DESPITE THE WIDESPREAD acceptance of the role of core groups in sexually transmitted disease (STD) transmission, the specific individuals who constitute the core groups have been difficult to identify and locate. Some have tried defining core group members as a function of disease incidence, others according to “high-risk” occupations, and yet others according to sexual behavior. 1 The attempt to identify core group members as targets for interventions begs the question, however, of whether identified core group members continue to engage in core behaviors or core occupations over time. In this issue of the journal, Humble et al. 2 find that core status defined by sex behavior changes over time in a cohort of young men and women.
The authors report on the sex behaviors of a birth cohort of women and men from Dunedin, New Zealand, as they aged from 18 to 26 years of age. The participants were interviewed at ages 18, 21, and 26 years and were asked about number of heterosexual partners, condom use, concurrent partnerships (at the beginning of their current relationship), and history of STDs. Participants were defined as being members of the core group based on numbers of partners in 2 different ways. The first core group definition comprised those with 5 or more sex partners in the year before the assessment at years 18, 21, and 26. The second definition specified core group members to be those with an average of 5 or more sex partners per year between sex debut and age 21 and/or between ages 21 and 26.
Using the first definition of core, a sizable portion of the cohort was in the core at one of the time points, especially for the men. Among 991 cohort members who responded at least once, 15% of women and 26% of men were in the core group at age 18, 21, or 26 using the first definition. However, only 0.5% of women and 0.9% of men were in the core group at all ages. Between the ages of 18 and 21, the migration out of the core group, using the first definition, was estimated to be 25% per year for women and 21% per year for men. Between the ages of 21 and 26, the rates of outmigration were 17% and 15% per year for women and men, respectively. Proportions fitting the core definition increased for both men and women between the ages of 18 and 21, then decreased by age 26. These data paint a clear picture of a fluid core from and into which people migrate easily.
The data also raise the question of how to define the core. Is the core group specifically those few people who are consistently engaged in the same behaviors, or is the core anyone who is exhibiting core behaviors at a given time? Humble et al. validated their core definitions with reported rates of STDs and high-risk behaviors. They found that the second core status definition was associated with higher self-reported cumulative incidence of STDs in women for the period between sex debut and age 21 and in men between ages 21 and 26. For men between sex debut and age 21, rates of disease were higher in the core group than in the noncore group, but not statistically significantly. There were too few women in the core group between ages 21 and 26 for an analysis to be done. Both core definitions were associated with higher rates of concurrency at all ages (defined as having another sex partner at the beginning of the current relationship). Membership in the core group at all ages, for both definitions, was associated with younger age of sex debut.
These results confirm those of Stigum et al. who also found that core group membership varied over time in a Norwegian population-based sample. 3 Among 8445 participants, the migration from the core group was estimated at 12% per year. In models of the effect of outmigration on the prevalence of infection, Stigum et al. found that this level of outmigration increased prevalence of chlamydia and HIV in the noncore group, but would have no effect on prevalence of gonorrhea in the noncore group. Despite this evidence showing that people migrate in and out of the core, core groups seem to be stable geographically. In fact, the notion of core groups evolved from studies showing geographic clustering of illness. 4–6 The idea that the core is not always the same people, combined with the fact that core groups seem to be geographically stable, suggests that places could be more important than people in defining core groups.
The importance of place has gained currency following the publication of studies showing that rates of disease are related to features of the environments in which people live. For example, in a study of New Orleans neighborhoods, Cohen et al. found that gonorrhea rates were associated with deteriorated physical conditions, after adjusting for poverty. 7 The same authors, in an ecologic analysis of 107 U.S. cities, found that the percentage of boarded-up housing was a statistically significant predictor of gonorrhea rates independently of sociodemographic factors, including income, education, and employment. 8 An analysis of syphilis rates in North Carolina found that disease incidence was twice as high in counties intersected by Interstate 95 than in non-I-95 counties after adjusting for race, age, sex, poverty, large cities, and drug activity, suggesting that proximity to the highway was an important risk factor for disease transmission independently of individual characteristics. 9
One reason why disease rates are thought to be linked to features of place is because physically decaying neighborhoods are places where high-risk sex behaviors could be more likely to take place as a result of the fact that mechanisms of social control could have broken down along with the physical environment. 5,6 The physical environment can deteriorate both as a result of the lack of resources of community members and because of government policies. In this way, Wallace has linked soaring rates of violence, substance abuse, and HIV in New York City to policies of “planned shrinkage” in which municipal services were actively cut back in minority communities. 10
The idea that core group membership could be a characteristic of places rather than people has important implications for interventions. 11 To identify members of core groups, it might be necessary to target particular places that enable or allow high-risk behaviors. We have used this “risk space” approach in a recent syphilis outbreak response in which we targeted sex partner meeting locations rather than members of social networks or people residing in certain areas to find infected individuals. 12 The idea was that certain meeting places were associated with the type of sex encounters that placed individuals at risk for syphilis. This method enabled the identification of new cases that had not been identified through traditional case-finding methods.
A place-based approach to STD prevention also implies a need for investigation into the particular features of places that are conducive to high-risk behaviors. The results of such studies would hopefully pave the way for interventions addressing the structural features of local environments that place some people at particular risk for STD infection.
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