Investigating ethnic inequalities in the incidence of sexually transmitted infections: mathematical modelling study

The potential impact of a vaccine on Neisseria gonorrhoeae prevalence among heterosexuals living in a high prevalence setting

BACKGROUND

Treatment of Neisseria gonorrhoeae is under threat with the emergence and spread of antimicrobial resistance. Thus, there is a growing interest in the development of a gonorrhoea vaccine. We used mathematical modelling to assess the impact of a hypothetical vaccine in controlling gonorrhoea among heterosexuals living in a setting of relatively high N. gonorrhoeae prevalence (∼3 %).

METHODS

We developed a mathematical model of N. gonorrhoeae transmission among 15-49-year-old heterosexuals, stratified by age and sex, and calibrated to prevalence and sexual behaviour data from South Africa as an example of a high prevalence setting for which we have data available. Using this model, we assessed the potential impact of a vaccine on N. gonorrhoeae prevalence in the entire population. We considered gonorrhoea vaccines having differing impacts on N. gonorrhoeae infection and transmission and offered to different age-groups.

RESULTS

The model predicts that N. gonorrhoeae prevalence can be reduced by ∼50 % in 10 years following introduction of a vaccine if annual vaccination uptake is 10 %, vaccine efficacy against acquisition of infection is 25 % and duration of protection is 5 years, with vaccination available to the entire population of 15-49-year-olds. If only 15-24-year-olds are vaccinated, the predicted reduction in prevalence in the entire population is 25 % with equivalent vaccine characteristics and uptake. Although predicted reductions in prevalence for vaccination programmes targeting only high-activity individuals and the entire population are similar over the same period, vaccinating only high-activity individuals is more efficient as the cumulative number of vaccinations needed to reduce prevalence in the entire population by 50 % is ∼3 times lower for this programme.

CONCLUSION

Provision of a gonorrhoea vaccine could lead to substantial reductions in N. gonorrhoeae prevalence in a high prevalence heterosexual setting, even with moderate annual vaccination uptake of a vaccine with partial efficacy.

Demographic and behavioural factors associated with antimicrobial susceptibility to azithromycin and ceftriaxone in Neisseria gonorrhoeae

Antimicrobial resistance of Neisseria gonorrhoeae (NG) is of global public health concern. The aim of this study was to explore demographic and behavioural factors associated with antimicrobial susceptibility of NG to ceftriaxone and azithromycin. Gonococcal isolates (n = 391) from clients attending the Auckland Sexual Health Service, New Zealand, from July 2014 - June 2015 (n = 206), and July 2017 - June 2018 (n = 185), were tested for susceptibility to ceftriaxone and azithromycin. Laboratory data were linked with behavioural and demographic data. Geometric mean azithromycin MICs increased across the two time periods (0.239 mg/L in 2014/15 to 0.347 mg/L in 2017/18, p < 0.001), and ceftriaxone MICs decreased (0.007 mg/L in 2014/15 to 0.005 mg/L in 2017/18, p < 0.001). Demographic and behavioural factors were not associated with differences in ceftriaxone MICs; however azithromycin MICs were higher in men who have sex with men (0.356 mg/L) compared with the heterosexual study population (0.192 mg/L, p < 0.001) and were lower in Pacific peoples (0.201 mg/L, p < 0.001) and Māori (0.244 mg/L, p = 0.05) compared with those of European ethnicity (0.321 mg/L). Our findings show that azithromycin MICs increased in our region between 2014 and 2018; associations were seen with sexual orientation and ethnicity.

Estimated Impact of Screening on Gonorrhea Epidemiology in the United States: Insights From a Mathematical Model

The burden of gonorrhea infections in the United States is high. There are marked disparities by race/ethnicity and sexual orientation. We quantified the impact of screening and treatment on gonorrhea rates in the US population aged 15 to 39 years for the period 2000 to 2015 and estimated the impact that alternative screening strategies might have had over the same period.

METHODS

We developed a national-level transmission model that divides the population by race/ethnicity, preferred gender of sex partners, age, gender, and sexual activity level. We compared our fitted model ("base case") to 4 alternative strategies: (i) no screening, (ii) full adherence to current screening guidelines, (iii) annual universal screening, or (iv) enhanced screening in groups with the highest infection burden. Main outcomes were incidence, infections averted, and incidence rate ratios by race/ethnicity. Mean values and 95% credible intervals were calculated from 1000 draws from parameter posterior distributions.

