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Modelling Sexual Interactions: Sexual behaviour and the spread of sexually transmitted infections on dynamic networks
Stockholm University, Faculty of Science, Department of Mathematics.ORCID iD: 0000-0003-1701-9325
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis we develop statistical and mathematical models to study different factors of relevance for the spread of sexually transmitted infections (STIs). Two special interest groups for STI interventions are considered: sexually active youths and men who have sex with men (MSM). The statistical models developed make it possible to estimate individuals’ dispositions towards sexual behaviours related to the spread of STIs: condom use and anal sex. To study the spread of an infection in a population we use mathematical models. The mathematical models in this thesis give insights into the transmission process of HIV among MSM in Sweden—a population at high risk for HIV infection.

The focus of the first paper is on mechanisms giving rise to observed sexual behaviour, such as condom use, among sexually active youths in Sweden. We study the sexual dispositions of individuals and how these interact and generate the observed sexual outcomes.

The second paper concerns the sexual behaviour of MSM in Sweden and the transmission process of HIV within this population. The population is modelled by a stochastic dynamic network model that incorporates both steady partnerships and casual contacts. We model the spread of an infection where individuals are susceptible, infectious or diagnosed (unable to transmit) and derive the basic reproduction number R0, the probability of a major outbreak, and the endemic prevalence.

The third paper further develops the dynamic network model of the second paper. The model now takes into account that individuals may be sexually high-active or sexually low-active. The division into two activity groups makes it possible to study a preventive intervention against HIV that is only targeted to sexually high-active. The intervention studied is pre-exposure prophylaxis for HIV (PrEP), i.e. that the antiviral drugs tenofovir-emtricitabine are taken by individuals with negative HIV serostatus to prevent getting infected by HIV. We study the PrEP coverage needed to reduce the observed HIV prevalence of 5% to a value close to 0%.

In the fourth and final paper we focus on condom dispositions among MSM. The disposition models from the first paper are extended to better fit an MSM population and are additionally extended to be used for more types of sexual behaviour data.

Place, publisher, year, edition, pages
Stockholm: Department of Mathematics, Stockholm University , 2019. , p. 40
Keywords [en]
Mathematical modelling, Sexually transmitted infections, Egocentric network analysis, Dynamic networks, Sexual behaviour, HIV, Statistical inference
National Category
Probability Theory and Statistics
Research subject
Mathematical Statistics
Identifiers
URN: urn:nbn:se:su:diva-172941ISBN: 978-91-7797-857-2 (print)ISBN: 978-91-7797-858-9 (electronic)OAI: oai:DiVA.org:su-172941DiVA, id: diva2:1351822
Public defence
2019-11-28, sal 14, hus 5, Kräftriket, Roslagsvägen 101, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-11-05 Created: 2019-09-16 Last updated: 2019-10-22Bibliographically approved
List of papers
1. Inferring individual sexual action dispositions from egocentric network data on dyadic sexual outcomes
Open this publication in new window or tab >>Inferring individual sexual action dispositions from egocentric network data on dyadic sexual outcomes
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2018 (English)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 13, no 11, article id e0207116Article in journal (Refereed) Published
Abstract [en]

In this paper we present a family of models that allows us to estimate egos' unobserved action dispositions from a joint behavioural outcome of a dyadic social interaction process of both egos' and alters' action dispositions. The method is put to test on a data set containing two different types of dyadic activities of high relevance for the spread of sexually transmitted infections (STI), condom use and anal sex. The data consists of individuals older than 15 years old who visited one of the nine youth clinics in the Vastra Gotaland region of Sweden between February 2010 and March 2011 for STI testing. This is hence a group of special interest for STI interventions. We cannot find any difference in condom disposition between women and men. Condoms are initially used more often in less risky types of relationships, especially if the partner ends up as a main partner. When studying the disposition towards anal sex we do however find a difference between men and women. Women are more against practising anal sex than men while the majority of men are neutral towards anal sex.

Keywords
Behavior, Human sexual behavior, Sexually transmitted diseases, Heterosexuals, Probability distribution, Social influence, Network analysis, Sweden
National Category
Sociology Mathematics
Research subject
Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-162851 (URN)10.1371/journal.pone.0207116 (DOI)000449909200046 ()30418994 (PubMedID)
Available from: 2018-12-28 Created: 2018-12-28 Last updated: 2019-10-07Bibliographically approved
2. A dynamic network model to disentangle the roles of steady and casual partners for HIV transmission among MSM
Open this publication in new window or tab >>A dynamic network model to disentangle the roles of steady and casual partners for HIV transmission among MSM
2019 (English)In: Epidemics, ISSN 1755-4365, E-ISSN 1878-0067, Vol. 27, p. 66-76Article in journal (Refereed) Published
Abstract [en]

