Concurrency of partnerships, consistency with data, and control of sexually transmitted infections

Staged HIV transmission and treatment in a dynamic model with long-term partnerships

The transmission dynamics of HIV are closely tied to the duration and overlap of sexual partnerships. We develop an autonomous population model that can account for the possibilities of an infection from either a casual sexual partner or a long-term partner who was either infected at the start of the partnership or has been newly infected since the onset of the partnership. The impact of the long-term partnerships on the rate of infection is captured by calculating the expected values of the rate of infection from these extended contacts. The model includes three stages of infectiousness: acute, chronic, and virally suppressed. We calculate HIV incidence and the fraction of new infections attributed to casual contacts and long-term partnerships allowing for variability in condom usage, the effect of achieving and maintaining viral suppression, and early intervention by beginning HAART during the acute phase of infection. We present our results using data on MSM HIV transmission from the CDC in the U.S. While the acute stage is the most infectious, the majority of the new infections will be transmitted by long-term partners in the chronic stage when condom use is infrequent as is common in long-term relationships. Time series analysis of the solution, as well as parameter sensitivity analysis, are used to determine effective intervention strategies.

Knowledge and risk practices related to HIV infection in the general population, young men, and MSM in three Brazilian cities in 2019

The study aimed to describe knowledge and risk practices related to HIV infection in three Brazilian cities in the general population, men 15 to 24 years of age living without a partner, and men that reported sex with other men (MSM) at least once in life. This was a cross-sectional household-based study with three-stage cluster sampling (census tracts, households, individuals) stratified by sex, age group (15-24; 25-34; 35-44; 45-59), and conjugal status in the individual selection. We estimated the proportions and 95% confidence intervals (95%CI) of indicators of knowledge, HIV testing, sexual behavior, and self-rated risk. We analyzed 5,764 individuals in Campo Grande, 3,745 in Curitiba, and 3,900 in Florianópolis. Low levels of knowledge were found for preventive methods, especially PrEP. Unprotected sex practices were frequent in the three municipalities. Lifetime HIV test rates were 57.2% (95%CI: 55.1-59.2) in Curitiba, 64.3% (95%CI: 62.7-66.0) in Campo Grande, and 65.9% (95%CI: 64.0-67.7) in Florianópolis. Among men 15-24 years of age, the proportions of stimulant drug use and unprotected sexual practices were higher than in the other age groups. Lifetime HIV test rates exceeded 80% in MSM. More than 30% of MSM were receptive partners in anal sex without condoms, and fewer than 5% assessed their risk as high. More effective communication strategies are needed on prevention of HIV infection, including increased knowledge that could motivate safer sexual practices.

Conhecimento e práticas de risco à infecção pelo HIV na população geral, homens jovens e HSH em três municípios brasileiros em 2019

O objetivo do estudo foi descrever o conhecimento e práticas de risco à infecção pelo HIV na amostra total de cada município, entre homens de 15 a 24 anos que vivem sem companheiro(a), e homens que fizeram sexo com homems (HSH) pelo menos uma vez na vida em três cidades brasileiras. Foi realizado estudo de corte transversal de base domiciliar com amostragem por conglomerados em três estágios (setores censitários, domicílios, indivíduos), com estratificação por sexo, faixa etária (15-24; 25-34; 35-44; 45-59) e vive com companheiro(a) na seleção do indivíduo. Estimaram-se proporções e intervalos de 95% de confiança (IC95%) de indicadores de conhecimento, testagem do HIV, comportamento sexual e autoavaliação do risco. Foram analisados 5.764 indivíduos em Campo Grande, 3.745 em Curitiba e 3.900 em Florianópolis. Baixo nível de conhecimento foi encontrado para os métodos de prevenção, sobretudo para profilaxia pré-exposição (PrEP). Práticas de sexo desprotegido foram frequentes nos três municípios. As proporções de teste de HIV na vida foram 57,2% (IC95%: 55,1-59,2) em Curitiba, 64,3% (IC95%: 62,7-66,0) em Campo Grande, e 65,9% (IC95%: 64,0-67,7) em Florianópolis. Entre homens de 15-24 anos, proporções de uso de drogas estimulantes e práticas sexuais desprotegidas foram mais altas que nos demais grupos etários. Entre os HSH, as proporções de teste de HIV na vida foram superiores a 80%. Mais de 30% foram parceiros receptivos no sexo anal sem uso de preservativo, e menos de 5% avaliam seu risco como alto. É preciso adotar estratégias de comunicação mais eficazes sobre a prevenção da infecção do HIV, incluindo a ampliação de conhecimentos que poderiam motivar práticas sexuais mais seguras.

