Partner notification as a tool for research in HIV epidemiology: behaviour change, transmission risk and incidence trends

HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention

Background The human immunodeficiency virus (HIV) infectiousness of anal intercourse (AI) has not been systematically reviewed, despite its role driving HIV epidemics among men who have sex with men (MSM) and its potential contribution to heterosexual spread. We assessed the per-act and per-partner HIV transmission risk from AI exposure for heterosexuals and MSM and its implications for HIV prevention.

Methods Systematic review and meta-analysis of the literature on HIV-1 infectiousness through AI was conducted. PubMed was searched to September 2008. A binomial model explored the individual risk of HIV infection with and without highly active antiretroviral therapy (HAART).

Results A total of 62 643 titles were searched; four publications reporting per-act and 12 reporting per-partner transmission estimates were included. Overall, random effects model summary estimates were 1.4% [95% confidence interval (CI) 0.2–2.5)] and 40.4% (95% CI 6.0–74.9) for per-act and per-partner unprotected receptive AI (URAI), respectively. There was no significant difference between per-act risks of URAI for heterosexuals and MSM. Per-partner unprotected insertive AI (UIAI) and combined URAI–UIAI risk were 21.7% (95% CI 0.2–43.3) and 39.9% (95% CI 22.5–57.4), respectively, with no available per-act estimates. Per-partner combined URAI–UIAI summary estimates, which adjusted for additional exposures other than AI with a ‘main’ partner [7.9% (95% CI 1.2–14.5)], were lower than crude (unadjusted) estimates [48.1% (95% CI 35.3–60.8)]. Our modelling demonstrated that it would require unreasonably low numbers of AI HIV exposures per partnership to reconcile the summary per-act and per-partner estimates, suggesting considerable variability in AI infectiousness between and within partnerships over time. AI may substantially increase HIV transmission risk even if the infected partner is receiving HAART; however, predictions are highly sensitive to infectiousness assumptions based on viral load.

Conclusions Unprotected AI is a high-risk practice for HIV transmission, probably with substantial variation in infectiousness. The significant heterogeneity between infectiousness estimates means that pooled AI HIV transmission probabilities should be used with caution. Recent reported rises in AI among heterosexuals suggest a greater understanding of the role AI plays in heterosexual sex lives may be increasingly important for HIV prevention.

Sexual transmission of HIV-1

HIV-1 transmission occurs in a limited number of ways all of which are preventable. Overall, the risk of HIV-1 transmission following a single sexual exposure is low especially in comparison with other sexually transmitted infections (STIs); with estimates of the average probability of male to female HIV-1 transmission only 0.0005-0.0026 per coital act. The risk of acquiring HIV-1 from a single contact varies enormously and is dependant upon the infectiousness of the HIV-1 positive individual and the susceptibility to HIV-1 of their sexual partner. An understanding of the determinants of HIV-1 transmission is important not only to assess the infection risk to an individual when exposed to the virus (e.g. to determine the provision of post exposure prophylaxis), but also to make accurate predictions on the potential spread of HIV-1 infection in a population and to direct appropriate targeted prevention strategies. In this review article we summarise the current literature on the major worldwide source of HIV-1 acquisition, sexual transmission. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

Systematic review of orogenital HIV-1 transmission probabilities

Background The objective was to assess the risk of HIV transmission from orogenital intercourse (OI).

Methods Systematic review of the literature on HIV-1 infectiousness through OI conducted according to MOOSE guidelines for reviews of observational studies. The PubMed database and bibliographies of relevant articles were searched to July 2007.

Results Of the titles, 56 214 were searched from which 10 potentially appropriate studies were identified; two additional studies were identified through bibliographies and one through discussion with experts. There were 10 included studies, providing estimates of transmission probabilities per-partner (n = 5), incidence per-partner (n = 3), per-study participant (n = 3, following initially seronegative individuals whose partners are of unknown serostatus) and per-act (n = 3). Only four of 10 studies reported non-zero estimates: two per-partner estimates (20%, 95% CI: 6–51, n = 10 and a model-based estimate, 1%, range 0.85–2.3%), one per-study participant estimate (0.37%, 95% CI: 0.10–1.34%) and one per-act estimate (0.04%, 95% CI: 0.01–0.17%). Upper bounds for the 95% CI for zero estimates tended to be relatively large due to the small study sample sizes: 9.0, 12.1 and 2.8% for per-partner; 4.7, 9.6 and 1.8 per 100 person-years for incidence per-partner; 4.4% per-study participant and 0.45 and 0.02% for per-act. Given the small number of studies, a meta-analysis was not considered appropriate.

