A Spreadsheet for AIDS: Estimating Heterosexual Injection Drug User Population Size from AIDS Statistics in San Francisco

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Abstract
A single-page computer spreadsheet can be used to back-calculate the size of the population at risk from the reported number of AIDS cases and HIV seropositivity levels for that population. However, Cohen (1988:35) has cautioned that "this method requires some heroic assumptions, and is, therefore, fraught with difficulty." Slight variations in the definition of AIDS, in the progression rate, and in reported seropositivity rates used as data in the spreadsheet can make enormous differences in the results obtained through back-calculation. Despite the limitations of the method, an estimate of the possible size of the IDU population can be derived from back-calculation, with careful consideration of ethnographic realities taken into account. In San Francisco, the present authors believe that there were approximately 13,000 heterosexual IDUs as of the end of 1989. Further demographic divisions by ethnicity, age, sex, and even neighborhood could be made using the same techniques, if AIDS cases and seropositivity levels could be obtained for each variable. Table II predicts a cumulative 705 AIDS diagnoses among San Francisco heterosexual IDUs by the end of 1993, or nearly six times as many as reported through 1988. This prediction is based on an assumption of 2.5% seroconversion per year (1989-1993) and on modest progression-rate increments of 5%, 5%, 4%, 3%, and 3% in the eleventh through fifteenth years after HIV infection. Thus, it shows the magnitude of the epidemic that San Francisco will shortly face and emphasizes the need to act vigorously to prevent further HIV contagion among IDUs as well as from them to their heterosexual partners (drug using or not).(ABSTRACT TRUNCATED AT 250 WORDS)