Age differences between sexual partners, behavioural and demographic correlates, and HIV infection on Likoma Island, Malawi

Age-disparate and intergenerational sex partnerships and HIV: the role of gender norms among adolescent girls and young women in Malawi

BACKGROUND

Age-mixing (age-disparate [5-9 years difference] and intergenerational [≥ 10 years difference]) partnerships are hypothesized drivers of HIV in adolescent girls and young women (AGYW; 15-24 years). These partnerships are often associated with increased gender inequities which undermine women's agency and assertiveness. We assessed whether age-mixing partnerships were associated with HIV in Malawi and if endorsement of inequitable gender norms modifies this relationship.

METHODS

We analyzed data from the Malawi Population-based HIV Impact Assessment, a nationally representative household survey conducted in 2015-2016. Participants underwent HIV testing and completed questionnaires related to actively endorsed gender norms and sexual risk behavior. We used multivariate logistic regression and multiplicative interaction to assess associations among AGYW who reported the age of their primary sex partner from the last year.

RESULTS

The analysis included 1,958 AGYW (mean age = 19.9 years, SD = 0.1), 459 (23.4%) and 131 (6.7%) of whom reported age-disparate and intergenerational partnerships, respectively. AGYW in age-mixing partnerships accounted for 13% of all AGYW and were older, more likely to reside in urban areas, to be married or cohabitating with a partner, and to have engaged in riskier sexual behavior compared with AGYW in age-concordant partnerships (p < 0.05). HIV prevalence among AGYW in age-disparate and intergenerational partnerships was 6.1% and 11.9%, respectively, compared with 3.2% in age-concordant partnerships (p < 0.001). After adjusting for residence, age, education, employment, wealth quintile, and ever been married or cohabitated as married, AGYW in age-disparate and intergenerational partnerships had 1.9 (95% CI: 1.1-3.5) and 3.4 (95% CI: 1.6-7.2) greater odds of HIV, respectively, compared with AGYW in age-concordant partnerships. Among the 614 (31% of the study group) who endorsed inequitable gender norms, AGYW in age-disparate and intergenerational partnerships had 3.5 (95% CI: 1.1-11.8) and 6.4 (95% CI: 1.5-27.8) greater odds of HIV, respectively, compared with AGYW in age-concordant partnerships.

CONCLUSIONS

In this Malawi general population survey, age-mixing partnerships were associated with increased odds of HIV among AGYW. These findings highlight inequitable gender norms as a potential focus for HIV prevention and could inform interventions targeting structural, cultural, and social constraints of this key group.

The epidemiology of HIV population viral load in twelve sub-Saharan African countries

BACKGROUND

We examined the epidemiology and transmission potential of HIV population viral load (VL) in 12 sub-Saharan African countries.

METHODS

We analyzed data from Population-based HIV Impact Assessments (PHIAs), large national household-based surveys conducted between 2015 and 2019 in Cameroon, Cote d'Ivoire, Eswatini, Kenya, Lesotho, Malawi, Namibia, Rwanda, Tanzania, Uganda, Zambia, and Zimbabwe. Blood-based biomarkers included HIV serology, recency of HIV infection, and VL. We estimated the number of people living with HIV (PLHIV) with suppressed viral load (<1,000 HIV-1 RNA copies/mL) and with unsuppressed viral load (viremic), the prevalence of unsuppressed HIV (population viremia), sex-specific HIV transmission ratios (number female incident HIV-1 infections/number unsuppressed male PLHIV per 100 persons-years [PY] and vice versa) and examined correlations between a variety of VL metrics and incident HIV. Country sample sizes ranged from 10,016 (Eswatini) to 30,637 (Rwanda); estimates were weighted and restricted to participants 15 years and older.

RESULTS

The proportion of female PLHIV with viral suppression was higher than that among males in all countries, however, the number of unsuppressed females outnumbered that of unsuppressed males in all countries due to higher overall female HIV prevalence, with ratios ranging from 1.08 to 2.10 (median: 1.43). The spatial distribution of HIV seroprevalence, viremia prevalence, and number of unsuppressed adults often differed substantially within the same countries. The 1% and 5% of PLHIV with the highest VL on average accounted for 34% and 66%, respectively, of countries' total VL. HIV transmission ratios varied widely across countries and were higher for male-to-female (range: 2.3-28.3/100 PY) than for female-to-male transmission (range: 1.5-10.6/100 PY). In all countries mean log10 VL among unsuppressed males was higher than that among females. Correlations between VL measures and incident HIV varied, were weaker for VL metrics among females compared to males and were strongest for the number of unsuppressed PLHIV per 100 HIV-negative adults (R2 = 0.92).