RESULTS

The calibrated model reproduced observed trends in gonorrhea, including disparities in infection burden by race/ethnicity. We estimated that screening for gonorrhea from 2000 to 2015 averted 30% (95% credible intervals, 18-44%) of total infections that would otherwise have occurred. All alternative active screening strategies were estimated to further reduce, but not eliminate, gonorrhea infections relative to the base case, with differential impacts on the subpopulations of interest.

CONCLUSIONS

Our model results suggest that screening has reduced gonorrhea incidence in the US population. Additional reductions in infection burden may have been possible over this period with increased screening, but elimination was unlikely.

Ethnic variations in sexual partnerships and mixing, and their association with STI diagnosis: findings from a cross-sectional biobehavioural survey of attendees of sexual health clinics across England

OBJECTIVES

Ethnic differences in partnership types and sexual mixing patterns may contribute to elevated STI diagnosis rates among England's Black Caribbean (BC) population. We examined the differences between BC and White British/Irish (WBI) sexual health clinic (SHC) attendees' reported partnerships and sexual mixing, and whether these differences could explain ethnic inequalities in STI, focusing on attendees reporting only opposite-sex partners (past year).

METHODS

We surveyed attendees at 16 SHCs across England (May to September 2016), and linked their survey responses to routinely collected data on diagnoses of bacterial STI or trichomoniasis ±6 weeks of clinic attendance ('acute STI'). Behaviourally-heterosexual BC and WBI attendees (n=1790) reported details about their ≤3 most recent opposite-sex partners (past 3 months, n=2503). We compared BC and WBI attendees' reported partnerships and mixing, in gender-stratified analyses, and used multivariable logistic regression to examine whether they independently explained differences in acute STI.

RESULTS

We observed differences by ethnic group. BC women's partnerships were more likely than WBI women's partnerships to involve age-mixing (≥5 years age difference; 31.6% vs 25.5% partnerships, p=0.013); BC men's partnerships were more often 'uncommitted regular' (35.4% vs 20.7%) and less often casual (38.5% vs 53.1%) than WBI men's partnerships (p<0.001). Acute STI was higher among BC women than WBI women (OR: 2.29, 95% CI 1.24 to 4.21), with no difference among men. This difference was unaffected by partnerships and mixing: BC women compared with WBI women adjusted OR: 2.31 (95% CI 1.30 to 4.09) after adjusting for age and partner numbers; 2.15 (95% CI 1.07 to 4.31) after additionally adjusting for age-mixing, ethnic-mixing and recent partnership type(s).

CONCLUSION

We found that differences in sexual partnerships and mixing do not appear to explain elevated risk of acute STI diagnosis among behaviourally-heterosexual BC women SHC attendees, but this may reflect the measures used. Better characterisation of 'high transmission networks' is needed, to improve our understanding of influences beyond the individual level, as part of endeavours to reduce population-level STI transmission.

A dynamic power-law sexual network model of gonorrhoea outbreaks

Human networks of sexual contacts are dynamic by nature, with partnerships forming and breaking continuously over time. Sexual behaviours are also highly heterogeneous, so that the number of partners reported by individuals over a given period of time is typically distributed as a power-law. Both the dynamism and heterogeneity of sexual partnerships are likely to have an effect in the patterns of spread of sexually transmitted diseases. To represent these two fundamental properties of sexual networks, we developed a stochastic process of dynamic partnership formation and dissolution, which results in power-law numbers of partners over time. Model parameters can be set to produce realistic conditions in terms of the exponent of the power-law distribution, of the number of individuals without relationships and of the average duration of relationships. Using an outbreak of antibiotic resistant gonorrhoea amongst men have sex with men as a case study, we show that our realistic dynamic network exhibits different properties compared to the frequently used static networks or homogeneous mixing models. We also consider an approximation to our dynamic network model in terms of a much simpler branching process. We estimate the parameters of the generation time distribution and offspring distribution which can be used for example in the context of outbreak reconstruction based on genomic data. Finally, we investigate the impact of a range of interventions against gonorrhoea, including increased condom use, more frequent screening and immunisation, concluding that the latter shows great promise to reduce the burden of gonorrhoea, even if the vaccine was only partially effective or applied to only a random subset of the population.

It's the network, stupid: a population's sexual network connectivity determines its STI prevalence

There is little consensus as to why sexually transmitted infections (STIs), including HIV and bacterial vaginosis (BV) are more prevalent in some populations than others. Using a broad definition of sexual network connectivity that includes both structural and conductivity-related factors, we argue that the available evidence suggests that high prevalence of traditional STIs, HIV and BV can be parsimoniously explained by these populations having more connected sexual networks. Positive feedback, whereby BV and various STIs enhance the spread of other STIs, then further accentuates the spread of BV, HIV and other STIs. We review evidence that support this hypothesis and end by suggesting study designs that could further evaluate the hypothesis, as well as implications of this hypothesis for the prevention and management of STIs.

