Sexually Transmitted Diseases:
Just One More Day: The GAP as Population Level Determinant and Risk Factor for STI Spread
Aral, Sevgi O. PhD, MS, MA
From the Division of Sexually Transmitted Disease Prevention, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia
The author thanks Patricia Jackson for her outstanding support in the preparation of the article.
Correspondence: Sevgi O. Aral, PhD, MS, MA, Division of STD Prevention, National Center for HIV/AIDS, Viral Hepatitis, STD and TB Prevention, Centers for Disease Control and Prevention, 1600 Clifton Road, NE, Mailstop E07, Atlanta, GA 30333. E-mail: Saral@cdc.gov.
Received for publication February 15, 2008, and accepted March 18, 2008.
Patterns of sexual contact among individuals determine the spread of sexually transmitted infections (STIs) in populations. Increasingly, over the past 2 decades, specific aspects of sexual contact patterns have been identified as foci of attention in attempts to explain STI spread. The distribution of sex partner numbers, including its determinants and consequences, has emerged as a major explanatory sexual contact pattern parameter.1,2 Empirical and theoretical studies have highlighted the importance of sexual mixing patterns, sexual networks, timing of sex partnerships, sequential order among sex partners, and the duration of time between sex partnerships in STI epidemiology.3–10 An important determinant of transmission for STIs is the length of time (“gap”) between the end of an individual’s partnership with 1 sex partner and the start of their next partnership.9,10 Gaps can be either positive, indicating a nonzero interval between 2 partnerships, or negative, indicating overlapping or concurrent partnerships.9,10 Concurrent partnerships are known to accelerate the spread of STI in populations11; in addition, partners’ concurrent partnerships are a risk factor for STIs in individuals.12 However, even serially monogamous partnerships can be effectively concurrent if the gap between them is shorter than the infectious period. Transmission occurs only if the infected individual has effective contact with a susceptible person during the infectious period, a finite period after acquisition.9,10
In this issue of the journal, Chen et al.13 report on the findings of a mathematical modeling study, which suggests that patterns of partnership duration and gaps between partnerships are critical in sustaining the transmission of gonorrhea. Partnerships of short to medium duration, coupled with gap lengths short enough to ensure onward transmission and assortative mixing between people with similar gaps and partnership duration were able to reproduce the patterns of prevalence of gonorrhea observed in the United Kingdom. Surprisingly, Chen et al. found that the very short partnership lengths (those of single day duration) play a less important role in gonorrhea transmission. Thus, individuals with the highest partner change rates may not always be the most critical group in STI transmission. The results of the current study call for a redefinition of the “core group” as consisting of the small number of individuals who have short gaps with the appropriate partnership lengths estimated to be 8.5% of the UK population. Findings from empirical studies of gonorrhea are consistent with these conclusions and further support a definition of the core in terms of gap and partnership lengths. Whether this same core-group definition is applicable in the case of all other STI or all other bacterial STI remains an empirical (or, in the case of mathematical modeling studies, theoretical) question. The redefinition of the “core group” calls for an adjustment in the approach to behavioral interventions. According to the new model, reduction in the proportion of people having single day partnerships may have no beneficial effect on gonococcal transmission; reduction in the proportion of people having short and midterm partnerships has a modest effect; reduction in the proportion of people having short gaps between partnerships has by far the most substantial beneficial effect on gonorrhea transmission.
The study by Chen et al., just like all studies, has its limitations. However, despite the limitations of the study, the current article poses a number of interesting challenges and points to several important future directions, which may enhance our understanding of STI epidemiology and our practice of STI prevention.
First, the gap emerges as an individual level risk factor for STI in general and gonococcal infection in particular. Future research may indicate that, as in the case of concurrency,14 partner’s gap is more important as a risk factor than the person’s own gap. Probability of acquiring an infection may be associated with partner’s gap, whereas the probability of transmitting an infection may be associated with one’s own gap. Second, at the population level, core groups defined as small numbers of individuals who have short gaps with appropriate partnership lengths may play an important role in the complex systems of STI transmission.15 Future empirical and theoretical research would help further refine our understanding of the importance of this new definition of core groups in STI spread.
The gap, although logically closely related to the concept of concurrency or partnership overlap, may be relevant to a larger segment of the population compared with concurrency and hence more important epidemiologically at both the individual and the population levels. In the United States, majority of women report nonoverlapping partnerships; approximately 12% of all reproductive age women and about one fourth of sexually active reproductive age women with 2 or more partners report concurrent partnerships.9 Concurrency is more prevalent among men, but only 11% of men report concurrent partnerships during the past year.16 Moreover, the gap may be differentially relevant to different age-sex groups; it may be more relevant to women compared with men because a greater majority of women report nonoverlapping sex partnerships. However, the gap may be particularly relevant to adolescents because they report shorter gaps compared with older persons.
Finally, the gap is important in STI epidemiology in that gaps that are shorter than the infectious period are associated with transmission risk. The length of the infectious period varies across STIs.17 Bacterial STI have limited infectious periods; the relevance of the gap for these infections is clear.9,10,13 To the extent that viral STI have limited periods during which infectivity is particularly elevated,18 the gap may be relevant for these infections as well. The relative importance of the gap may vary by specific STI depending on the infectiousness of the pathogen; the critical length of the gap may vary in accordance with the length of the infectious period. Questions remain about social demographic and behavioral correlates of short gaps. Do people in different socioeconomic strata or different racial-ethnic groups have shorter or longer gaps? Do people who engage in other risk behaviors such as drug use have shorter gaps? Do sexual minorities have shorter or longer gaps? Are poverty and/or discrimination correlated with gap length? Another domain which needs to be explored in future research is sexual mixing across groups with different gap length and partnership duration. Chen et al.13 assumed that sexual mixing across such groups was assortative. Although this assumption makes intuitive sense, its validity needs to be empirically supported. Mixing patterns may vary across societies and even across population groups in the same society. Further, the extent to which a mixing pattern is assortative is also variable. For example, although heterosexual mixing across age groups is strongly assortative, heterosexual mixing across race-ethnicity groups is even more strongly assortative.3 Future research may shed light on all these issues.
The gap concept has implications for prevention efforts as well. First, short gaps, one’s own or partners’, may put individuals at risk of STI transmission and acquisition. It may be important to ensure that risk assessment algorithms include items that cover these 2 parameters. This would be a particularly relevant consideration for adolescents, young adults, women, and people whose other risk behaviors suggest that they may practice or (sexually) mix with others who practice short gaps. Second, the findings of Chen et al. and other studies on the gap suggest that behavioral interventions target persons with short gaps and medium length partnerships. This suggestion deviates from the established prevention wisdom and may need to be evaluated empirically. The suggested direction may lead to creative approaches. Yet, it is important to note that the suggested demographic target groups—adolescents, young adults, and women are in line with and not in contradiction to suggestions based on other approaches. Third, based on the findings of the current study, the risk behaviors to be focused on include gap length and partnership duration. More specifically, for STI prevention, a change in behavior that prolongs the period between consecutive partners is most desirable; secondarily, a change in behavior towards longer partnerships may be helpful. These new targets for behavior change have implications for the composition of public health messages. Both in counseling individual patients and in social marketing of prevention messages to populations it may be important to include a suggestion to wait “just one more day (week … month)” before initiating sexual intercourse with a new partner.
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