Purpose: Persistence of sexually transmitted infections (STI) in a population is due to the activities of a small proportion of the population with STI, who transmit infection on average to one or more susceptible sex partners during an infectious period. Within these groups, the average number of transmissions by infectious people to susceptible people in a closed group is measured by the reproductive number; a threshold, above which endemic infection is likely occur and below which, in the rest of the population, it is unlikely to occur. We hypothesized that people with repeated bacterial STI's and their sex partners include the theoretical core group and that they differ from singly infected noncore individuals.
Methods: Data on infected individuals and nominated sex partners for the years 1990-1992 were extracted from the notifiable disease and health insurance registries in Manitoba, Canada. Individuals with repeated gonorrhea, chlamydia and coinfected infections were compared using logistic regression, and reproductive numbers were calculated using sex partner data.
Results: Of the three groups, the coinfected were youngest, and they were largely of aboriginal descent and had the lowest incomes. Repeaters were older; they had higher incomes, and there were fewer aboriginal people in this group. Chlamydia repeaters had even higher incomes, and this group comprised the least number of aboriginal people. The reproductive numbers showed the same gradient; (1.09, 1.01, and 2.41, respectively.)
Conclusions: These data show that these groups do not contribute equally to STI endemicity and indicate that sexual network structure affects epidemic thresholds.