It's the network, stupid: a population's sexual network connectivity determines its STI prevalence

There is little consensus as to why sexually transmitted infections (STIs), including HIV and bacterial vaginosis (BV) are more prevalent in some populations than others. Using a broad definition of sexual network connectivity that includes both structural and conductivity-related factors, we argue that the available evidence suggests that high prevalence of traditional STIs, HIV and BV can be parsimoniously explained by these populations having more connected sexual networks. Positive feedback, whereby BV and various STIs enhance the spread of other STIs, then further accentuates the spread of BV, HIV and other STIs. We review evidence that support this hypothesis and end by suggesting study designs that could further evaluate the hypothesis, as well as implications of this hypothesis for the prevention and management of STIs.

Epidemiological, behavioural, and clinical factors associated with antimicrobial-resistant gonorrhoea: a review

Antimicrobial-resistant Neisseria gonorrhoeae is a global public health problem in the 21st century. N. gonorrhoeae has developed resistance to all classes of antibiotics used for empirical treatment, and clinical treatment failure caused by extensively resistant strains has been reported. Identifying specific factors associated with an increased risk of antimicrobial-resistant N. gonorrhoeae might help to develop strategies to improve antimicrobial stewardship. In this review, we describe the findings of 24 studies, published between 1989 and 2017, that examined epidemiological, behavioural, and clinical factors and their associations with a range of antimicrobial agents used to treat gonorrhoea. Antimicrobial-resistant N. gonorrhoeae is more common in older than younger adults and in men who have sex with men compared with heterosexual men and women. Antimicrobial-resistant N. gonorrhoeae is less common in some black minority and Aboriginal ethnic groups than in the majority white population in high-income countries. The factors associated with antimicrobial-resistant gonorrhoea are not necessarily those associated with a higher risk of gonorrhoea.

Ethnicity and sexual risk in heterosexual people attending sexual health clinics in England: a cross-sectional, self-administered questionnaire study

Objectives

In the UK, people of black ethnicity experience a disproportionate burden of HIV and STI. We aimed to assess the association of ethnicity with sexual behaviour and sexual health among women and heterosexual men attending genitourinary medicine (GUM) clinics in England.

Methods

The Attitudes to and Understanding of Risk of Acquisition of HIV is a cross-sectional, self-administered questionnaire study of HIV negative people recruited from 20 GUM clinics in England, 2013–2014. Modified Poisson regression with robust SEs was used to calculate adjusted prevalence ratios (aPR) for the association between ethnicity and various sexual risk behaviours, adjusted for age, study region, education and relationship status.

Results

Questionnaires were completed by 1146 individuals, 676 women and 470 heterosexual men. Ethnicity was recorded for 1131 (98.8%) participants: 550 (48.6%) black/mixed African, 168 (14.9%) black/mixed Caribbean, 308 (27.2%) white ethnic groups, 105 (9.3%) other ethnicity. Compared with women from white ethnic groups, black/mixed African women were less likely to report condomless sex with a non-regular partner (aPR (95% CI) 0.67 (0.51 to 0.88)), black/mixed African and black/mixed Caribbean women were less likely to report two or more new partners (0.42 (0.32 to 0.55) and 0.44 (0.29 to 0.65), respectively), and black/mixed Caribbean women were more likely to report an STI diagnosis (1.56 (1.00 to 2.42)). Compared with men from white ethnic groups, black/mixed Caribbean men were more likely to report an STI diagnosis (1.91 (1.20 to 3.04)), but did not report risk behaviours more frequently. Men and women of black/mixed Caribbean ethnicity remained more likely to report STI history after adjustment for sexual risk behaviours.

Discussion

Risk behaviours were reported less frequently by women of black ethnicity; however, history of STI was more prevalent among black/mixed Caribbean women. In black/mixed Caribbean men, higher STI history was not explained by ethnic variation in reported risk behaviours. The association between STI and black/mixed Caribbean ethnicity remained after adjustment for risk behaviours.

