EFFECT OF CHANGING PARTNERSHIP FORMATION RATES ON THE SPREAD OF SEXUALLY-TRANSMITTED DISEASES AND HUMAN-IMMUNODEFICIENCY-VIRUS

Core population groups play an important role in the spread of sexuall y transmitted diseases. Subjects in a core group may change their beha vior over time and ''migrate'' to the noncore. The authors examined th e effects of such migration on the prevalence of gonorrhea, chlamydia, and human immunodeficiency virus (HIV) using a mathematical model. Th e size of the core and the migration rate from the core to the noncore were estimated from population-based sexual survey data on 8,445 Norw egians collected in 1987 and 1992. Sixty-four percent of the sample wa s considered without risk of contracting a sexually transmitted diseas e. The core group made up 2.5% of the remaining sample. The migration rate from the core was estimated at 12% per year. The three types of i nfections analyzed exemplify three different patterns of the effect of migration on infection prevalence in the core/noncore groups: gonorrh ea = no effect/no effect, chlamydia = no effect/increase, and HIV = de crease/increase. Migration affects the basic reproductive ratio of dis eases with a long infectious period more than that of diseases with a short infectious period, For HIV, this means that the later stages of infection contribute less to the basic reproductive ratio in the prese nce of migration, The results are qualitative and show that detailed k nowledge about mixing, migration, transmission rates, and duration of infectiousness is necessary to make accurate predictions.