Conclusions There are currently insufficient data to estimate precisely the risk from OI exposure. The low risk of transmission evident from identified studies means that more and larger studies would be required to provide sufficient evidence to derive more precise estimates.

The effectiveness of HIV partner counseling and referral services in increasing identification of HIV-positive individuals a systematic review

Partner counseling and referral services (PCRS) are part of the spectrum of care for HIV-positive people and their sexual or needle-sharing partners. Referral includes notifying partners of exposure, after which they are (ideally) tested and receive prevention or risk reduction counseling or enter into care (if they test positive). Using The Guide to Community Preventive Services's methods for systematic reviews, the effectiveness of PCRS was evaluated, including partner notification, in identifying a population at high risk of HIV infection and in increasing testing in those populations. In this review, PCRS efforts using provider referral were found to be effective in reaching a population with a high prevalence of HIV. Nine studies qualified for the review. In these studies, a range of one to eight partners was identified per index case (a person newly diagnosed with HIV who has partners who should be notified); a mean of 67% of identified partners were found and notified of their potential exposure to HIV, and a mean of 63% of those notified were tested (previously known "positives" were not tested). Of those tested, a mean of 20% were HIV positive. Therefore, even given that not all partners could be found and notified and that some who could be found did not accept testing, 1% to 8% of people named as potentially exposed and not previously known to be HIV positive were identified as HIV positive through partner notification (although these people were not necessarily infected by the index case). Evidence was insufficient to determine whether PCRS, including partner notification, was also effective in changing behavior or reducing transmission because available studies did not generally report on these outcomes. Little empirical evidence was available to assess potential harm of the interventions, but current studies have not shown substantial harms. Based on Community Guide rules of evidence, sufficient evidence shows that PCRS with partner notification by a public health professional ("provider referral") effectively increases identification of a high-prevalence target population for HIV testing.

Uncertainties in predicting the demographic impact of AIDS

A model giving the demographic impact of AIDS is analysed to examine the sensitivity of the projections when various complicating features are included. The model deals with age and sexual partner change rate as continuous variables and uses a device to specify arbitrary correlations between the ages of the people who form sexual partnerships. The device ensures consistency, in that the amount of partner formation is the same regardless of whether the partnerships are counted from the point of view of males or females. Arbitrary correlation between partner change rate and fertility is also permitted. The results show the uncertainty in model predictions that population growth will reduce over the next 20 years to approximately zero in parts of East Africa severely affected by the AIDS epidemic. The main sources of uncertainty in the model predictions are assumptions concerning the correlation between ages in a partnership, the correlation between partner change rate and fertility, the incubation period of AIDS, and the variability of the female partner change rate.

Economic evaluation of an HIV prevention intervention for gay and bisexual male adolescents

The objective of this study was to evaluate the cost-effectiveness of an HIV prevention intervention for gay and bisexual male adolescents. The intervention included individualized risk assessment and counseling, peer education, optional HIV testing, and referrals to needed services. From 1989 to 1994, 501 male volunteers, 13 to 21 years of age, who self-identified as gay/bisexual or as having had sex with men, completed preintervention and postintervention surveys to assess changes in HIV risk behavior. An HIV transmission model was constructed to project the HIV seroprevalence in the target population over a 10-year period from the self-reported number of partners for unprotected anal intercourse. Cost-effectiveness was analyzed from a societal perspective. Total costs of the intervention, including medical treatment costs saved, were projected to be $1.1 million U.S. for the 10-year period. The number of HIV infections averted and the quality-adjusted life years (QALYs) saved were projected to be 13 and 180, respectively. An incremental cost-effectiveness ratio was projected to be $6180 U.S. per QALY saved. The intervention was found to be cost-effective from the societal perspective. In addition, HIV prevalence in the target population was projected to be 6.1% without and 5.6% with intervention by the end of the 10-year period. This study highlights that an HIV prevention program can be cost-effective even if the effects on behavior are partial and short term.