CONCLUSIONS

Despite higher proportions of viral suppression, female unsuppressed PLHIV outnumbered males in all countries examined. Unsuppressed male PLHIV have consistently higher VL and a higher risk of transmitting HIV than females. Just 5% of PLHIV account for almost two-thirds of countries' total VL. Population-level VL metrics help monitor the epidemic and highlight key programmatic gaps in these African countries.

A multi-country cross-sectional study to assess predictors of daily versus on-demand oral pre-exposure prophylaxis in youth from South Africa, Uganda and Zimbabwe

INTRODUCTION

Sub-Saharan Africa (SSA) carries the burden of the HIV epidemic, especially among adolescents and young people (AYP). Little is known about pre-exposure prophylaxis (PrEP) uptake and preferences among AYP in SSA. We describe preferences for daily and on-demand PrEP among AYP in South Africa, Uganda and Zimbabwe.

METHODS

A cross-sectional survey was conducted in 2019 among 13- to 24-year olds, capturing socio-demographics, HIV risk behaviours and preferences for daily or on-demand PrEP. Logistic regression models were used to estimate odds ratios, adjusting for site, sex and age.

RESULTS AND DISCUSSION

A total of 1330 participants from Cape Town (n = 239), Johannesburg (n = 200), Entebbe (n = 491) and Chitungwiza (n = 400) were enrolled; 673 (51%) were male, and the median age was 19 years (interquartile range 17-22 years). Of 1287 participants expressing a preference, 60% indicated a preference for on-demand PrEP with differences by site (p < 0.001), sex (p < 0.001) and age group (p = 0.003). On-demand PrEP was most preferred in Entebbe (75%), among males (65%) versus females (54%) and in older participants (62% in 18- to 24-year-olds vs. 47% in 13- to 15-year-olds). After adjusting for site, sex and age group, preference for on-demand PrEP decreased as sex frequency over the past month increased (p-trend = 0.004) and varied with the number of partners in the last 6 months, being least popular among those reporting four or more partners (p = 0.02). Participants knowing further in advance that they were likely to have sex were more likely to prefer on-demand PrEP (p-trend = 0.02). Participants having a larger age gap with their most recent partner and participants whose last partner was a transactional sex partner or client were both less likely to prefer on-demand compared to daily PrEP (p = 0.05 and p = 0.09, respectively). Participants who knew their most recent partner was living with HIV or who did not know the HIV status of their most recent partner were less likely to prefer on-demand PrEP (p = 0.05).

CONCLUSIONS

Our data show that AYP in four SSA communities prefer on-demand over daily PrEP options, with differences seen by site, age and sex. PrEP demand creation needs to be reviewed, optimized and tailored to socio-demographic differences and designed in conjunction with AYP.

Exposure to Sex Education and Its Effects on Adolescent Sexual Behavior in Nigeria

Sexual behavior during adolescence fundamentally steers the future life of both girls and boys, and it should be guided with appropriate education, especially as it also represents a key factor to be considered in attainment of sustainable developmental goals. The study assessed the effect of exposure to sex education on adolescents' sexual behavior. The primary and cross-sectional survey data used for this study were analyzed using basic descriptive statistics and binary logistic regression analytical technique. The results, among others, highlighted that the most common sources of first sex education among the respondents are school (54.6%), family (21.6%), social media (9.5%), and others like television (7.6%) and books or magazines (4.9%). Frequency of discussion on sexual matters is positively associated with the use of protection such as condom (β = 0.261; p ≤ 0.01). The study gave support to the increasing pursuit of sexuality education. However, since adolescents' needs could vary by demographics, streamlining sex education need by age and sex characteristics could enhance its effectiveness.

