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Masuda N, Miller JC, Holme P. Concurrency measures in the era of temporal network epidemiology: a review. J R Soc Interface 2021; 18:20210019. [PMID: 34062106 PMCID: PMC8169215 DOI: 10.1098/rsif.2021.0019] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2021] [Accepted: 05/11/2021] [Indexed: 01/19/2023] Open
Abstract
Diseases spread over temporal networks of interaction events between individuals. Structures of these temporal networks hold the keys to understanding epidemic propagation. One early concept of the literature to aid in discussing these structures is concurrency-quantifying individuals' tendency to form time-overlapping 'partnerships'. Although conflicting evaluations and an overabundance of operational definitions have marred the history of concurrency, it remains important, especially in the area of sexually transmitted infections. Today, much of theoretical epidemiology uses more direct models of contact patterns, and there is an emerging body of literature trying to connect methods to the concurrency literature. In this review, we will cover the development of the concept of concurrency and these new approaches.
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Affiliation(s)
- Naoki Masuda
- Department of Mathematics, State University of New York at Buffalo, New York, NY, USA
- Computational and Data-Enabled Science and Engineering Program, State University of New York at Buffalo, New York, NY, USA
| | - Joel C. Miller
- School of Engineering and Mathematical Sciences, La Trobe University, Bundoora, Australia
| | - Petter Holme
- Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
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Wong NS, Lee MP, Wong KH, Tsang OTY, Lee SS. The differential impacts of non-locally acquired infections and treatment interventions on heterosexual HIV transmission in Hong Kong. PLoS One 2020; 15:e0237433. [PMID: 32790778 PMCID: PMC7425942 DOI: 10.1371/journal.pone.0237433] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2019] [Accepted: 07/03/2020] [Indexed: 11/18/2022] Open
Abstract
Introduction Heterosexual infections have contributed to a high proportion of the HIV burden in Asia and Eastern Europe. Human mobility and non-local infections are important features in some cities/countries. An understanding of the determinants of the sustained growth of the heterosexual HIV epidemics would enable the potential impacts of treatment-based interventions to be assessed. Methods We developed a compartmental model for heterosexual HIV transmissions, parameterized by clinical and surveillance data (1984–2014) in Hong Kong. HIV sequence data were included for examining genetic linkages and clustering pattern. We performed sensitivity analyses to evaluate effects of high-risk sexual partnership and proportions of non-locally acquired infections. Four hypothetical interventions (a) immediate treatment, (b) enhancement of retention in care, (c) HIV testing campaigns, and (d) test-and-treat strategy, were examined. Results Data of 2174 patients (723 female and 1451 male) diagnosed with HIV between 1984 and 2012 in Hong Kong were collected for model parameterization. Among 1229 sequences of non-MSM (men who have sex with men) patients, 70% were isolates and 17% were either dyads or triads. In base-case scenario, the total estimated number of new infections in 2012–2023 would be 672 for male and 452 for female. Following 100% retention in care intervention, the total proportion of averted new infections in 2012–2023 would be 7% for male and 10% for female. HIV testing campaign in 2012 and 2017 followed by 100% immediate treatment strategy would avert 5% and 9% of male and female new infections, respectively. In the epidemic model, an increase of high-risk sexual partnership from 6% to 9% would increase the epidemic growth (annual number of newly diagnosed and newly infected cases) by about 10%. If no non-locally acquired infection occurred as from 2012, the epidemic growth would slump. To control the heterosexual epidemic, periodic HIV testing at 5-year intervals with immediate treatment would avert 5–13% of annual new infections in 2013–2023. Conclusions Enhanced HIV testing with immediate treatment is most effective in controlling the heterosexual epidemic, the impacts of which might however be attenuated by any increase of non-locally acquired infection, assuming little variations of high risk partnership over time.
