Reduction in AIDS virus transmission: seroconversion in San Francisco 1982-1985 [abstract]

A probability sample of 809 homosexual-bisexual men drawn from the 12 square km area of San Francisco where the AIDS epidemic has been most severe has been enrolled in a prospective epidemiologic study (San Francisco Mens Health Study [S.F.M.H.S.]). At entry in 1984 49% were seropositive for antibodies to the AIDS-associated retrovirus. It can be estimated from the data that for the period June 1982 through May 1984 the average annual seroconversion rate was 17%. However from June 1984 through January 1985 10 study participants seroconverted yielding an annual conversion rate of 4.0% a substantial decline. These are the 1st seroconversion rates derived from a representative sample of a defined population. A nested case-control study of 10 observed seroconverters and 30 seronegatives from the S.F.M.H.S. revealed that during the 6 months prior to conversion seroconverters had continued high risk sexual practices such as multiple partners (median number 14 among converters compared to 1 among controls) and receptive anal intercourse (90% among converters compared to 50% among controls). Substantial reductions in these same practices are revealed among controls during the 2nd 6 months of observation. In general seroconversion rates in an epidemic increase as the prevalence of seropositivity increases given that the number of exposures remains constant. Nevertheless the findings from this study indicate that transmission of AIDS virus infection in the homosexual-bisexual population of San Francisco has declined to a low level. This decline attributable largely to change in sexual practices has favorable implications for controlling the AIDS epidemic in San Francisco and elsewhere. (full text)