The determinants and impacts of age-disparate relationships on women in Zimbabwe: A life course perspective

Age-disparate relationships (ADR) with older men have been studied mostly in the context of HIV and found to be associated with increased HIV prevalence among young women in sub-Saharan Africa. Less is known about the impact of ADR on the broader life course of women. The objectives of this study are to identify the factors associated with being in ADR and estimate the association between ADR and a set of life outcomes in Manicaland, Zimbabwe. We used data from a general population open-cohort survey from 1998 to 2013 in Manicaland. We applied binary logistic regression models to estimate the odds ratios for association between socio-demographic determinants and ADR and multinomial logistic regression models to estimate the association between ADR and women's life outcomes. We found that women with less education, younger age at first sex and first marriage were more likely to be in ADR, and women in ADR have male partners with less education and less skilled employment. In terms of life and relationship outcomes, women in ADR had mostly negative life outcomes compared to women not in ADR. Future policies and research on ADR in women should reflect these complexities and study a wider range of life outcomes, beyond the commonly studied narrower topics such as HIV.

Assessing the uncertainty around age-mixing patterns in HIV transmission inferred from phylogenetic trees

Understanding age-mixing patterns in Human Immunodeficiency Virus (HIV) transmission networks can enhance the design and implementation of HIV prevention strategies in sub-Saharan Africa. Due to ethical consideration, it is less likely possible to conduct a benchmark study to assess which sampling strategy, and sub-optimal sampling coverage which can yield best estimates for these patterns. We conducted a simulation study, using phylogenetic trees to infer estimates of age-mixing patterns in HIV transmission, through the computation of proportions of pairings between men and women, who were phylogenetically linked across different age groups (15-24 years, 25-39 years, and 40-49 years); and the means, and standard deviations of their age difference. We investigated also the uncertainty around these estimates as a function of the sampling coverage in four sampling strategies: when missing sequence data were missing completely at random (MCAR), and missing at random (MAR) with at most 30%-50%-70% of women in different age groups being in the sample. The results suggested that age-mixing patterns in HIV transmission can be unveiled from proportions of phylogenetic pairings between men and women across age groups; and the mean, and standard deviation of their age difference. A 55% sampling coverage was sufficient to provide the best values of estimates of age-mixing patterns in HIV transmission with MCAR scenario. But we should be cautious in interpreting proportions of men phylogenetically linked to women because they may be overestimated or underestimated, even at higher sampling coverage. The findings showed that, MCAR was the best sampling strategy. This means, it is advisable not to use sequence data collected in settings where we can find a systematic imbalance of age and gender to investigate age-mixing in HIV transmission. If not possible, ensure to take into consideration the imbalance in interpreting the results.

Variations in HIV Risk by Young Women's Age and Partner Age Disparity in Rural South Africa (HPTN 068)

BACKGROUND

Nearly all population-level research showing positive associations between age-disparate partnerships and HIV acquisition among adolescent girls and young women (AGYW) has classified age disparity as ≥5 or ≥10 years. We describe variations in 1-year risk of HIV infection after exposure to sexual partner(s) of continuous age disparities.

METHODS

Longitudinal data from the HPTN 068 randomized trial in South Africa were used to estimate 1-year risk of HIV infection at various age pairings. The parametric g-formula was used to estimate risk at up to 5 annual time points, stratified by maximum partner age difference, maximum partner age, and AGYW age.

RESULTS

AGYW reported an older partner in 86% of 5351 age pairings. The 1-year risk of HIV infection rapidly increased with maximum partner age difference among girls ages 13-14 years, from 0·01 with a same-age partner, to 0·21 with a partner 10 years older, and 0·24 with a partner 15 years older. A gradual increase occurred among AGYW ages 15-16 years, up to 0·13 with a partner 15 years older, and 0·09 among AGYW 17-18 years with partners 8-11 years older. Risk of HIV infection among AGYW ages 19-21 years remained relatively constant across maximum partner age differences.

CONCLUSIONS

Age differences between AGYW and their sexual partners have a greater effect on HIV-risk infection in younger compared with older AGYW. Considering both the age of an AGYW and her sexual partners provides granular insight into identifying key groups for HIV transmission prevention efforts.

Social, Behavioral, and Cultural factors of HIV in Malawi: Semi-Automated Systematic Review

BACKGROUND

Demographic and sociobehavioral factors are strong drivers of HIV infection rates in sub-Saharan Africa. These factors are often studied in qualitative research but ignored in quantitative analyses. However, they provide in-depth insight into the local behavior and may help to improve HIV prevention.