A protocol for a cluster randomised feasibility study of an adolescent incentive intervention to increase uptake of HPV vaccination among girls

BACKGROUND

Uptake of the human papillomavirus (HPV) vaccine in the UK is good, but there are pockets of the community who remain unprotected. Immunisation teams usually require written parental consent for a girl to receive the vaccine. Evidence suggests that uptake of the vaccine might be improved by promoting consent form return (if returned, forms are likely to grant consent). Incentivising girls to return consent forms is a promising approach to promoting consent form return. Before testing the efficacy of an incentive intervention in a randomised controlled trial (RCT), we must first establish whether the RCT is feasible. In this randomised feasibility study, we aim to establish the feasibility of conducting a cluster RCT of an adolescent incentive intervention to increase uptake of HPV vaccination.

METHODS

At least six schools will be randomised to either an incentive intervention arm or a standard invitation arm. Girls in standard invitation arm schools will receive the usual HPV vaccine programme invitation materials. Girls attending schools in the incentive intervention arm will receive the standard invitation and will also be told that they will receive an incentive if they return their consent form (regardless of whether consent is granted or denied). The incentive is being entered into a prize draw to win a retail voucher. Feasibility objectives include estimating the schools' and parents' willingness to participate in the study and be randomised; response rates to questionnaires; the extent of missing data; the girls' and parents' attitudes towards the incentive offered; school staff experiences of participating, fidelity to the trial procedures, data on any unintended consequences and the possible mechanisms of action, and proof-of-concept evidence of the effect of the intervention on consent form return rates and uptake of the vaccine. Analysis of feasibility outcomes will primarily be descriptive. Consent form return rates and uptake of the vaccine will be presented by trial arm without comparison.

DISCUSSION

Incentivising HPV vaccine consent form return may promote HPV vaccine uptake. This study will provide the evidence needed to establish whether testing this incentive intervention using a RCT design in the future is feasible.

TRIAL REGISTRATION

ISRCTN72136061.

Patterns of sexual mixing with respect to social, health and sexual characteristics among heterosexual couples in England: analyses of probability sample survey data

Patterns of sexual mixing are major determinants of sexually transmitted infection (STI) transmission, in particular the extent to which high-risk populations mix with low-risk populations. However, patterns of mixing in the general population are poorly understood. We analysed data from a national probability sample survey of households, the Health Survey for England 2010. A total of 943 heterosexual couples living together, where at least one partner was aged between 16–44 years, were included. We used correlation coefficients to measure the strength of similarities between partners with respect to demographic characteristics, general health, health behaviours and sexual history. Males were on average 2 years older than their female partners, although this age difference ranged from a median of 0 years in men aged 16–24 years to a median of 2 years in men aged 35–44 years. A positive correlation between partners was found for all demographic characteristics. With respect to general health and health behaviours, a strongly positive correlation was found between men and women in reporting alcohol consumption at ⩾3 days a week and smoking. Men typically reported greater numbers of sexual partners than their female partner, although men and women with more partners were more likely to mix with each other. We have been able to elucidate the patterns of sexual mixing between men and women living together in England. Mixing based on demographic characteristics was more assortative than sexual characteristics. These data can better inform mathematical models of STI transmission.

Who mixes with whom among men who have sex with men? Implications for modelling the HIV epidemic in southern India

In India, the identity of men who have sex with men (MSM) is closely related to the role taken in anal sex (insertive, receptive or both), but little is known about sexual mixing between identity groups. Both role segregation (taking only the insertive or receptive role) and the extent of assortative (within-group) mixing are known to affect HIV epidemic size in other settings and populations. This study explores how different possible mixing scenarios, consistent with behavioural data collected in Bangalore, south India, affect both the HIV epidemic, and the impact of a targeted intervention. Deterministic models describing HIV transmission between three MSM identity groups (mostly insertive Panthis/Bisexuals, mostly receptive Kothis/Hijras and versatile Double Deckers), were parameterised with behavioural data from Bangalore. We extended previous models of MSM role segregation to allow each of the identity groups to have both insertive and receptive acts, in differing ratios, in line with field data. The models were used to explore four different mixing scenarios ranging from assortative (maximising within-group mixing) to disassortative (minimising within-group mixing). A simple model was used to obtain insights into the relationship between the degree of within-group mixing, R0 and equilibrium HIV prevalence under different mixing scenarios. A more complex, extended version of the model was used to compare the predicted HIV prevalence trends and impact of an HIV intervention when fitted to data from Bangalore. With the simple model, mixing scenarios with increased amounts of assortative (within-group) mixing tended to give rise to a higher R0 and increased the likelihood that an epidemic would occur. When the complex model was fit to HIV prevalence data, large differences in the level of assortative mixing were seen between the fits identified using different mixing scenarios, but little difference was projected in future HIV prevalence trends. An oral pre-exposure prophylaxis (PrEP) intervention was modelled, targeted at the different identity groups. For intervention strategies targeting the receptive or receptive and versatile MSM together, the overall impact was very similar for different mixing patterns. However, for PrEP scenarios targeting insertive or versatile MSM alone, the overall impact varied considerably for different mixing scenarios; more impact was achieved with greater levels of disassortative mixing.