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Affiliation(s)
- Ngai Sze Wong
- Stanley Ho Centre for Emerging Infectious Diseases, The Chinese University of Hong Kong, Shatin, Hong Kong, People’s Republic of China
- Institute for Global Health & Infectious Diseases, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States of America
- University of North Carolina Project-China, Guangzhou, Guangdong, China
| | - Man Po Lee
- Department of Medicine, Queen Elizabeth Hospital, Kowloon, Hong Kong, People’s Republic of China
| | - Ka Hing Wong
- Special Preventive Programme, Department of Health, Hong Kong Special Administrative Region Government, Hong Kong, People’s Republic of China
| | - Owen T. Y. Tsang
- Department of Medicine and Geriatrics, Princess Margaret Hospital, Lai King, Kowloon, Hong Kong, People’s Republic of China
| | - Shui Shan Lee
- Stanley Ho Centre for Emerging Infectious Diseases, The Chinese University of Hong Kong, Shatin, Hong Kong, People’s Republic of China
- * E-mail:
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3
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Hansson D, Strömdahl S. Estimating individual action dispositions using binary and frequency egocentric sexual network data. STAT NEERL 2020. [DOI: 10.1111/stan.12207] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Disa Hansson
- Department of Mathematics Stockholm University Stockholm Sweden
| | - Susanne Strömdahl
- Department of Medical Sciences, Section of Infectious Diseases Uppsala University Uppsala Sweden
- Department of Public Health Sciences Karolinska Institutet Stockholm Sweden
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4
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Mulberry N, Rutherford AR, Wittenberg RW, Williams BG. HIV control strategies for sex worker-client contact networks. J R Soc Interface 2019; 16:20190497. [PMID: 31551046 DOI: 10.1098/rsif.2019.0497] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Controlling the spread of HIV among hidden, high-risk populations such as survival sex workers and their clients is becoming increasingly important in the ongoing fight against HIV/AIDS. Several sociological and structural factors render general control strategies ineffective in these settings; instead, focused prevention, testing and treatment strategies which take into account the nature of survival sex work are required. Using a dynamic bipartite network model of sexual contacts, we investigate the optimal distribution of treatment and preventative resources among sex workers and their clients; specifically, we consider control strategies that randomly allocate antiretroviral therapy and pre-exposure prophylaxis within each subpopulation separately. Motivated by historical data from a South African mining community, three main asymmetries between sex workers and clients are considered in our model: relative population sizes, migration rates and partner distributions. We find that preventative interventions targeted at female sex workers are the lowest cost strategies for reducing HIV prevalence, since the sex workers form a smaller population and have, on average, more sexual contacts. However, the high migration rate among survival sex workers limits the extent to which prevalence can be reduced using this strategy. To achieve a further reduction in HIV prevalence, testing and treatment in the client population cannot be ignored.
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Affiliation(s)
- Nicola Mulberry
- Department of Mathematics, Simon Fraser University, Burnaby, British Columbia, Canada
| | - Alexander R Rutherford
- Department of Mathematics and SFU Big Data, Simon Fraser University, Burnaby, British Columbia, Canada
| | - Ralf W Wittenberg
- Department of Mathematics, Simon Fraser University, Burnaby, British Columbia, Canada
| | - Brian G Williams
- South African Centre for Epidemiological Modelling and Analysis, University of Stellenbosch, Stellenbosch, South Africa
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Coital Frequency and Male Concurrent Partnerships During Pregnancy and Postpartum in Agbogbloshie, Ghana. AIDS Behav 2019; 23:1508-1517. [PMID: 30673896 DOI: 10.1007/s10461-019-02403-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
During pregnancy and postpartum, women in high HIV prevalence regions continue to be at high risk for acquiring HIV, due to both behavioral and biological mechanisms, despite declines in coital frequency as a pregnancy advances. We estimated differences in rates of partnership concurrency for men with and without pregnant or postpartum sexual partners. We used monthly retrospective panel data from Ghana from three perspectives: couple-level data, female reports of pregnancy and male partner concurrency, and male reports of concurrent partnerships and female partner pregnancy. Coital frequency increased during the first trimester and then declined with advancing pregnancy. However, in all three analyses, there was no compelling evidence that men with pregnant or postpartum partners had additional concurrent partnerships. Our findings suggest that even though women's sexual activity likely declines during pregnancy and postpartum, they may not be at increased risk of HIV/STI due to their partners seeking additional partnerships.
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Miller JC, Slim AC. Saturation effects and the concurrency hypothesis: Insights from an analytic model. PLoS One 2017; 12:e0187938. [PMID: 29136021 PMCID: PMC5685581 DOI: 10.1371/journal.pone.0187938] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2017] [Accepted: 10/27/2017] [Indexed: 01/06/2023] Open
Abstract
Sexual partnerships that overlap in time (concurrent relationships) may play a significant role in the HIV epidemic, but the precise effect is unclear. We derive edge-based compartmental models of disease spread in idealized dynamic populations with and without concurrency to allow for an investigation of its effects. Our models assume that partnerships change in time and individuals enter and leave the at-risk population. Infected individuals transmit at a constant per-partnership rate to their susceptible partners. In our idealized populations we find regions of parameter space where the existence of concurrent partnerships leads to substantially faster growth and higher equilibrium levels, but also regions in which the existence of concurrent partnerships has very little impact on the growth or the equilibrium. Additionally we find mixed regimes in which concurrency significantly increases the early growth, but has little effect on the ultimate equilibrium level. Guided by model predictions, we discuss general conditions under which concurrent relationships would be expected to have large or small effects in real-world settings. Our observation that the impact of concurrency saturates suggests that concurrency-reducing interventions may be most effective in populations with low to moderate concurrency.