HIV is a sexually transmitted infection (STI) whose transmission process is highly dependent on the sexual network structure of the population under consideration. Most sexual behaviour data is egocentric in nature. We develop a stochastic dynamic sexual network model that utilises this type of egocentric network data. The model incorporates both steady and casual sex partners, and can be seen as a stochastic form of a generalised pair-formation model. We model the spread of an infection where individuals are susceptible, infectious, or successfully treated (and unable to transmit) and derive analytical expressions for several epidemiological quantities. We use sexual behaviour and HIV prevalence data that was gathered among 403 MSM at an STI clinic in Stockholm. To accurately capture transmission dynamics for this population, we need to explicitly model both casual sex partners and steady partnerships. Our model yields an estimate for the mean time until diagnosis followed by successful treatment that is in line with literature. This study indicates that small reductions in the time to diagnosis, and thereby, beginning of treatment, may substantially reduce HIV prevalence. Moreover, we find that moderate increases in condom use with casual sex partners have greater impact on reducing prevalence than the same increases in condom use with steady sex partners. This result demonstrates the relative importance of casual contacts on the HIV transmission dynamics among MSM in Sweden. Our results highlight the importance of HIV testing and condom-use interventions, and the role that casual and steady partners play in this, in order to turn the epidemiological trend in Sweden towards decreased HIV incidence.

Keywords
Mathematical models, Dynamic networks, Sexual behaviour, HIV, Steady and casual partners
National Category
Infectious Medicine Mathematics
Research subject
Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-170112 (URN)10.1016/j.epidem.2019.02.001 (DOI)000469416700008 ()30738786 (PubMedID)
Available from: 2019-07-01 Created: 2019-07-01 Last updated: 2019-10-07Bibliographically approved
3. Introducing pre-exposure prophylaxis to prevent HIV acquisition among men who have sex with men in Sweden: insights from a mathematical pair-formation model
Open this publication in new window or tab >>Introducing pre-exposure prophylaxis to prevent HIV acquisition among men who have sex with men in Sweden: insights from a mathematical pair-formation model
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Objectives Since 2017, the Public health Agency of Sweden recommend that pre-exposure prophylaxis for HIV (PrEP) should be offered to high-risk individuals, in particular to men who have sex with men (MSM). The objective of this study is to develop a mathematical model investigating the effect of introducing PrEP to MSM in Sweden.

Design A pair-formation model, including steady and casual sex partners, is developed to study the impact of introducing PrEP. Two groups are included in the model: sexually high-active MSM and sexually low-active MSM. Three mixing assumptions between the groups are considered.

Setting A gay-friendly MSM HIV/STI-testing clinic in Stockholm, Sweden. This clinic started offering PrEP to MSM in October 2018.

Participants The model is calibrated according to detailed sexual behaviour data gathered in 2015 among 403 MSM.

Results By targeting sexually high-active MSM, a PrEP coverage of 3.5% of the MSM population (10% of all high-actives) would result in the long-term prevalence to drop considerably (close to 0%). While targeting only low-actives would require a PrEP coverage of 35% for a similar reduction. The main effect of PrEP is the reduced susceptibility, whereas the increased HIV-testing rate (every 3rd month) among PrEP users plays a lesser role.

Conclusions To create a multifaceted picture of the effects of interventions against HIV, we need models that include the different stages of HIV infection and real-world data on detailed sexual behaviour to calibrate the mathematical models. Our findings conclude that targeting HIV high-risk individuals, within HIV risk populations such as MSM, with PrEP programmes could greatly decrease the long-term HIV prevalence in Sweden. Therefore, risk stratification of individuals is of importance in PrEP implementation programmes, to ensure optimising the effect and cost-effectiveness of such programmes.

Keywords
HIV & AIDS, Infectious diseases, Pre-exposure prophylaxis, Mathematical models, Transmission dynamics, Sexual networks
National Category
Probability Theory and Statistics
Research subject
Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-172938 (URN)
Available from: 2019-09-14 Created: 2019-09-14 Last updated: 2019-09-16Bibliographically approved
4. Estimating individual action dispositions using binary and frequency egocentric sexual network data
Open this publication in new window or tab >>Estimating individual action dispositions using binary and frequency egocentric sexual network data
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The degree distribution of nodes in a sexual network has been under thorough investigation, as has its implications for the spread of sexually transmitted infections. However, not only the structure of the network is of importance in regulating the propagation of an infection. Two nodes connected by an edge may take actions that reduce the transmission probability through that edge. Condom use is one such action. In this paper, we derive models for individual action dispositions, and how they together generate an outcome on the edge connecting two nodes. We derive two main models, one where two connected nodes generate one outcome together (suitable for casual sex partners), and one where several outcomes is allowed (suitable for steady sex partners). We model different disposition distributions and different rules on how the dispositions generate outcomes, using an egocentric network data set on condom use behaviour.

Keywords
Egocentric network analysis, Social networks, Sexual behaviour, Sexually transmitted infections
National Category
Probability Theory and Statistics
Research subject
Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-172939 (URN)
Available from: 2019-09-14 Created: 2019-09-14 Last updated: 2019-09-16Bibliographically approved

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