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Different mixing scenarios are explored for 3 groups of role-segregated MSM.

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Models show that the mixing scenario affects both R₀ and endemic HIV prevalence.

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When models are fit to data, predicted HIV trends are unaffected by mixing.

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Impact of targeted (but not non-targeted) interventions can be affected by mixing.

Cross-cultural differences and sexual risk behavior of emerging adults

PURPOSE

The authors examined population-specific risk factors that increase emerging adults' risk of acquiring sexually transmitted infections (STIs), including the human papillomavirus (HPV).

DESIGN AND METHOD

A cross-sectional sample of 335 diverse, emerging adults ages 18 to 24 years was recruited from a health center at a large university in the Southeastern United States. The mean age was 20.6 ± 1.9 years, majority were females (74.0%), and 61.0% were Hispanic.

RESULTS

Findings revealed inconsistent condom use, reasons for not using condoms, and a need for more culturally specific intervention strategies.

DISCUSSION AND CONCLUSIONS

Healthcare providers should identify culturally specific reasons for inconsistent condom use, examine cultural and geographic differences in sexual risk behaviors among groups and communities, and modify communication, educational programs, and interventions accordingly.

IMPLICATIONS FOR PRACTICE

By adopting a multicultural approach to the control of STIs, nurses can address specific cultural attitudes and behaviors that may influence exposure to STIs, including HPV.

Simulation models for socioeconomic inequalities in health: a systematic review

Background: The emergence and evolution of socioeconomic inequalities in health involves multiple factors interacting with each other at different levels. Simulation models are suitable for studying such complex and dynamic systems and have the ability to test the impact of policy interventions in silico. Objective: To explore how simulation models were used in the field of socioeconomic inequalities in health. Methods: An electronic search of studies assessing socioeconomic inequalities in health using a simulation model was conducted. Characteristics of the simulation models were extracted and distinct simulation approaches were identified. As an illustration, a simple agent-based model of the emergence of socioeconomic differences in alcohol abuse was developed. Results: We found 61 studies published between 1989 and 2013. Ten different simulation approaches were identified. The agent-based model illustration showed that multilevel, reciprocal and indirect effects of social determinants on health can be modeled flexibly. Discussion and Conclusions: Based on the review, we discuss the utility of using simulation models for studying health inequalities, and refer to good modeling practices for developing such models. The review and the simulation model example suggest that the use of simulation models may enhance the understanding and debate about existing and new socioeconomic inequalities of health frameworks.

Racial disparities in HIV prevalence and risk behaviors among injection drug users and members of their risk networks

In the United States, racial differences in the prevalence and incidence of HIV infection and AIDS diagnoses are dramatic. These differences are large, have been recognized for nearly 20 years, and are as yet not well investigated. These disparities show no signs of diminishing and, in fact, are widening, particularly among drug users and women. Most observers of the racial disparities in prevalence and incidence of HIV infections and AIDS diagnoses in the United States have concluded that these disparities exist because prevention messages, supplies, and/or interventions do not effectively reach those at greatest risk of infection. In essence, such interpretations suggest that Blacks and Latinos continue to practice more risk behaviors than Whites. There are much data to suggest that this is, in fact, not true. Evidence from 232 'index' injection drug users and 465 of their drug and sexual network members participating in HIV Prevention Trials Network 037 is presented. These data describe lower use and/or access to drug treatment and needle exchange programs by Black injectors. In addition, data indicate the coexistence of increased prevalence of HIV in the networks of uninfected Black drug users and fewer associated risk behaviors in the networks of Black and Latino indices compared with networks of White indices. Understanding racial disparities in HIV is a critical challenge; yet, risk behaviors alone do not explain observed disparities in infection rates.

Predicting individual affect of health interventions to reduce HPV prevalence

Recently, human papilloma virus (HPV) has been implicated to cause several throat and oral cancers and HPV is established to cause most cervical cancers. A human papilloma virus vaccine has been proven successful to reduce infection incidence in FDA clinical trials, and it is currently available in the USA. Current intervention policy targets adolescent females for vaccination; however, the expansion of suggested guidelines may extend to other age groups and males as well. This research takes a first step toward automatically predicting personal beliefs, regarding health intervention, on the spread of disease. Using linguistic or statistical approaches, sentiment analysis determines a text's affective content. Self-reported HPV vaccination beliefs published in web and social media are analyzed for affect polarity and leveraged as knowledge inputs to epidemic models. With this in mind, we have developed a discrete-time model to facilitate predicting impact on the reduction of HPV prevalence due to arbitrary age- and gender-targeted vaccination schemes.