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Affiliation(s)
- Joel C. Miller
- Institute for Disease Modeling, Bellevue, WA, United States of America
- * E-mail:
| | - Anja C. Slim
- School of Mathematical Sciences, Monash University, Clayton, VIC, Australia
- School of Earth, Atmosphere, and the Environment, Monash University, Clayton, VIC, Australia
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Onaga T, Gleeson JP, Masuda N. Concurrency-Induced Transitions in Epidemic Dynamics on Temporal Networks. PHYSICAL REVIEW LETTERS 2017; 119:108301. [PMID: 28949155 DOI: 10.1103/physrevlett.119.108301] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/16/2017] [Indexed: 06/07/2023]
Abstract
Social contact networks underlying epidemic processes in humans and animals are highly dynamic. The spreading of infections on such temporal networks can differ dramatically from spreading on static networks. We theoretically investigate the effects of concurrency, the number of neighbors that a node has at a given time point, on the epidemic threshold in the stochastic susceptible-infected-susceptible dynamics on temporal network models. We show that network dynamics can suppress epidemics (i.e., yield a higher epidemic threshold) when the node's concurrency is low, but can also enhance epidemics when the concurrency is high. We analytically determine different phases of this concurrency-induced transition, and confirm our results with numerical simulations.
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Affiliation(s)
- Tomokatsu Onaga
- Department of Physics, Kyoto University, Kyoto 606-8502, Japan
- MACSI, Department of Mathematics and Statistics, University of Limerick, Limerick V94 T9PX, Ireland
| | - James P Gleeson
- MACSI, Department of Mathematics and Statistics, University of Limerick, Limerick V94 T9PX, Ireland
| | - Naoki Masuda
- Department of Engineering Mathematics, University of Bristol, Woodland Road, Bristol BS8 1UB, United Kingdom
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Pair formation models for sexually transmitted infections: A primer. Infect Dis Model 2017; 2:368-378. [PMID: 29928748 PMCID: PMC6002071 DOI: 10.1016/j.idm.2017.07.002] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2017] [Revised: 07/19/2017] [Accepted: 07/24/2017] [Indexed: 01/01/2023] Open
Abstract
For modelling sexually transmitted infections, duration of partnerships can strongly influence the transmission dynamics of the infection. If partnerships are monogamous, pairs of susceptible individuals are protected from becoming infected, while pairs of infected individuals delay onward transmission of the infection as long as they persist. In addition, for curable infections re-infection from an infected partner may occur. Furthermore, interventions based on contact tracing rely on the possibility of identifying and treating partners of infected individuals. To reflect these features in a mathematical model, pair formation models were introduced to mathematical epidemiology in the 1980's. They have since been developed into a widely used tool in modelling sexually transmitted infections and the impact of interventions. Here we give a basic introduction to the concepts of pair formation models for a susceptible-infected-susceptible (SIS) epidemic. We review some results and applications of pair formation models mainly in the context of chlamydia infection.
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Cassels S, Jenness SM, Biney AAE, Dodoo FNA. Geographic mobility and potential bridging for sexually transmitted infections in Agbogbloshie, Ghana. Soc Sci Med 2017; 184:27-39. [PMID: 28501018 PMCID: PMC5525941 DOI: 10.1016/j.socscimed.2017.05.003] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2016] [Revised: 04/11/2017] [Accepted: 05/02/2017] [Indexed: 11/24/2022]
Abstract
Short-term mobility can significantly influence the spread of infectious disease. In order for mobile individuals to geographically spread sexually transmitted infections (STIs), individuals must engage in sexual acts with different partners in two places within a short time. In this study, we considered the potential of mobile individuals as bridge populations - individuals who link otherwise disconnected sexual networks and contributed to ongoing STI transmission. Using monthly retrospective panel data, we examined associations between short-term mobility and sexual partner concurrency in Agbogbloshie, Ghana. We also examined bridging by the location of sex acts and the location of sexual partners in concurrent triads, and whether mobile individuals from our sample were more likely to be members of geographic bridging triads. Although reported rates of sexual partnership concurrency were much higher for men compared to women, mobility was only associated with increased concurrency for women. Additionally, this association held for middle-distance mobility and short-duration trips for women. Taking into account the location of sex acts and the location of sexual partners, about 22% of men (21.7% and 22.4% for mobile and non-mobile men, respectively) and only 3% of women (1.4% and 3.3% for mobile and non-mobile women, respectively) were potential bridges for STIs over the last year. Our results highlight the gendered nature of mobility and sexual risk behavior, reflecting the normative social context that encourages women to conceal certain types of sexual behavior.