OBJECTIVE

To obtain a comprehensive overview of the sociobehavioral factors influencing HIV prevalence and incidence in Malawi, we systematically reviewed the literature using a newly programmed tool for automatizing part of the systematic review process.

METHODS

Due to the choice of broad search terms ("HIV AND Malawi"), our preliminary search revealed many thousands of articles. We, therefore, developed a Python tool to automatically extract, process, and categorize open-access articles published from January 1, 1987 to October 1, 2019 in the PubMed, PubMed Central, JSTOR, Paperity, and arXiV databases. We then used a topic modelling algorithm to classify and identify publications of interest.

RESULTS

Our tool extracted 22,709 unique articles; 16,942 could be further processed. After topic modelling, 519 of these were clustered into relevant topics, of which 20 were kept after manual screening. We retrieved 7 more publications after examining the references so that 27 publications were finally included in the review. Reducing the 16,942 articles to 519 potentially relevant articles using the software took 5 days. Several factors contributing to the risk of HIV infection were identified, including religion, gender and relationship dynamics, beliefs, and sociobehavioral attitudes.

CONCLUSIONS

Our software does not replace traditional systematic reviews, but it returns useful results to broad queries of open-access literature in under a week, without a priori knowledge. This produces a "seed dataset" of relevance that could be further developed. It identified known factors and factors that may be specific to Malawi. In the future, we aim to expand the tool by adding more social science databases and applying it to other sub-Saharan African countries.

Risk of HIV infection among adolescent girls and young women in age-disparate relationships in sub-Saharan Africa

OBJECTIVE

To determine the association between age-disparate relationships and risk of HIV infection among adolescent girls and young women (AGYW) aged 15-24 years.

DESIGN

Systematic review and meta-analysis of published studies until January 5, 2020 in sub Saharan Africa (SSA).

METHODS

We searched several electronic databases, grey literature, and hand searched reference list of included studies to identify eligible studies for data abstraction. We assessed the quality of included studies using Newcastle-Ottawa Scale for nonrandomized studies. The DerSimonian-Laird random effects model was used to pool the overall results using risk ratios (RR), presented in a forest plot with 95% confidence interval (CI) and predictive interval. Heterogeneity was assessed with Cochrane's Q-test and quantified with I values. Publication bias was checked with funnel plots and Egger's test.

RESULTS

We included 24 studies with an overall sample size of 33 390. Data show that age-disparate relationships were significantly associated with unprotected sexual intercourse (pooled RR, 1.57; 95% CI, 1.34-1.83; 95% predictive interval, 1.22-2.02), and higher risk for HIV infection (pooled RR, 1.39; 95 CI, 1.21-1.60; 95% predictive interval, 0.80-2.42). Studies included in pooling risk of unprotected sexual intercourse were largely homogeneous (I-value= 0.0, P = 0.79) whereas those for HIV infection were heterogeneous (I- value = 89.0%, P < 0.01). We found no publication bias and no study influenced the meta-analytic results.

CONCLUSION

Age-disparate relationships among AGYW are associated with increased risk of unprotected sexual intercourse and HIV infection in SSA. HIV prevention interventions should target this sub-population.

Structural Design and Data Requirements for Simulation Modelling in HIV/AIDS: A Narrative Review

Born out of a necessity for fiscal sustainability, simulation modeling is playing an increasingly prominent role in setting priorities for combination implementation strategies for HIV treatment and prevention globally. The design of a model and the data inputted into it are central factors in ensuring credible inferences. We executed a narrative review of a set of dynamic HIV transmission models to comprehensively synthesize and compare the structural design and the quality of evidence used to support each model. We included 19 models representing both generalized and concentrated epidemics, classified as compartmental, agent-based, individual-based microsimulation or hybrid in our review. We focused on four structural components (population construction; model entry, exit and HIV care engagement; HIV disease progression; and the force of HIV infection), and two analytical components (model calibration/validation; and health economic evaluation, including uncertainty analysis). While the models we reviewed focused on a variety of individual interventions and their combinations, their structural designs were relatively homogenous across three of the four focal components, with key structural elements influenced by model type and epidemiological context. In contrast, model entry, exit and HIV care engagement tended to differ most across models, with some health system interactions-particularly HIV testing-not modeled explicitly in many contexts. The quality of data used in the models and the transparency with which the data was presented differed substantially across model components. Representative and high-quality data on health service delivery were most commonly not accessed or were unavailable. The structure of an HIV model should ideally fit its epidemiological context and be able to capture all efficacious treatment and prevention services relevant to a robust combination implementation strategy. Developing standardized guidelines on evidence syntheses for health economic evaluation would improve transparency and help prioritize data collection to reduce decision uncertainty.