Early sexual risk among black and minority ethnicity teenagers: a mixed methods study

PURPOSE

To determine how ethnic background influences early sexual activity among young adults.

METHODS

Quantitative data were collected during the Research with East London Adolescents Community Health Survey study, a population-based survey of young adults belonging to white and black and minority ethnic groups and residing in east London in 2001 (n = 2,689) and 2003 (n = 2,675). Qualitative data were obtained from 146 young adults between January and September 2003.

RESULTS

Black Caribbean, black African, white other, and mixed ethnicity young men were most likely to report high-risk sexual behaviors, that is, sexual debut at the age of ≤13 years, having unprotected sex, and having multiple sexual partners. There were marked variations within groups commonly collapsed as "black" or as "Muslim." Black Caribbean and black African young adults reported high rates of protective behaviors in addition to risk behaviors. Qualitative data confirmed variations in sexual behavior within ethnic groups. Longitudinally, risk of engaging in two or more high-risk sexual behaviors was predicted by low family support (OR: 2.8, 95% CI: 1.6-4.9), regular smoking (OR: 4.5, 95% CI: 1.7-12.0), and usage of illicit drugs (OR: 2.9, 95% CI: 1.5-5.8), with lower risk predicted by low peer support (OR: .3, 95% CI: .2-.6).

CONCLUSIONS

Young adults belonging to black and minority ethnic groups reported a wide variation in sexual risk behaviors. High levels of high-risk behaviors were reported in ethnic groups known to have high rates of sexually transmitted infections. Effective sexual health interventions should be started early and they must focus on sexual debut and partner choices as well as messages regarding safe sex.

Racial/ethnic and sexual behavior disparities in rates of sexually transmitted infections, San Francisco, 1999-2008

Background

Racial/ethnic minorities and men who have sex with men (MSM) represent populations with disparate sexually transmitted infection (STI) rates. While race-specific STI rates have been widely reported, STI rates among MSM is often challenging given the absence of MSM population estimates. We evaluated the race-specific rates of chlamydia and gonorrhea among MSM and non-MSM in San Francisco between 1999-2008.

Methods

2000 US Census data for San Francisco was used to estimate the number of African-American, Asian/Pacific Islander, Hispanic, and white males. Data from National HIV Behavioral Surveillance (NHBS) MSM 1, conducted in 2004, was used to estimate the total number of MSM in San Francisco and the size of race/ethnic sub-populations of MSM. Non-MSM estimates were calculated by subtracting the number of estimated MSM from the total number of males residing in San Francisco. Rates of MSM and non-MSM gonorrhea and chlamydia reported between 1999 and 2008 were stratified by race/ethnicity. Ratios of MSM and non-MSM rates of morbidity were calculated by race/ethnicity.

Results

Between 1999-2008, MSM accounted for 72% of gonorrhea cases and 51% of chlamydia cases. Throughout the study period, African-American MSM had the highest chlamydia rate with 606 cases per 100,000 in 1999 increasing to 2067 cases per 100,000 in 2008. Asian/Pacific Islander MSM consistently had the lowest rate among MSM with1003 cases per 100,000 in 2008. The ratio of MSM/non-MSM for chlamydia was highest among whites 11.6 (95% CI: 8.8-14.4) and Asian/Pacific Islanders 8.6 (95% CI: 6.2-11), and lowest among African-Americans 1.53 (95% CI: 1.2-1.9) and Hispanics 4.43 (95% CI: 2.8-6.0). Gonorrhea rates were similar for African-American, white, and Hispanic MSM between 2137-2441 cases per 100,000 in 2008. Asian/Pacific Islander MSM had the lowest gonorrhea rate with 865 cases per 100,000 in 2008. The ratio of MSM/non-MSM for gonorrhea was highest among whites 11.6 (95% CI: 8.8-14.4) and Asian/Pacific Islanders 8.6 (95% CI: 6.2-11), and lowest among African-Americans 1.53 (95% CI: 1.2-1.9) and Hispanics 4.43 (95% CI: 2.8-6.0).

Conclusions

For all racial/ethnic groups in San Francisco, MSM carried a substantially higher burden of STIs compared to non-MSM except among African-American men. These racial and sexual behavior disparities warrant further public health attention and resources.

