overall estimate that synthesizes all the data. There are three main sources of error in this approach: 1) most of the serosurveys are not population-based and are difficult to generalize beyond the venue in which the HIV testing was done; 2) coverage of geographic regions and specific subpopulations at risk is not complete; and 3) the sizes of the populations at risk are not known with any precision--the numbers of homosexual men and of injecting drug users in the United States, for example, are particularly difficult to estimate.

The second approach to estimating prevalence uses a mathematical model called "back calculation," which combines the available data on the numbers of reported AIDS cases and the incubation period distribution of AIDS (the mathematical function that estimates the probability of developing AIDS for each year following HIV infection) to derive how many HIV infections occurred during years past.(15) With information on past infections and AIDS cases, current HIV prevalence can be estimated. This technique requires fairly complete surveillance of AIDS cases and an accurate estimate of the incubation period distribution. It is limited by its inability to estimate HIV infections in recent years with any precision. More significantly, the large, and as yet largely unmodeled, effect of antiretroviral therapy on the incubation period has rendered back-calculation currently ineffective in estimating prevalence. The complexity of treatment regimens and their effects appear unlikely to be captured by an adjustment to the incubation distribution. For this reason, back calculation may no longer be a useful method of estimating HIV prevalence.

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Estimating HIV Incidence
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Incidence estimates are more difficult to obtain than prevalence figures, but they are more informative about the effects of prevention efforts and the future of the epidemic. HIV incidence estimates can be obtained from: 1) observing seroconversions in a longitudinal study; 2) inferring incidence from serial cross-sectional surveys; 3) using capture-recapture methods in serial surveys; 4) back-calculation from reported AIDS cases; and 5) identifying recent seroconverters from a cross-sectional sample using two HIV antibody tests of differing sensitivity for HIV antibodies.

The first method of estimating incidence is to enroll an HIV-negative population in a longitudinal, or cohort, study and to test the participants at regular intervals for new HIV infections, thereby deriving an incidence rate (number of new infections per total number of person-years of follow-up). Longitudinal studies with incident infections have been a valuable source of data.(16) Longitudinal studies are limited by the expense of conducting such a study, by the characteristics of the population enrolled, and the consideration that the longer the cohort is followed, the less likely it is that they are still representative of the population from which they were recruited. The individuals at highest risk, for example, are likely to get infected first, and if no new members are recruited to the cohort, it will represent a sample composed of individuals with progressively lower average risk over time.

The second method of estimating incidence is by conducting serial cross-sectional surveys in a population. This method does not directly estimate incidence, but incidence is indirectly estimated by the slope of the seroprevalence against time if the population being surveyed remains representative over time and if deaths and other losses to follow-up can be considered negligible. This approach has been suggested for estimating incidence from successive birth cohorts of recruits into the U.S. military.(17)

The third method is a variant on the cross-sectional survey approach that uses "capture-recapture," a methodology long used by biologists to study wildlife populations. It requires some sort of unique identifier, but not necessarily names, of individuals included in repeated surveys, so that the seroconverters among those repeatedly tested can be identified. The uniqueness of the identifier and potential biases in what members of the population are being repeatedly tested have to be considered. This method was used to estimate incidence rates among injecting drug users in San Francisco by repeated testing in both clinic and street settings over a 5-year period while asking participants to receive their test results under a unique identifier constructed from the day of the month of their birth and their parents' first names.(18)

The fourth method uses "back calculation," described above in the discussion of measuring prevalence. The fifth method is relatively new. It uses two HIV enzyme immunoassays: one is a current, highly sensitive test and the other has been made insensitive ("detuned"), in order to identify recent seroconverters from a single cross-sectional sample. As the quantity and avidity of antibody in peripheral blood increases progressively in the first weeks and months after HIV infection, a newly infected person will test positive on the sensitive assay and negative on the "detuned," as it is often called, or less sensitive assay. If the average time a newly infected person will be positive on the first test and negative on the second is known, an annualized incidence rate can be extrapolated from the cross-sectional samples. This method was tested against observed incidence rates in the San Francisco Men's Health Study, a 10-year cohort study, and the estimate from the dual assay method agreed well with the observed incidence over the 10-year period although the rates for individual years did not all agree well.(19) One source of variation with this method is the viral subtype (clade) of HIV being tested. The average window of time captured by the two assays also needs to be determined and validated separately for assays of different manufacture. False positive seroconversions can occur in individuals with late-stage HIV infection, in which antibody levels decline, and in persons receiving antiretroviral treatment. Despite these limitations, this method has grown in use because it is the only method that allows an incidence estimate from a single cross-sectional sample. It is described by CDC as the serological testing algorithm for recent HIV seroconversion or STARHS.(20)