Age-disparate partnerships and HSV-2 among adolescent girls and young women in South Africa: implications for HIV infection risk

OBJECTIVE

There is an urgent need to understand high HIV-infection rates among young women in sub-Saharan Africa. While age-disparate partnerships have been characterised with high-risk sexual behaviours, the mechanisms through which these partnerships may increase HIV-risk are not fully understood. This study assessed the association between age-disparate partnerships and herpes simplex virus type-2 (HSV-2) infection, a factor known to increase HIV-infection risk.

METHODS

Cross-sectional face-to-face questionnaire data, and laboratory HSV-2 and HIV antibody data were collected among a representative sample in the 2014/2015 household survey of the HIV Incidence Provincial Surveillance System in KwaZulu-Natal, South Africa. Among 15-24-year-old women who reported having ever had sex (n=1550), the association between age-disparate partnerships (ie, male partner ≥5 years older) and HSV-2 antibody status was assessed using multivariable Poisson regression models with robust variance. Analyses were repeated among HIV-negative women.

RESULTS

HSV-2 prevalence was 55% among 15-24-year-old women. Women who reported an age-disparate partnership with their most recent partner were more likely to test HSV-2 positive compared with women with age-similar partners (64% vs 51%; adjusted prevalence ratio (aPR):1.19 (95% CI 1.07 to 1.32, p<0.01)). HSV-2 prevalence was also significantly higher among HIV-negative women who reported age-disparate partnerships (51% vs 40 %; aPR:1.25 (95% CI 1.05 to 1.50, p=0.014)).

CONCLUSIONS

Results indicate that age-disparate partnerships are associated with a greater risk of HSV-2 among young women. These findings point towards an additional mechanism through which age-disparate partnerships could increase HIV-infection risk. Importantly, by increasing the HSV-2 risk, age-disparate partnerships have the potential to increase the HIV-infection risk within subsequent partnerships, regardless of the partner age-difference in those relationships.

Who are the male sexual partners of adolescent girls and young women? Comparative analysis of population data in three settings prior to DREAMS roll-out

BACKGROUND

The DREAMS (Determined Resilient Empowered AIDS-free, Mentored and Safe) Partnership aims to reduce HIV incidence among adolescent girls and young women (AGYW,15-24y) with a core package of evidence-based interventions. Some interventions, including voluntary HIV counselling and testing and circumcision, will be targeted at the male sexual partners of AGYW. A priority of DREAMS is to characterise the male partners for effective targeting.

METHODS

Using population-based data (2010-2015) in three DREAMS impact evaluation settings in Kenya and South Africa, we describe the demographic characteristics and sexual behaviour of male partners reported by AGYW, and the characteristics of males who report sexual activity with AGYW.

RESULTS

In all settings, over 90% of recent male partners reported by AGYW were aged <35 years. Median ages of spousal and non-spousal partners were 29 and 23 years respectively in uMkhanyakude (rural South Africa) and 21 and 20 years respectively in Nairobi (urban Kenya). Most males reporting an AGYW partner had never been married (89%) and many were in school (39%). Most male partners reported only 1 AGYW partner in the past year; in Gem (rural Kenya) and Nairobi 25%-29% reported 2+(AGYW or older female) partners. Concurrent partners were reported by 16% of male partners in Gem and 3-4% in uMkhanyakude. Two thirds of male partners in Gem reported testing for HIV in the past 6 months and under half in uMkhanyakude reported testing for HIV in the past year. Almost all (96%) partners in Nairobi were circumcised, compared to 45% in Gem and 43% in uMkhanyakude.

CONCLUSIONS

With almost all AGYW's sexual partners aged 15-34 years, this is an appropriate target group for DREAMS interventions. Encouraging young men to reduce their number of partners and concurrency, and uptake prevention and treatment services such as HIV testing, circumcision and ART is crucial in the effort to reduce HIV among both AGYW and young men.