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Affiliation(s)
- Susan Cassels
- Department of Geography, University of California, Santa Barbara, CA 93106-4060, United States.
| | - Samuel M Jenness
- Department of Epidemiology, Rollins School of Public Health, Emory University, 1518 Clifton Road, Atlanta, GA 30322, United States
| | - Adriana A E Biney
- Regional Institute for Population Studies, University of Ghana, P.O. Box LG 96, Legon, Accra, Ghana
| | - F Nii-Amoo Dodoo
- The Pennsylvania State University & University of Ghana, 211 Oswald Tower, University Park, PA 16802, United States
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Forward reachable sets: Analytically derived properties of connected components for dynamic networks. ACTA ACUST UNITED AC 2017; 5:328-354. [DOI: 10.1017/nws.2017.10] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractFormal analysis of the emergent structural properties of dynamic networks is largely uncharted territory. We focus here on the properties of forward reachable sets (FRS) as a function of the underlying degree distribution and edge duration. FRS are defined as the set of nodes that can be reached from an initial seed via a path of temporally ordered edges; a natural extension of connected component measures to dynamic networks. Working in a stochastic framework, we derive closed-form expressions for the mean and variance of the exponential growth rate of the FRS for temporal networks with both edge and node dynamics. For networks with node dynamics, we calculate thresholds for the growth of the FRS. The effects of finite population size are explored via simulation and approximation. We examine how these properties vary by edge duration and different cross-sectional degree distributions that characterize a range of scientifically interesting normative outcomes (Poisson and Bernoulli). The size of the forward reachable set gives an upper bound for the epidemic size in disease transmission network models, relating this work to epidemic modeling (Ferguson, 2000; Eames, 2004).
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11
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Leung KY, Powers KA, Kretzschmar M. Gender asymmetry in concurrent partnerships and HIV prevalence. Epidemics 2017; 19:53-60. [PMID: 28169133 DOI: 10.1016/j.epidem.2017.01.003] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2016] [Revised: 01/08/2017] [Accepted: 01/15/2017] [Indexed: 01/26/2023] Open
Abstract
The structure of the sexual network of a population plays an essential role in the transmission of HIV. Concurrent partnerships, i.e. partnerships that overlap in time, are important in determining this network structure. Men and women may differ in their concurrent behavior, e.g. in the case of polygyny where women are monogamous while men may have concurrent partnerships. Polygyny has been shown empirically to be negatively associated with HIV prevalence, but the epidemiological impacts of other forms of gender-asymmetric concurrency have not been formally explored. Here we investigate how gender asymmetry in concurrency, including polygyny, can affect the disease dynamics. We use a model for a dynamic network where individuals may have concurrent partners. The maximum possible number of simultaneous partnerships can differ for men and women, e.g. in the case of polygyny. We control for mean partnership duration, mean lifetime number of partners, mean degree, and sexually active lifespan. We assess the effects of gender asymmetry in concurrency on two epidemic phase quantities (R0 and the contribution of the acute HIV stage to R0) and on the endemic HIV prevalence. We find that gender asymmetry in concurrent partnerships is associated with lower levels of all three epidemiological quantities, especially in the polygynous case. This effect on disease transmission can be attributed to changes in network structure, where increasing asymmetry leads to decreasing network connectivity.
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Affiliation(s)
- Ka Yin Leung
- Utrecht University, PO Box 80010, 3508 TA Utrecht, The Netherlands; University Medical Center Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands.
| | - Kimberly A Powers
- The University of North Carolina at Chapel Hill, 2105D McGavran-Greenberg Hall, Campus Box 7435, Chapel Hill, NC 27599-7435, USA.
| | - Mirjam Kretzschmar
- University Medical Center Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands; National Institute of Public Health and the Environment, PO Box 1, 3720 BA Bilthoven, The Netherlands.