Using the Kalman filter and dynamic models to assess the changing HIV/AIDS epidemic

Many factors, including therapy and behavioral changes, have modified the course of the HIV/AIDS epidemic in recent years. To include these modifications in HIV/AIDS models, in the absence of appropriate external data sources, changes over time in the parameters can be incorporated by a recursive estimation technique such as the Kalman filter. The Kalman filter accounts for stochastic fluctuations in both the model and the data and provides a means to assess any parameter modifications included in new observations. The Kalman filter approach was applied to a simple differential model to describe the observed HIV/AIDS epidemic in the homo/bisexual male community in Paris (France). This approach gave quantitative information on the time-evolution of some parameters of major epidemiological significance (average transmission rate, mean incubation rate, and basic reproduction rate), which appears quite consistent with the recent epidemiological literature.

Using existing health care systems to respond to the AIDS epidemic: research and recommendations for Chile

Chile is a country with a relatively low prevalence of HIV infection, where successful prevention has the potential to change the future course of the epidemic. A controversial national prevention strategy based upon public education has emerged in response to characterizations of the epidemic as well-dispersed with a growing involvement of heterosexuals. This characterization is not consistent with the observed facts. There is a comparatively well-organized health care system in Santiago that is doing a good job of detecting HIV infection and already has in place the elements of a targeted intervention scheme. Chile should place priority on the use of the existing health care infrastructure for implementing both the traditional public health interventions for sexually transmitted diseases (contact tracing and partner notification) and the AIDS-necessitated strategy of focused counseling and education.

The impact of sexual mixing patterns on the spread of AIDS

It has previously been demonstrated that the distribution of sexual partner change rates may not be sufficient behavioral information to predict the spread of AIDS. A more adequate description of the sexual network through which AIDS may spread will require information on who mixes with whom. Previous models indicated that a highly "assortive" pattern of sexual mixing, in which people of high partner change rates nearly always partner similar people, would result in a relatively rapid epidemic but one that would ultimately be rather small as it would be largely confined to those with high partner change rates. However, in earlier models it was necessary to arbitrarily vary either the partnering patterns or the distribution of partner change rates as the epidemic progressed in order to ensure consistency in the partnering patterns. Consequently, the "pure" effects of assortiveness on the size of the epidemic could not be assessed. In this paper a new model is described in which consistency in partnering patterns is maintained as the epidemic selectively depletes those with higher partner change rates. However, this model maintains consistency without arbitrary adjustments to either the assortive nature of partnering or important features of the distribution of the partner change rate. It is then possible to test the effects of assortiveness on the outcome of the epidemic without important confounding effects from the mathematical device used to maintain consistency in partnering. A practical sensitivity test on the impact of assortiveness on model predictions shows that, contrary to expectations, increasing assortiveness can, under certain circumstances, lead to larger epidemics.

Modelling the effect of treatment and behavioral change in HIV transmission dynamics

In this paper we analyze a model for the HIV-infection transmission in a male homosexual population. In the model we consider two types of infected individuals. Those that are infected but do not know their serological status and/or are not under any sort of clinical/therapeutical treatment, and those who are. The two groups of infectives differ in their incubation time, contact rate with susceptible individuals, and probability of disease transmission. The aim of this article is to study the roles played by detection and changes in sexual behavior in the incidence and prevalence of HIV. The analytical results show that there exists a unique endemic equilibrium which is globally asymptotically stable under a range of parameter values whenever a detection/treatment rate and an indirect measure of the level of infection risk are sufficiently large. However, any level of detection/treatment rate coupled with a decrease of the transmission probability lowers the incidence rate and prevalence level in the population. In general, only significant reductions in the transmission probability (achieved through, for example, the adoption of safe sexual practices) can contain effectively the spread of the disease.

Partner notification for HIV infection in the United Kingdom: a look back on seven years experience in Newcastle upon Tyne

OBJECTIVE

To examine the outcome of partner notification for HIV infection.

DESIGN

Retrospective analysis of medical, health adviser and counsellor records.

SETTING

Teaching hospital, Newcastle upon Tyne.

PATIENTS

All newly diagnosed cases of human immunodeficiency virus (HIV) infection and their sexual partners.

MAIN OUTCOME MEASURES

Attendance of contact at genitourinary medicine clinics for counselling and testing. Seropositivity rate of people attending as a result of partner notification.

RESULTS

Of the 80 partners attending as a result of partner notification 79 were tested. Twenty-five of these (31.6%) were seropositive. This was 21.9% of our newly diagnosed caseload. Seventy-five attended following patient referral and five as a result of provider referral. Discrepancies between districts in policies of provider referral prevented two partners being notified.

CONCLUSIONS

Partner notification is an effective method of ensuring that people with a very high risk of HIV infection have access to counselling and medical care. Complete integration of notification services throughout the UK is required.