A sixth approach does not estimate HIV incidence per se but uses the number of reported AIDS cases in the youngest age range of adult cases, ages 13-25, as a surrogate for recent trends in incidence.(21) The justification for this approach is that onset of sexual and drug-using risk behavior in the teenage years (or later) leads to the inference that AIDS cases in this age group will be predominately those with a short incubation time from infection to AIDS and that therefore most of the cases reflect relatively recent infections (less than, say, 5 years on average). As a rough indicator of recent infection trends this approach has some validity, but even 5 years on average is not as recent as one would like the data to be.

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Current Estimates of Prevalence and Incidence
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A wide variety of estimates of HIV prevalence appeared in the medical and the popular literature in the first decade of the epidemic, ranging up to 3 million infected.(22) Many estimates, including the Public Health Service estimate that the number infected ranged between 1 and 1.5 million, were later shown to be too high by the subsequent numbers of diagnosed AIDS cases.(23) Something approaching a consensus on the number infected in the range of about 600,000 to 900,000 was reached in the early 1990s by the convergence of prevalence estimates that used the method of back calculation and those that extrapolated from serosurveys of subpopulations.(14,24)

Testing blood samples from the National Health and Nutrition Examination Survey (NHANES III, 1988-1994), a probability sample of U.S. households, proved not to be a satisfactory way to estimate prevalence as the survey significantly underrepresents the high risk groups of MSM and IDU where the majority of infections lie.(25) A sensitivity analysis of the bias in participation rates for this survey estimated that the number of 461,000 obtained from testing NHANES III blood samples might have been too low by 190,000 persons (giving a national estimate of 650,000 at the midpoint of 1991).

The minimum number of HIV-infected persons in the United States given by the total of persons reported living with AIDS and living with HIV to CDC through December 2001 was 506,154. It is likely that there are at least another 200,000 or so additional persons with HIV infection not captured by the reporting system. Many have been tested at anonymous sites or reside in states without named HIV reporting, and many are infected but have not been tested. CDC estimated HIV prevalence in the United States in 2000 between 850,000 - 950,000.(26) A lower estimate is obtained if the 650,000 estimate for 1991 (midyear of sample collection) from the adjusted NHANES III data is projected forward to 2002 by the difference between annual AIDS deaths and the estimate of 40,000 new HIV infections each year. That calculation gives a prevalence of approximately 720,000 by the beginning of 2002. The current trend of about 20,000 deaths each year implies that prevalence is increasing by about 20,000 per year if the incidence is 40,000 per year. In applying this kind of calculation, the reader should bear in mind that the incidence estimate is quite approximate, that incidence may be increasing in some populations, and that deaths are somewhat underreported.

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HIV-2 Infection in the United States
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Nearly all cases of HIV infection in the United States are due to HIV-1. Reports of HIV-2 infection are rare. The first known case was a West African woman identified in 1987. Through June 30, 1995, 62 persons had been identified with HIV-2 infection.(15) Of the 62 HIV-2-infected individuals, 9 were born in the United States, 42 in Africa, and 2 in Europe; for 9 individuals, the nationality was unknown. Of the 9 U.S. natives, 6 were adults of whom 4 had traveled in West Africa or had a sex partner from West Africa; 3 were infants born to women of unknown national origin. A 1996 serosurvey of 832 patients at a New York hospital that serves a community with a high percentage of West African immigrants did not find any HIV-2 infections that were confirmed by Western blot analysis.(27) In 1998 the third HIV-2 antibody-positive blood donor was reported. This person was a native of an HIV-2 endemic country.(28) Thus, HIV-2 infection in the United States continues to be quite rare and is still largely seen in persons from West Africa or who have had sexual contact with Africans.