A Bayesian approach to uncertainty analysis of sexually transmitted infection models

OBJECTIVES

To propose a Bayesian approach to uncertainty analysis of sexually transmitted infection (STI) models, which can be used to quantify uncertainty in model assessments of policy options, estimate regional STI prevalence from sentinel surveillance data and make inferences about STI transmission and natural history parameters.

METHODS

Prior distributions are specified to represent uncertainty regarding STI parameters. A likelihood function is defined using a hierarchical approach that takes account of variation between study populations, variation in diagnostic accuracy as well as random binomial variation. The method is illustrated using a model of syphilis, gonorrhoea, chlamydial infection and trichomoniasis in South Africa.

RESULTS

Model estimates of STI prevalence are in good agreement with observations. Out-of-sample projections and cross-validations also show that the model is reasonably well calibrated. Model predictions of the impact of interventions are subject to significant uncertainty: the predicted reductions in the prevalence of syphilis by 2020, as a result of doubling the rate of health seeking, increasing the proportion of private practitioners using syndromic management protocols and screening all pregnant women for syphilis, are 43% (95% CI 3% to 77%), 9% (95% CI 1% to 19%) and 6% (95% CI 4% to 7%), respectively.

CONCLUSIONS

This study extends uncertainty analysis techniques for fitted HIV/AIDS models to models that are fitted to other STI prevalence data. There is significant uncertainty regarding the relative effectiveness of different STI control strategies. The proposed technique is reasonable for estimating uncertainty in past STI prevalence levels and for projections of future STI prevalence.

A metapopulation modelling framework for gonorrhoea and other sexually transmitted infections in heterosexual populations

Gonorrhoea continues to be a public health problem in the UK, and is the second most common bacterial sexually transmitted infection (STI) after chlamydia. In the UK, gonorrhoea is disproportionately concentrated in epidemiologically distinct subpopulations, with much higher incidence rates in young people, some ethnic minorities and inner city subpopulations. The original model of STI transmission proposed by Hethcote and Yorke explained some of these features through the concept of the 'core group'. Since then, several authors have modified the original model approach to include multiple sexual activity classes, but found this modelling approach to be inadequate when applied to low-prevalence settings such as the UK. We present a metapopulation framework for modelling gonorrhoea and other STIs. The model proposes that the epidemiology of gonorrhoea is largely driven by subpopulations with higher than average concentrations of individuals with high sexual risk activity. We show how this conceptualization of gonococcal epidemiology overcomes key limitations associated with some of the prior efforts to model gonorrhoea. We also use the model to explain several epidemiological features of gonorrhoea, such as its asymmetric distribution across subpopulations, and the contextual risk experienced by members of at-risk subpopulations. Finally, we extend the model to explain the distribution of other STIs, using chlamydia as an example of a more ubiquitous bacterial STI.

Heterosexual behavior patterns and the spread of HIV/AIDS: the interacting effects of rate of partner change and sexual mixing

OBJECTIVES

This study evaluates whether the influence of sexual mixing patterns on the human immunodeficiency virus (HIV) epidemic curve is sensitive to the prevailing rates of sexual partner change in a population.

STUDY DESIGN

A biobehavioral macrosimulation model is employed to assess the interacting dynamics of the rates of sexual partner change and patterns of sexual mixing between population subgroups. HIV spread is simulated under 2 rates of partner change scenarios and under various degrees of assortativeness in sexual mixing patterns.

RESULTS

With high rates of partner change, disassortativeness in sexual mixing tends to increase the overall size of the HIV epidemic. However, when relatively low rates of partner change are simulated, disassortative mixing yields a smaller epidemic. This pattern is further influenced by the underlying sexual transmission probabilities of HIV.

CONCLUSIONS

Each of the various determinants of the sexual spread of HIV must not be considered in isolation. Instead, the interactive nature of those determinants should be accounted for in discussions of HIV epidemic dynamics.

Marijuana use and sexually transmitted infections in young women who were teenage mothers

PURPOSE

Teenage pregnancy and marijuana use are associated with higher risk of contracting sexually transmitted infections (STIs). In this study, we examined the role of early and current marijuana use as it related to STI risk in a sample of young women who were pregnant teenagers, using a variety of statistical models.

METHODS

We recruited 279 pregnant adolescents, ages 12-18, from an urban prenatal clinic as part of a study that was developed to evaluate the long-term effects of prenatal substance exposure. Six years later, they were asked about their substance use and sexual history. The association of early and late marijuana use to lifetime sexual partners and STIs was examined, and then structural equation modeling (SEM) was used to illustrate the associations among marijuana use, number of sexual partners, and STIs.