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Rozhnova G, van der Loeff MFS, Heijne JCM, Kretzschmar ME. Impact of Heterogeneity in Sexual Behavior on Effectiveness in Reducing HIV Transmission with Test-and-Treat Strategy. PLoS Comput Biol 2016; 12:e1005012. [PMID: 27479074 PMCID: PMC4968843 DOI: 10.1371/journal.pcbi.1005012] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2015] [Accepted: 06/02/2016] [Indexed: 12/19/2022] Open
Abstract
The WHO’s early-release guideline for antiretroviral treatment (ART) of HIV infection based on a recent trial conducted in 34 countries recommends starting treatment immediately upon an HIV diagnosis. Therefore, the test-and-treat strategy may become more widely used in an effort to scale up HIV treatment and curb further transmission. Here we examine behavioural determinants of HIV transmission and how heterogeneity in sexual behaviour influences the outcomes of this strategy. Using a deterministic model, we perform a systematic investigation into the effects of various mixing patterns in a population of men who have sex with men (MSM), stratified by partner change rates, on the elimination threshold and endemic HIV prevalence. We find that both the level of overdispersion in the distribution of the number of sexual partners and mixing between population subgroups have a large influence on endemic prevalence before introduction of ART and on possible long term effectiveness of ART. Increasing heterogeneity in risk behavior may lead to lower endemic prevalence levels, but requires higher coverage levels of ART for elimination. Elimination is only feasible for populations with a rather low degree of assortativeness of mixing and requires treatment coverage of almost 80% if rates of testing and treatment uptake by all population subgroups are equal. In this case, for fully assortative mixing and 80% coverage endemic prevalence is reduced by 57%. In the presence of heterogeneity in ART uptake, elimination is easier to achieve when the subpopulation with highest risk behavior is tested and treated more often than the rest of the population, and vice versa when it is less. The developed framework can be used to extract information on behavioral heterogeneity from existing data which is otherwise hard to determine from population surveys. HIV is endemic in populations of MSM in Western countries. As ART reduces transmission risk, increased testing and treatment rates are expected to lower HIV incidence. However, concerns are that in MSM populations changing risk behavior may counteract the impact of ART on transmission. Using a mathematical model, we investigated how heterogeneity in sexual behavior influences the possible effects of a test-and-treat strategy on HIV prevalence and in particular the prospects of eliminating HIV from these populations. We demonstrated that behavioral heterogeneity plays an important role in determining the impact of ART on reducing HIV transmission. Knowledge of behavioral heterogeneity is key in setting intervention goals in populations of MSM.
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Affiliation(s)
- Ganna Rozhnova
- Julius Center for Health Sciences and Primary Care, University Medical Centre Utrecht, Utrecht, The Netherlands
- * E-mail:
| | - Maarten F. Schim van der Loeff
- Department of Infectious Disease Control, Public Health Service Amsterdam, Amsterdam, The Netherlands
- Center of Infection and Immunity Amsterdam, Academic Medical Center, Amsterdam, The Netherlands
| | - Janneke C. M. Heijne
- Centre for Infectious Disease Control, National Institute of Public Health and the Environment, Bilthoven, The Netherlands
| | - Mirjam E. Kretzschmar
- Julius Center for Health Sciences and Primary Care, University Medical Centre Utrecht, Utrecht, The Netherlands
- Centre for Infectious Disease Control, National Institute of Public Health and the Environment, Bilthoven, The Netherlands
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Disease dynamics and costly punishment can foster socially imposed monogamy. Nat Commun 2016; 7:11219. [PMID: 27044573 PMCID: PMC4832056 DOI: 10.1038/ncomms11219] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2015] [Accepted: 03/01/2016] [Indexed: 11/24/2022] Open
Abstract
Socially imposed monogamy in humans is an evolutionary puzzle because it requires costly punishment by those who impose the norm. Moreover, most societies were—and are—polygynous; yet many larger human societies transitioned from polygyny to socially imposed monogamy beginning with the advent of agriculture and larger residential groups. We use a simulation model to explore how interactions between group size, sexually transmitted infection (STI) dynamics and social norms can explain the timing and emergence of socially imposed monogamy. Polygyny dominates when groups are too small to sustain STIs. However, in larger groups, STIs become endemic (especially in concurrent polygynist networks) and have an impact on fertility, thereby mediating multilevel selection. Punishment of polygynists improves monogamist fitness within groups by reducing their STI exposure, and between groups by enabling punishing monogamist groups to outcompete polygynists. This suggests pathways for the emergence of socially imposed monogamy, and enriches our understanding of costly punishment evolution. Many human societies transitioned from polygyny to socially imposed monogamy as group sizes increased. Using a simulation model, the authors show that sexually transmitted infections impose heavier fitness penalties on polygynists as group size grows, enabling monogamists who punish polygyny to thrive in large groups.
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