摘要：Objectives . We measured and compared the concentration of primary and secondary syphilis, gonorrhea, chlamydial infection, and genital herpes in a large county with urban, suburban, and rural settings. Methods . We geocoded sexually transmitted infections reported to King County, Washington health department in 2000–2001 to census tract of residence. We used a model-based approach to measure concentration with Lorenz curves and Gini coefficients. Results . Syphilis exhibited the highest level of concentration (estimated Gini coefficient = 0.68, 95% confidence interval [CI] = 0.64, 0.78), followed by gonorrhea (estimated Gini coefficient=0.57; 95% CI=0.54, 0.60), chlamydial infection (estimated Gini coefficient = 0.45; 95% CI = 0.40, 0.43), and herpes (estimated Gini coefficient=0.26; 95% CI=0.22, 0.29). Conclusions . Geographically targeted interventions may be most appropriate for syphilis and gonorrhea. For less-concentrated infections, control strategies must reach a wider portion of the population. In a model introduced by Wasserheit and Aral, 1 an epidemic of a sexually transmitted infection (STI) progresses through identifiable phases influenced by dynamic interactions among several factors, including characteristics of the pathogen, sexual behaviors in a population, and control and prevention activities. Prevention and control strategies may need to be phase specific to most effectively control the spread of STIs. 1, 2 A limitation to applying phase-specific interventions is that methodologies for identifying the stages of an epidemic are not well developed. Wasserheit and Aral hypothesized that both “spread networks” (e.g., groups with higher rates of sexual partner change, concurrent sexual relationships, or reduced access to or use of health care and prevention services) and “maintenance networks” (e.g., groups with lower rates of sexual partner change and concurrency, representing a greater share of the population) are involved during the initial growth and hyperendemic phases of an epidemic, but spread networks dominate in subsequent decline and endemic phases. 1 As incidence decreases in these later phases, infection becomes more concentrated within spread networks. 1 Wasserheit 3 and Elliot et al. 4 suggested that measuring the concentration of an STI may be useful in determining an epidemic’s phase; they introduced the application of Lorenz curves and Gini coefficients, measures of inequality most often used in economics and sociology, into the STI literature. Both measures are based on the distribution of some commodity—income, wealth, or an STI—across a population. To date, statistical tests have not been applied to compare Gini coefficients or Lorenz curves to one another, for example, to compare the patterns of concentration of different STIs or to assess trends in concentration of any given STI over time. We set out to measure and compare the concentration of 4 STIs in a large county with urban, suburban, and rural settings. We hypothesized that the concentration of the STIs would be influenced by the intrinsic characteristics of the pathogen and the disease (i.e., efficiency of transmission and duration of infectivity with no intervention), by patterns of sexual behavior in the population, and by intervention activities. Thus, STIs that often produce symptoms that prompt health care seeking and curative therapy and that have long been the target of effective, wide-coverage sexually transmitted disease (STD) control programs (i.e., syphilis and gonorrhea), will become more heavily concentrated in smaller segments of the population than would be the case for an STI that is more often asymptomatic and that has been addressed with control efforts of shorter duration (e.g., chlamydial infection) or incurable STIs such as genital herpes, characterized by prolonged infectivity and not specifically addressed by an effective control program. We tested these hypotheses by examining the levels of concentration of syphilis, gonorrhea, chlamydial infection, and herpes in the population of King County, Washington, for the years 2000 and 2001.