RESULTS

Bivariate analyses revealed a dose-response effect of early and current marijuana use on STIs in young adulthood. Early and current marijuana use also predicted a higher number of lifetime sexual partners. However, the effect of early marijuana use on STIs was mediated by lifetime number of sexual partners in the SEM, whereas African-American race, more externalizing problems, and a greater number of sexual partners were directly related to more STIs.

CONCLUSIONS

Adolescent pregnancy, early marijuana use, mental health problems, and African-American race were significant risk factors for STIs in young adult women who had become mothers during adolescence. Pregnant teenage girls should be screened for early drug use and mental health problems, because they may benefit the most from the implementation of STI screening and skill-based prevention programs.

'People make assumptions about our communities': sexual health amongst teenagers from black and minority ethnic backgrounds in East london

OBJECTIVES

(1) To explore sexual behaviour and relationships amongst Black and minority ethnic (BME) teenagers in East London. (2) To examine how these relationships are shaped by culture, gender, peer norms and religion. (3) To describe the implications for sexual health policy and practice in urban, multicultural areas.

DESIGN

This report draws primarily on the qualitative arm of a mixed methods study which collected data from 126 young people, aged 15-18, largely through focus groups in the London boroughs of Hackney, Newham and Tower Hamlets.

RESULTS

Previous research has reported culture influencing the patterning of risk/protection amongst BME groups. Our data suggest that this is mediated by gender, religion and youth. Religion reportedly influenced young women's sexual behaviour in multiple ways. Young people described gendered norms in meeting and flirting with partners, and the role of mobile phones and peer pressure.

CONCLUSION

Our paper suggests culture, gender, religion and youth influence BME teenagers in aspects of sexual relationships, and that these social markers may have different contextual meanings for individuals. The multiplicity of factors affecting attitudes/behaviour requires a range of contraceptive, counselling, screening and sex education services available for all teenagers, although delivery patterns may differ in response to differing needs.

The role of compensatory mutations in the emergence of drug resistance

Pathogens that evolve resistance to drugs usually have reduced fitness. However, mutations that largely compensate for this reduction in fitness often arise. We investigate how these compensatory mutations affect population-wide resistance emergence as a function of drug treatment. Using a model of gonorrhea transmission dynamics, we obtain generally applicable, qualitative results that show how compensatory mutations lead to more likely and faster resistance emergence. We further show that resistance emergence depends on the level of drug use in a strongly nonlinear fashion. We also discuss what data need to be obtained to allow future quantitative predictions of resistance emergence.

Sexual bridging socially and over time: a simulation model exploring the relative effects of mixing and concurrency on viral sexually transmitted infection transmission

BACKGROUND

Sexual partnerships between people at higher and lower risk for sexually transmitted infections (STIs) (i.e., bridging) occur through dissortative mixing and concurrent partnerships, yet the relative effects of these network patterns on population STI spread are poorly understood.

GOAL

Using a stochastic model, the authors investigated the impact of mixing and concurrency on the spread of a persistent viral STI.

STUDY DESIGN

A total of 1,050 populations were simulated of 1,000 subjects over 400 weeks with varied concurrency levels and mixing patterns. STI prevalence and the average number of secondary transmissions per subject were analyzed with regression.

RESULTS

Mixing had a greater impact on prevalence for all groups, whereas concurrency was significant for only the lowest activity group. Mixing patterns moderated the magnitude of concurrency's impact on secondary transmissions.

CONCLUSIONS

Through connecting subgroups of differential risk, sexual mixing facilitates dissemination of STIs throughout a population. Concurrency expedites transmission by shortening the time between sexual contacts among infected and susceptible persons, particularly during the highly infectious period.

Modelling the spread of HIV/AIDS in China: the role of sexual transmission

The study presented here is an exploration of the implications of patterns of sexual behaviour for the spread of HIV in China, using a bio-behavioural macrosimulation model. To reflect the uncertainty surrounding key parameters, analyses of varied scenarios are used to show a range of possible outcomes consistent with variations in selected biological and behavioural inputs. The latter are estimated from a nationwide survey of sexual behaviour recently conducted in China, a country with an emerging HIV/AIDS epidemic, where it is feared that HIV/AIDS will spread to the general population via heterosexual transmission. The results highlight the primacy of the levels and distribution of sexual activity in the population. They offer some guidelines for understanding and interpreting the potential implications of current and prospective changes in sexual behaviour for the spread of HIV/AIDS in the world's largest population, and also highlight the need to collect better data on sexual behaviour for the estimation of key model inputs.