HIV Risk Among Adolescent Girls and Young Women in Age-Disparate Partnerships: Evidence From KwaZulu-Natal, South Africa

BACKGROUND

Evidence on the role of age-disparate partnerships in high HIV-infection rates among young women in sub-Saharan Africa remains inconclusive. This study examined the HIV-infection risk associated with age-disparate partnerships among 15- to 24-year-old women in a hyperendemic setting in South Africa.

METHODS

Face-to-face questionnaire, and laboratory HIV and viral load data were collected during 2014-2015 among a representative sample (15-49 years old) in KwaZulu-Natal. The association between age-disparate partnerships (age difference ≥5 years) and HIV status among 15- to 24-year-old women (N = 1459) was assessed using multiple logistic regression analyses. Data from the male sample on all on-going partnerships (N = 1229) involving 15- to 24-year-old women were used to assess whether young women's age-disparate male partners were more likely to have a viral load ≥1000 copies per milliliter, a marker of HIV-infection risk.

RESULTS

Women reporting an age disparity in any of their 3 most recent partnerships were more likely to test HIV positive compared to women with only age-similar partners [adjusted odds ratio (aOR): 1.58, 95% confidence interval (CI): 1.20 to 2.09, P < 0.01]. Among partnerships men reported with 15- to 24-year-old women, the age-disparate male partners were more likely to be HIV positive and have a viral load ≥1000 copies per milliliter (aOR: 2.05, 95% CI: 1.30 to 3.24, P < 0.01) compared with age-similar partners. Results were similar for each category of age disparity: partners 5-9 years older (aOR: 2.01, 95% CI: 1.18 to 3.43, P = 0.010) and those ≥10 years older (aOR: 2.17, 95% CI: 1.01-4.66, P = 0.048).

CONCLUSIONS

Results indicate that age-disparate partnerships increase young women's HIV risk, although conclusive evidence was not ascertained. Interventions addressing risk from age-disparate sexual partnering, including expanding antiretroviral treatment among older partners, may help to reduce HIV incidence among young women.

The role of age-mixing patterns in HIV transmission dynamics: Novel hypotheses from a field study in Cape Town, South Africa

BACKGROUND

Age-disparate relationships are thought to put young women at increased risk of HIV, though current evidence is inconclusive. Studying population-level age-mixing patterns as well as individual-level measures of age difference variation may provide insight into the persistence and magnitude of the epidemic in South Africa.

METHODS

We used data from a survey in Cape Town (n = 506) to describe age-mixing dynamics in the four population strata of HIV negative and HIV positive male and female participants. Mixed-effects models were used to calculate the average increase in partner age for each year increase in age of participant, the average partner age for 15 year olds, and the between-subject and the within-subject standard deviation of partner ages. We conducted 2000 bootstrap replications of the models. Using negative binomial models, we also explored whether HIV status was associated with participants having a larger range in partner ages.

RESULTS

HIV positive women had large variability in partner ages at the population level, and at the individual level had nearly three times the expected range of partner ages compared to HIV negative women. This pattern may increase the potential for HIV transmission across birth cohorts and may partially explain the persistence of the epidemic in South Africa. Young men, who have been previously absent from the age-disparity discourse, also choose older partners who may be putting them at increased risk of HIV infection due to the high HIV prevalence among older age categories of women.

Age difference between heterosexual partners in Britain: Implications for the spread of Chlamydia trachomatis

Heterosexual partners often differ in age. Integrating realistic patterns of sexual mixing by age into dynamic transmission models has been challenging. The effects of these patterns on the transmission of sexually transmitted infections (STI) including Chlamydia trachomatis (chlamydia), the most common bacterial STI are not well understood. We describe age mixing between new heterosexual partners using age- and sex-specific data about sexual behavior reported by people aged 16-63 years in the 2000 and 2010 British National Surveys of Sexual Attitudes and Lifestyles. We incorporate mixing patterns into a compartmental transmission model fitted to age- and sex-specific, chlamydia positivity from the same surveys, to investigate C. trachomatis transmission. We show that distributions of ages of new sex partners reported by women and by men in Britain are not consistent with each other. After balancing these distributions, new heterosexual partnerships tend to involve men who are older than women (median age difference 2, IQR -1, 5 years). We identified the most likely age combinations of heterosexual partners where incident C. trachomatis infections are generated. The model results show that in >50% of chlamydia transmitting partnerships, at least one partner is ≥25 years old. This study illustrates how sexual behavior data can be used to reconstruct detailed sexual mixing patterns by age, and how these patterns can be integrated into dynamic transmission models. The proposed framework can be extended to study the effects of age-dependent transmission on incidence in any STI.

