By the end of 1987, nearly 50,000 cases of acquired immunodeficiency syndrome (AIDS) had been reported since 1981, 20,745 in the past year alone. Black and Hispanic adults and children have reported rates 3 to 12 times as high as whites. This can be largely attributed to higher reported rates in black and Hispanic intravenous (IV) drug abusers, their sex partners, and infants. In 1986, reported AIDS deaths increased adult male and female mortality in the United States by an estimated 0.7 and 0.07%, respectively, with much greater increases in selected age groups or areas of the country. The greatest variation in infection with the human immunodeficiency virus (HIV) (0 to 70%) has been found in surveys of IV drug abusers, while surveys of homosexual men reveal infection rates of 20 to 50%. Infection with HIV ranged from 0 to 2.6% in limited sexually transmitted disease clinic surveys of heterosexual men and women without a history of IV drug abuse or known sexual contact with persons at increased risk. The modes of HIV transmission are now well understood, but a large amount of biologic variability in efficiency of transmission remains to be explained. The period between initial infection with HIV and the development of AIDS is variable, but the risk for disease progression increases with duration of infection.
The reported incidence of acquired immune deficiency syndrome (AIDS) continues to increase in countries throughout the world. On the basis of a polynomial model for extrapolation, the cumulative number of cases diagnosed and reported since 1981 in the United States is expected to double during the next year with over 12,000 additional cases projected to be diagnosed by July 1986. The annual incidence rates for single (never-married) men in Manhattan and San Francisco, intravenous drug users in New York City and New Jersey, and persons with hemophilia A ranged from 261 to 350 per 100,000 population during 1984. For single men aged 25 to 44 years in Manhattan and San Francisco, AIDS was the leading cause of premature mortality in 1984 as measured by years of potential life lost. Infection with HTLV-III/LAV is considerably more common than reported AIDS in high-risk populations and can persist at least for several years, so the presence of specific antibody should be considered presumptive evidence of current infection. The screening of donated blood and plasma for antibody to HTLV-III/LAV and use of safer clotting factor concentrates should greatly reduce HTLV-III/LAV transmission through blood and blood products. Most HTLV-III/LAV infections occur through sexual transmission, use of contaminated needles, and as a result of infected mothers passing the virus to newborns. Continued research commitment is needed to develop an HTLV-III/LAV vaccine and therapy for this infection. In the interim, widespread community efforts are needed to minimize transmission.
The incubation period, representing the interval between the date of exposure and the date of diagnosis, can be firmly ascertained in transfusion-associated cases of acquired immunodeficiency syndrome (AIDS). However, because the observation period of all transfusion-infected persons may be short compared with the average incubation period for AIDS, many cases with long incubation periods have not yet been diagnosed. Thus, the simple average of 2.6 years tends to underestimate the true mean. To correct for this underestimation bias, we assumed that the underlying distribution of the incubation periods is a member of a broad class of probability densities. Then, by maximum likelihood techniques, the mean incubation period for transfusion-associated AIDS was estimated to be 4.5 years, with the 90% confidence interval ranging from 2.6 to 14.2 years. The logng incubation period has important consequences for infected individuals and implications for public health intervention and prevention policy.Transfusion-associated acquired immunodeficiency syndrome (AIDS) provides a unique opportunity to study the incubation period ofthis disease because the date ofexposure to the virus can be accurately determined through epidemiologic investigations that identify which blood donor, and donation date, was associated with transmission of infection (1). We define the incubation period as the period from transfusion to the diagnosis of the first opportunistic disease associated with AIDS. The date of diagnosis is used as a closure for this period, rather than the more conventional date ofonset of symptoms, because the earliest symptoms are typically nonspecific and their dates of onset are poorly remembered.During the initial phase of a new disease epidemic, use of the simple average of the incubation periods of the observed cases is subject to two types of bias due to length-biased sampling. One bias can result from any omission of patients who had such short incubation periods that their illnesses went undiagnosed as AIDS before the existence of transfusion-associated AIDS was first recognized; this constitutes left censoring in our sample. As a consequence of this bias, the traditional estimate, the mean incubation periods of the observed cases, might tend to overestimate the true mean. On the other hand, another source of bias can arise from the fact that our sample does not include those exposed persons who will have such long incubation periods that they have not yet been diagnosed; this constitutes right censoring, which might cause the traditional estimate to underestimate the true mean.Our estimate would now suggest that the right censoring exerts a greater impact on the estimate and the simple average tends to represent an underestimation. Moreover, immediately following a period when the rate of exposure is increasing, as certainly was occurring with AIDS during 1978-1984, the degree of underestimation due to the right censoring is even greater, since the absolute number of patients with short incubation perio...
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