Screening of blood donors from 1987 through 1989 failed to identify any persons with HIV-2 infection.(29) The CDC tested 31,533 clients from STD clinics, drug treatment centers, and HIV testing sites for HIV-2 and found only two seropositives (0.006%); both were male, heterosexual, and black.(30) Beginning in 1992, the Food and Drug Administration recommended that whole blood and blood components be screened with combination HIV-1/HIV-2 enzyme immunoassays.(31) Screening of blood and plasma donors from 1992 to 1995 detected the first two cases of HIV-2 infection among potential donors.(15) One potential male donor was born in France, had lived in western Africa, and had been vaccinated in Africa with needles wiped with cotton between patients. The second potential donor was female, born in the United States, had not traveled out of the country, denied injecting drug use, and had no known sexual partners born outside of the United States.

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Projections for the Future of the Epidemic in the United States
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Projections for the future of the epidemic have most often taken the form of trying to estimate how many new AIDS cases will be diagnosed (or reported) over some span of future years. AIDS cases can be projected in one of three ways. First, the numbers of recent AIDS cases can simply be extrapolated forward by using various regression techniques to model past trends and extend the model forward.(32) This approach was used for estimating the number of cases in the next 2 to 3 years with pretty good validity because the number of AIDS cases did not change rapidly from year to year in the past due to the long incubation period from infection to AIDS. Rapid changes in the incubation period due to the introduction of effective multi-drug therapy have rendered this technique not very useful in the 1995 to 1997 period when the new treatments were being introduced. Artifactual changes, such as new case definitions, are another source of sudden change in the number of cases, but otherwise, changes tend to be gradual, even if the rate of new HIV infections is changing rapidly in the same time period.

Another way to project AIDS cases is by using the "back-calculation" technique described above,(33) which "back calculates" the number of infections in the past that gave rise to the observed AIDS cases by using an estimate of the incubation distribution. With an estimate of the number of infections, a minimum number of future AIDS cases can be projected ("minimum" because future infections are not included in the model). This approach can include corrections for reporting delay, incomplete reporting, and treatment effects that alter the incubation period, but it provides very little information about the effects of recent trends in HIV infection rates on future AIDS cases. It can be a useful way to obtain intermediate range projections, but, as previously pointed out, adjustment for the substantial and complicated effects of multidrug antiretroviral therapy on the incubation distribution has not heretofore been possible.

The third method of projecting AIDS cases is to build dynamic mathematical models that use a wider variety of information, such as data on risk behaviors, modes of transmission and their associated probabilities, and mixing patterns among subgroups of the population, as well as AIDS cases and the incubation period, to model the size of the epidemic in the future.(34) These models have greater potential for long-term projections, but they usually have more parameters, the values of which may be uncertain, so the results are less certain. The three approaches provide a hierarchy of decreasing confidence in the projections that is counterbalanced by the increasing length of time in the future to which they apply.

Prospective studies of homosexual/bisexual men in San Francisco and other U.S. cities showed large declines in the annual HIV incidence rates in the 1980s. The San Francisco Men's Health Study reported a dramatic reduction in seroconversion, from rates of 10 to 20% per year in the early 1980s to less than 1% per year from 1987 to 1988.(16,35) Similar declines were seen in other cities. In the four-city Multicenter AIDS Cohort Study of homosexual and bisexual men, HIV seroconversion rates dropped from 4% per 6 months to between 0.5 and 1.2% per 6 months between 1984 and 1989.(36) These annual incidence rates of 1% per year or slightly below continued during subsequent years. Studies initiated in the 1990s of younger homosexual men found somewhat higher infection rates among men in their twenties, but these rates were still far below the high rates of the early 1980s. The San Francisco Young Men's Health Study found about 1.5% per year seroconverting during the period from 1993 to 1997 among men who were 18 to 29 years old when they were recruited to the study in 1993.(37)

These data led to the inference that incidence rates among homosexual men in the major urban centers in the late 1980s and early 1990s were fairly constant at rates centering around 1% per year--that the epidemic had become endemic among homosexual men. A similar conclusion was reached from studies of injecting drug users where similarly low but relatively steady incidence rates were observed in a number of communities. Although infections were thought to be increasing in heterosexuals, they still accounted for a minority of new infections, and overall projections for the epidemic by the mid-1990s were that the number of new infections was relatively constant at about 40,000 per year, a number that was approximately balanced by the number of annual deaths. The prevalence of HIV infection, therefore, was thought to remain more or less constant year to year.