Partner age differences and associated sexual risk behaviours among adolescent girls and young women in a cash transfer programme for schooling in Malawi

Background

Age disparities in sexual relationships have been proposed as a key risk factor for HIV transmission in Sub-Saharan Africa, but evidence remains inconclusive. The SIHR study, a cluster randomised trial of a cash transfer programme in Malawi, found that young women in the intervention groups were less likely to have had a sexual partner aged 25 or older, and less likely to test positive for HIV and HSV-2 at follow-up compared to control groups. We examined the hypotheses that girls in the intervention groups had smaller age differences than control groups and that large age differences were associated with relationship-level HIV transmission risk factors: inconsistent condom use, sex frequency, and relationship duration.

Methods

We conducted an analysis of schoolgirls in the Schooling, Income, and Health Risk (SIHR) study aged 13-22 at baseline (n = 2907). We investigated the effects of study arm, trial stage and participant age on age differences in sexual relationships using a linear mixed-effects model. Cumulative-link mixed-effects models were used to estimate the effect of relationship age difference on condom use and sex frequency, and a Cox proportional hazard model was used to estimate the effect of relationship age difference on relationship duration. We controlled for the girl’s age, number of partners, study group and study round.

Results

Girls receiving cash transfers, on average, had smaller age differences in relationships compared to controls, though the estimated difference was not statistically significant (− 0.43 years; 95% CI: -1.03, 0.17). The older the participant was, the smaller her age differences (− 0.67 per 4-year increase in age; 95% CI: -0.99, − 0.35). Among controls, after the cash transfers had ended the average age difference was 0.82 years larger than during the intervention (95% CI: 0.43, 1.21), suggesting a possible indirect effect of the study on behaviour in the community as a whole. Across treatment groups, larger age differences in relationships were associated with lower levels of condom use, more frequent sex, and longer relationship durations.

Conclusions

Cash-transfer programmes may prevent HIV transmission in part by encouraging young women to form age-similar relationships, which are characterised by increased condom use and reduced sex frequency. The benefits of these programmes may extend to those who are not directly receiving the cash.

Electronic supplementary material

The online version of this article (10.1186/s12889-018-5327-7) contains supplementary material, which is available to authorized users.

Sexual partnership age pairings and risk of HIV acquisition in rural South Africa

OBJECTIVE

To quantify the contribution of specific sexual partner age groups to the risk of HIV acquisition in men and women in a hyperendemic region of South Africa.

DESIGN

We conducted a population-based cohort study among women (15-49 years of age) and men (15-55 years of age) between 2004 and 2015 in KwaZulu-Natal, South Africa.

METHODS

Generalized additive models were used to estimate smoothed HIV incidence rates across partnership age pairings in men and women. Cox proportional hazards regression was used to estimate the relative risk of HIV acquisition by partner age group.

RESULTS

A total of 882 HIV seroconversions were observed in 15 935 person-years for women, incidence rate = 5.5 per 100 person-years [95% confidence interval (CI), 5.2-5.9] and 270 HIV seroconversions were observed in 9372 person-years for men, incidence rate = 2.9 per 100 person-years (95% CI, 2.6-3.2). HIV incidence was highest among 15-24-year-old women reporting partnerships with 30-34-year-old men, incidence rate = 9.7 per 100 person-years (95% CI, 7.2-13.1). Risk of HIV acquisition in women was associated with male partners aged 25-29 years (adjusted hazard ratio; aHR = 1.44, 95% CI, 1.02-2.04) and 30-34 years (aHR = 1.50, 95% CI, 1.08-2.09) relative to male partners aged 35 and above. Risk of HIV acquisition in men was associated with 25-29-year-old (aHR = 1.72, 95% CI, 1.02-2.90) and 30-34-year-old women (aHR = 2.12, 95% CI, 1.03-4.39) compared to partnerships with women aged 15-19 years.

CONCLUSION

Age of sexual partner is a major risk factor for HIV acquisition in both men and women, independent of one's own age. Partner age pairings play a critical role in driving the cycle of HIV transmission.