Concurrency measures in the era of temporal network epidemiology: a review

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.

Improving pairwise approximations for network models with susceptible-infected-susceptible dynamics

Network models of disease spread play an important role in elucidating the impact of long-lasting infectious contacts on the dynamics of epidemics. Moment-closure approximation is a common method of generating low-dimensional deterministic models of epidemics on networks, which has found particular success for diseases with susceptible-infected-recovered (SIR) dynamics. However, the effect of network structure is arguably more important for sexually transmitted infections, where epidemiologically relevant contacts are comparatively rare and longstanding, and which are in general modelled via the susceptible-infected-susceptible (SIS)-paradigm. In this paper, we introduce an improvement to the standard pairwise approximation for network models with SIS-dynamics for two different network structures: the isolated open triple (three connected individuals in a line) and the k-regular network. This improvement is achieved by tracking the rate of change of errors between triple values and their standard pairwise approximation. For the isolated open triple, this improved pairwise model is exact, while for k-regular networks a closure is made at the level of triples to obtain a closed set of equations. This improved pairwise approximation provides an insight into the errors introduced by the standard pairwise approximation, and more closely matches both higher-order moment-closure approximations and explicit stochastic simulations with only a modest increase in dimensionality to the standard pairwise approximation.

A multilayer temporal network model for STD spreading accounting for permanent and casual partners

Sexually transmitted diseases (STD) modeling has used contact networks to study the spreading of pathogens. Recent findings have stressed the increasing role of casual partners, often enabled by online dating applications. We study the Susceptible-Infected-Susceptible (SIS) epidemic model –appropriate for STDs– over a two-layer network aimed to account for the effect of casual partners in the spreading of STDs. In this novel model, individuals have a set of steady partnerships (links in layer 1). At certain rates, every individual can switch between active and inactive states and, while active, it establishes casual partnerships with some probability with active neighbors in layer 2 (whose links can be thought as potential casual partnerships). Individuals that are not engaged in casual partnerships are classified as inactive, and the transitions between active and inactive states are independent of their infectious state. We use mean-field equations as well as stochastic simulations to derive the epidemic threshold, which decreases substantially with the addition of the second layer. Interestingly, for a given expected number of casual partnerships, which depends on the probabilities of being active, this threshold turns out to depend on the duration of casual partnerships: the longer they are, the lower the threshold.

Assessing the cost-effectiveness of HPV vaccination strategies for adolescent girls and boys in the UK

BACKGROUND

Human papillomavirus (HPV) is the most widespread sexually transmitted infection worldwide. It causes several health consequences, in particular accounting for the majority of cervical cancer cases in women. In the United Kingdom, a vaccination campaign targeting 12-year-old girls started in 2008; this campaign has been successful, with high uptake and reduced HPV prevalence observed in vaccinated cohorts. Recently, attention has focused on vaccinating both sexes, due to HPV-related diseases in males (particularly for high-risk men who have sex with men) and an equity argument over equalising levels of protection.

METHODS

We constructed an epidemiological model for HPV transmission in the UK, accounting for nine of the most common HPV strains. We complemented this with an economic model to determine the likely health outcomes (healthcare costs and quality-adjusted life years) for individuals from the epidemiological model. We then tested vaccination with the three HPV vaccines currently available, vaccinating either girls alone or both sexes. For each strategy we calculated the threshold price per vaccine dose, i.e. the maximum amount paid for the added health benefits of vaccination to be worth the cost of each vaccine dose. We calculated results at 3.5% discounting, and also 1.5%, to consider the long-term health effects of HPV infection.