The estimate that the epidemic was in something approximating a steady state in the mid-1990s began to change with the advent of more effective therapy in 1996. A rapid decline of the death rate resulted in an increase in the estimate of persons living with HIV infection. Along with this increase in prevalence in the late 1990s came reports from studies of homosexual men showing increasing proportions of men reporting anal sex without use of a condom.(38-40) Reports of increasing high-risk behavior were followed by reports of increasing HIV incidence in some cities and by an increase in sexually transmitted diseases in homosexual men.(41-43) These reports of increasing risk and increasing HIV infections are not reflected in CDC's estimates of the number of annual infections shown in Table 4. Those estimates from 1998 through 2001 are about 40,000 per year, the same number that was being estimated in the late 1980s and early 1990s. The lack of good national data on infection rates makes it difficult to judge any of these estimates, but they may be too low if there is in fact a significant increase in incidence rates among homosexual men. Similar increases have not been reported for injecting drug users.

In sum, we currently do not have good projections of the future of the epidemic in the U.S. or of the present rate of new infections. Given that projections from a decade ago of somewhere around 40,000 infections per year seem to have been borne out by new AIDS cases in the 1990s, we can anticipate those numbers as a minimum likely projection for the next few years with the sobering possibility that they may be on the increase, at least among homosexual men, who still account for the largest proportion of new AIDS cases.

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References

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1.   Pneumocystis pneumonia--Los Angeles. MMWR Morb Mortal Wkly Rep. 1981 Jun 5;30(21):250-2.
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2.   Kaposi's sarcoma and Pneumocystis pneumonia among homosexual men--New York City and California. MMWR Morb Mortal Wkly Rep. 1981 Jul 3;30(25):305-8.
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3.   Masur H, Michelis MA, Greene JB, Onorato I, Stouwe RA, Holzman RS, Wormser G, Brettman L, Lange M, Murray HW, Cunningham-Rundles S. An outbreak of community-acquired Pneumocystis carinii pneumonia: initial manifestation of cellular immune dysfunction. N Engl J Med. 1981 Dec 10;305(24):1431-8.
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4.   Opportunistic infections and Kaposi's sarcoma among Haitians in the United States. MMWR Morb Mortal Wkly Rep. 1982 Jul 9;31(26):353-4, 360-1.
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5.   Update on acquired immune deficiency syndrome (AIDS) among patients with hemophilia A. MMWR Morb Mortal Wkly Rep. 1982 Dec 10;31(48):644-6, 652.
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6.   CDC. Update on acquired immune deficiency syndrome (AIDS)--United States. MMWR Morb Mortal Wkly Rep. 1982 Sep 24;31(37):507-8,513-4.
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7.   Sonnabend J, Witkin SS, Purtilo DT. Acquired immunodeficiency syndrome, opportunistic infections, and malignancies in male homosexuals. A hypothesis of etiologic factors in pathogenesis. JAMA. 1983 May 6;249(17):2370-4.
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8.   Barre-Sinoussi F, Chermann JC, Rey F, Nugeyre MT, Chamaret S, Gruest J, Dauguet C, Axler-Blin C, Vezinet-Brun F, Rouzioux C, Rozenbaum W, Montagnier L. Isolation of a T-lymphotropic retrovirus from a patient at risk for acquired immune deficiency syndrome (AIDS). Science. 1983 May 20;220(4599):868-71.
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9.   Gallo RC, Salahuddin SZ, Popovic M, Shearer GM, Kaplan M, Haynes BF, Palker TJ, Redfield R, Oleske J, Safai B, et al. Frequent detection and isolation of cytopathic retroviruses (HTLV-III) from patients with AIDS and at risk for AIDS. Science. 1984 May 4;224(4648):500-3.
transparent image
10.   Levy JA, Hoffman AD, Kramer SM, Landis JA, Shimabukuro JM, Oshiro LS. Isolation of lymphocytopathic retroviruses from San Francisco patients with AIDS. Science. 1984 Aug 24;225(4664):840-2.
transparent image
11.   Guyader M, Emerman M, Sonigo P, Clavel F, Montagnier L, Alizon M. Genome organization and transactivation of the human immunodeficiency virus type 2. Nature. 1987 Apr 16-22;326(6114):662-9.
transparent image
12.   Huminer D, Rosenfeld JB, Pitlik SD. AIDS in the pre-AIDS era. Rev Infect Dis. 1987 Nov-Dec;9(6):1102-8.
transparent image
13.   Garry RF, Witte MH, Gottlieb AA, Elvin-Lewis M, Gottlieb MS, Witte CL, Alexander SS, Cole WR, Drake WL Jr. Documentation of an AIDS virus infection in the United States in 1968. JAMA. 1988 Oct 14;260(14):2085-7.
transparent image
14.   Holmberg SD. The estimated prevalence and incidence of HIV in 96 large US metropolitan areas. Am J Public Health. 1996 May;86(5):642-54.
transparent image
15.   CDC. Update: HIV-2 infection among blood and plasma donors--United States, June 1992-June 1995. MMWR Morb Mortal Wkly Rep. 1995 Aug 18;44(32):603-6.
transparent image
16.   Winkelstein W Jr, Samuel M, Padian NS, Wiley JA, Lang W, Anderson RE, Levy JA. The San Francisco Men's Health Study: III. Reduction in human immunodeficiency virus transmission among homosexual/bisexual men, 1982-86. Am J Public Health. 1987 Jun;77(6):685-9.
transparent image
17.   Brundage JF, Burke DS, Gardner LI, McNeil JG, Goldenbaum M, Visintine R, Redfield RR, Peterson M, Miller RN. Tracking the spread of the HIV infection epidemic among young adults in the United States: results of the first four years of screening among civilian applicants for U.S. military service. J Acquir Immune Defic Syndr. 1990;3(12):1168-80.
transparent image
18.   Moss AR, Vranizan K, Gorter R, Bacchetti P, Watters J, Osmond D. HIV seroconversion in intravenous drug users in San Francisco, 1985-1990. AIDS. 1994 Feb;8(2):223-31.
transparent image
19.   Janssen RS, Satten GA, Stramer SL, Rawal BD, O'Brien TR, Weiblen BJ, Hecht FM, Jack N, Cleghorn FR, Kahn JO, Chesney MA, Busch MP. New testing strategy to detect early HIV-1 infection for use in incidence estimates and for clinical and prevention purposes. JAMA. 1998 Jul 1;280(1):42-8.
transparent image
20.   Schwarcz S, Kellogg T, McFarland W, Louie B, Kohn R, Busch M, Katz M, Bolan G, Klausner J, Weinstock H. Differences in the temporal trends of HIV seroincidence and seroprevalence among sexually transmitted disease clinic patients, 1989-1998: application of the serologic testing algorithm for recent HIV seroconversion. Am J Epidemiol. 2001 May 15;153(10):925-34.
transparent image
21.   Recent trends in the HIV epidemic in adolescent and young adult gay and bisexual men. J Acquir Immune Defic Syndr Hum Retrovirol. 1997 Dec 15;16(5):374-9.