RESULTS

At 3.5% discounting, continuing to vaccinate girls remains highly cost-effective compared to halting vaccination, with threshold dose prices of £56-£108. Vaccination of girls and boys is less cost-effective (£25-£53). Compared to vaccinating girls only, adding boys to the programme is not cost-effective, with negative threshold prices (-£6 to -£3) due to the costs of administration. All threshold prices increase when using 1.5% discounting, and adding boys becomes cost-effective (£36-£47). These results are contingent on the UK's high vaccine uptake; for lower uptake rates, adding boys (at the same uptake rate) becomes more cost effective.

CONCLUSIONS

Vaccinating girls is extremely cost-effective compared with no vaccination, vaccinating both sexes is less so. Adding boys to an already successful girls-only programme has a low cost-effectiveness, as males have high protection through herd immunity. If future health effects are weighted more heavily, threshold prices increase and vaccination becomes cost-effective.

A sexually transmitted infection model with long-term partnerships in homogeneous and heterogenous populations

Population models for sexually transmitted infections frequently use a transmission model that assumes an inherent partnership length of zero. However, in a population with long-term partnerships, the infection status of the partners, the length of the partnership, and the exclusivity of the partnership significantly affect the rate of infection. We develop an autonomous population model that can account for the possibilities of an infection from either a casual sexual partner or a longtime partner who was either infected at the start of the partnership or was newly infected. The impact of the long-term partnerships on the rate of infection is captured by calculating the expected values of the rate of infection from these extended contacts. We present a new method to evaluate partner acquisition rates for casual or long-term partnerships which produces in a more realistic number of lifetime sexual partners. Results include a SI model with different infectiousness levels for the transmission of HIV and HSV-2 with acute and chronic/latent infection stages for homogeneous (MSM) and heterogeneous (WSM-MSW) groups. The accompanying reproduction number and sensitivity studies highlight the impact of both casual and long-term partnerships on infection spread. We construct an autonomous set of equations that handle issues usually ignored by autonomous equations and handled only through simulations or in a non-autonomous form. The autonomous formulation of the model allows for simple numerical computations while incorporating a combination of random instantaneous contacts between individuals and prolonged contacts between specific individuals.

Capturing sexual contact patterns in modelling the spread of sexually transmitted infections: Evidence using Natsal-3

BACKGROUND

Understanding the spread of sexually transmitted infections (STIs) in a population is of great importance to the planning and delivery of health services globally. The worldwide rise of HIV since the 1980's, and the recent increase in common STIs (including HPV and Chlamydia) in many countries, means that there is an urgent need to understand transmission dynamics in order to better predict the spread of such infections in the population. Unlike many other infections which can be captured by assumptions of random mixing, STI transmission is intimately linked to the number and pattern of sexual contacts. In fact, it is the huge variation in the number of new sexual partners that gives rise to the extremes of risk within populations which need to be captured in predictive models of STI transmission. Such models are vital in providing the necessary scientific evidence to determine whether a range of controls (from education to screening to vaccination) are cost-effective.

METHOD AND RESULTS

We use probability sample survey data from Britain's third National Survey of Sexual Attitudes and Lifestyles (Natsal-3) to determine robust distributions for the rate of new partnerships that involve condomless sex and can therefore facilitate the spread of STIs. Different distributions are defined depending on four individual-level characteristics: age, sex, sexual orientation, and previous sexual experience. As individual behaviour patterns can change (e.g. by remaining in a monogamous relationship for a long period) we allow risk-percentiles to be randomly redrawn, to capture longer term behaviour as measured by Natsal-3. We demonstrate how this model formulation interacts with the transmission of infection by constructing an individual-based SIS-P (Susceptible-Infected-Susceptible-Protected) transmission model for the spread of a generic STI, and observing overall population demographics when varying the transmission probability within a partnership, recovery rate and the level of population protection (e.g. from vaccination where applicable).