transparent image
22.  Masters WH, Johnson VE, Kolodny RC. Crisis: Heterosexual behavior in the age of AIDS. 1st ed. New York: Grove Press, 1988.
transparent image
23.   Estimates of HIV prevalence and projected AIDS cases: summary of a workshop, October 31-November 1, 1989. MMWR Morb Mortal Wkly Rep. 1990 Feb 23;39(7):110-2, 117-9.
transparent image
24.   Rosenberg PS. Scope of the AIDS epidemic in the United States. Science. 1995 Nov 24;270(5240):1372-5.
transparent image
25.   McQuillan GM, Khare M, Karon JM, Schable CA, Vlahov D. Update on the seroepidemiology of human immunodeficiency virus in the United States household population: NHANES III, 1988-1994. J Acquir Immune Defic Syndr Hum Retrovirol. 1997 Apr 1;14(4):355-60.
transparent image
26.  Fleming PL, Byers RH, Sweeney PA, Daniels D, Karon JM, Janssen RS. HIV Prevalence in the United States, 2000. Oral abstract, session 5, presentation 11, 9th Conference on Retroviruses and Opportunistic Infections, Seattle, WA, February 24-28, 2002.
transparent image
27.   Irwin KL, Olivo N, Schable C, Weber JT, Janssen R, Ernst J. Absence of human immunodeficiency virus type 2 infection among patients in a hospital serving a New York community at high risk for infection. Centers for Disease Control and Prevention/Bronx-Lebanon Hospital Center HIV Serosurvey Team. Transfusion. 1996 Aug;36(8):731-3.
transparent image
28.   Sullivan MT, Guido EA, Metler RP, Schable CA, Williams AE, Stramer SL. Identification and characterization of an HIV-2 antibody-positive blood donor in the United States. Transfusion. 1998 Feb;38(2):189-93.
transparent image
29.   Surveillance for HIV-2 infection in blood donors--United States, 1987-1989. MMWR Morb Mortal Wkly Rep. 1990 Nov 23;39(46):829-31.
transparent image
30.   Onorato IM, O'Brien TR, Schable CA, Spruill C, Holmberg SD. Sentinel surveillance for HIV-2 infection in high-risk US populations. Am J Public Health. 1993 Apr;83(4):515-9.
transparent image
31.   George JR, Rayfield MA, Phillips S, Heyward WL, Krebs JW, Odehouri K, Soudre R, De Cock KM, Schochetman G. Efficacies of US Food and Drug Administration-licensed HIV-1-screening enzyme immunoassays for detecting antibodies to HIV-2. AIDS. 1990 Apr;4(4):321-6.
transparent image
32.   Morgan WM, Curran JW. Acquired immunodeficiency syndrome: current and future trends. Public Health Rep. 1986 Sep-Oct;101(5):459-65.
transparent image
33.  Brookmeyer R, Gail MH. A method for obtaining short-term projections and lower bounds on the size of the AIDS epidemic. J Am Stat Assoc 1988;83:301-308.
transparent image
34.   Gail MH, Brookmeyer R. Methods for projecting course of acquired immunodeficiency syndrome epidemic. J Natl Cancer Inst. 1988 Aug 17;80(12):900-11.
transparent image
35.   Winkelstein W Jr, Wiley JA, Padian NS, Samuel M, Shiboski S, Ascher MS, Levy JA. The San Francisco Men's Health Study: continued decline in HIV seroconversion rates among homosexual/bisexual men. Am J Public Health. 1988 Nov;78(11):1472-4.
transparent image
36.   Kingsley LA, Zhou SY, Bacellar H, Rinaldo CR Jr, Chmiel J, Detels R, Saah A, VanRaden M, Ho M, Munoz A. Temporal trends in human immunodeficiency virus type 1 seroconversion 1984-1989. A report from the Multicenter AIDS Cohort Study (MACS). Am J Epidemiol. 1991 Aug 15;134(4):331-9.
transparent image
37.   Osmond DH, Page K, Wiley J, Garrett K, Sheppard HW, Moss AR, Schrager L, Winkelstein W. HIV infection in homosexual and bisexual men 18 to 29 years of age: the San Francisco Young Men's Health Study. Am J Public Health. 1994 Dec;84(12):1933-7.
transparent image
38.   Ekstrand ML, Stall RD, Paul JP, Osmond DH, Coates TJ. Gay men report high rates of unprotected anal sex with partners of unknown or discordant HIV status. AIDS. 1999 Aug 20;13(12):1525-33.
transparent image
39.   Koblin BA, Torian LV, Guilin V, Ren L, MacKellar DA, Valleroy LA. High prevalence of HIV infection among young men who have sex with men in New York City. AIDS. 2000 Aug 18;14(12):1793-800.
transparent image
40.   Valleroy LA, MacKellar DA, Karon JM, Rosen DH, McFarland W, Shehan DA, Stoyanoff SR, LaLota M, Celentano DD, Koblin BA, Thiede H, Katz MH, Torian LV, Janssen RS. HIV prevalence and associated risks in young men who have sex with men. Young Men's Survey Study Group. JAMA. 2000 Jul 12;284(2):198-204.
transparent image
41.   Gonorrhea among men who have sex with men--selected sexually transmitted diseases clinics, 1993-1996. MMWR Morb Mortal Wkly Rep. 1997 Sep 26;46(38):889-92.
transparent image
42.   Katz MH, Schwarcz SK, Kellogg TA, Klausner JD, Dilley JW, Gibson S, McFarland W. Impact of highly active antiretroviral treatment on HIV seroincidence among men who have sex with men: San Francisco. Am J Public Health. 2002 Mar;92(3):388-94.
transparent image
43.   Chen SY, Gibson S, Katz MH, Klausner JD, Dilley JW, Schwarcz SK, Kellogg TA, McFarland W. Continuing increases in sexual risk behavior and sexually transmitted diseases among men who have sex with men: San Francisco, Calif, 1999-2001, USA. Am J Public Health. 2002 Sep;92(9):1387-8.
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