Imperial College London


Faculty of MedicineSchool of Public Health

Research Fellow



+44 (0)20 7594 3290r.silhol




Medical SchoolSt Mary's Campus





Publication Type

27 results found

Kra AK, Fosto AS, Nguessan KN, Geoffroy O, Younoussa S, Kabemba OK, Gueye PA, Ndeye PD, Rouveau N, Boily M-C, Silhol R, dElbée M, Maheu-Giroux M, Vautier A, Larmarange Jet al., 2023, Can HIV self-testing reach first-time testers? A telephone survey among self-test end users in Côte d’Ivoire, Mali, and Senegal, BMC Infectious Diseases, Vol: 22

<jats:title>Abstract</jats:title><jats:sec> <jats:title>Background</jats:title> <jats:p>Coverage of HIV testing remains sub-optimal in West Africa. Between 2019 and 2022, the ATLAS program distributed ~400 000 oral HIV self-tests (HIVST) in Côte d’Ivoire, Mali, and Senegal, prioritising female sex workers (FSW) and men having sex with men (MSM), and relying on secondary redistribution of HIVST to partners, peers and clients to reach individuals not tested through conventional testing. This study assesses the proportion of first-time testers among HIVST users and the associated factors.</jats:p> </jats:sec><jats:sec> <jats:title>Methods</jats:title> <jats:p>A phone-based survey was implemented among HIVST users recruited using dedicated leaflets inviting them to anonymously call a free phone number. We collected socio-demographics, sexual behaviours, HIV testing history, HIVST use, and satisfaction with HIVST. We reported the proportion of first-time testers and computed associated factors using logistic regression.</jats:p> </jats:sec><jats:sec> <jats:title>Results</jats:title> <jats:p>Between March and June 2021, 2 615 participants were recruited for 50 940 distributed HIVST (participation rate: 5.1%). Among participants, 30% received their HIVST kit through secondary distribution (from a friend, sexual partner, family member, or colleague).</jats:p> <jats:p>The proportion who had never tested for HIV before HIVST (first-time testers) was 41%. The main factors associated with being a first-time tester were sex, age group, education level, condom use, and secondary distribution. A higher proportion was observed among those aged 24 years or less (55% vs 32% for 25–34, aOR: 0.37 [95%CI: 0.

Journal article

Silhol R, Nordsletten A, Maheu-Giroux M, Elmes J, Staunton R, Owen B, Shacklett B, McGowan I, Feliciano KG, van der Straten A, Eller LA, Robb M, Marrazzo J, Dimitrov D, Boily M-Cet al., 2023, The association between heterosexual anal intercourse and HIV acquisition in three prospective cohorts of women, AIDS and Behavior, Pages: 1-12, ISSN: 1090-7165

The extent to which receptive anal intercourse (RAI) increases the HIV acquisition risk of women compared to receptive vaginal intercourse (RVI) is poorly understood. We evaluated RAI practice over time and its association with HIV incidence during three prospective HIV cohorts of women: RV217, MTN-003 (VOICE), and HVTN 907. At baseline, 16% (RV 217), 18% (VOICE) of women reported RAI in the past 3 months and 27% (HVTN 907) in the past 6 months, with RAI declining during follow-up by around 3-fold. HIV incidence in the three cohorts was positively associated with reporting RAI at baseline, albeit not always significantly. The adjusted hazard rate ratios for potential confounders (aHR) were 1.1 (95% Confidence interval: 0.8-1.5) for VOICE and 3.3 (1.6-6.8) for RV 217, whereas the ratio of cumulative HIV incidence by RAI practice was 1.9 (0.6-6.0) for HVTN 907. For VOICE, the estimated magnitude of association increased slightly when using a time-varying RAI exposure definition (aHR = 1.2; 0.9-1.6), and for women reporting RAI at every follow-up survey (aHR = 2.0 (1.3-3.1)), though not for women reporting higher RAI frequency (> 30% acts being RAI vs. no RAI in the past 3 months; aHR = 0.7 (0.4-1.1)). Findings indicated precise estimation of the RAI/HIV association, following multiple RVI/RAI exposures, is sensitive to RAI exposure definition, which remain imperfectly measured. Information on RAI practices, RAI/RVI frequency, and condom use should be more systematically and precisely recorded and reported in studies looking at sexual behaviors and HIV seroconversions; standardized measures would aid comparability across geographies and over time.

Journal article

Stone J, Looker KJ, Silhol R, Turner KME, Hayes R, Coetzee J, Baral S, Schwartz S, Mayaud P, Gottlieb S, Boily M-C, Vickerman Pet al., 2023, The population impact of herpes simplex virus type 2 (HSV-2) vaccination on the incidence of HSV-2, HIV and genital ulcer disease in South Africa: a mathematical modelling study, EBioMedicine, Vol: 90, Pages: 1-13, ISSN: 2352-3964

BACKGROUND: Evidence suggests HSV-2 infection increases HIV acquisition risk and HIV/HSV-2 coinfection increases transmission risk of both infections. We analysed the potential impact of HSV-2 vaccination in South Africa, a high HIV/HSV-2 prevalence setting. METHODS: We adapted a dynamic HIV transmission model for South Africa to incorporate HSV-2, including synergistic effects with HIV, to evaluate the impact of: (i) cohort vaccination of 9-year-olds with a prophylactic vaccine that reduces HSV-2 susceptibility; (ii) vaccination of symptomatically HSV-2-infected individuals with a therapeutic vaccine that reduces HSV shedding. FINDINGS: An 80% efficacious prophylactic vaccine offering lifetime protection with 80% uptake could reduce HSV-2 and HIV incidence by 84.1% (95% Credibility Interval: 81.2-86.0) and 65.4% (56.5-71.6) after 40 years, respectively. This reduces to 57.4% (53.6-60.7) and 42.1% (34.1-48.1) if efficacy is 50%, 56.1% (53.4-58.3) and 41.5% (34.2-46.9) if uptake is 40%, and 29.4% (26.0-31.9) and 24.4% (19.0-28.7) if protection lasts 10 years. An 80% efficacious therapeutic vaccine offering lifetime protection with 40% coverage among symptomatic individuals could reduce HSV-2 and HIV incidence by 29.6% (21.8-40.9) and 26.4% (18.5-23.2) after 40 years, respectively. This reduces to 18.8% (13.7-26.4) and 16.9% (11.7-25.3) if efficacy is 50%, 9.7% (7.0-14.0) and 8.6% (5.8-13.4) if coverage is 20%, and 5.4% (3.8-8.0) and 5.5% (3.7-8.6) if protection lasts 2 years. INTERPRETATION: Prophylactic and therapeutic vaccines offer promising approaches for reducing HSV-2 burden and could have important impact on HIV in South Africa and other high prevalence settings. FUNDING: WHO, NIAID.

Journal article

Boily M-C, Barnabas R, Ronn MM, Bayer CJ, van Schalkwyk C, Soni N, Rao DW, Staadegaard L, Liu G, Silhol R, Brisson M, Johnson LF, Bloem P, Gottlieb S, Broutet N, Dalal Set al., 2022, Estimating the effect of HIV on cervical cancer elimination in South Africa: comparative modelling of the impact of vaccination and screening, EClinicalMedicine, Vol: 54, Pages: 1-18, ISSN: 2589-5370

BackgroundIn 2020, the World Health Organization (WHO) launched its initiative to eliminate cervical cancer as a public health problem. To inform global efforts for countries with high HIV and cervical cancer burden, we assessed the impact of human papillomavirus (HPV) vaccination and cervical cancer screening and treatment in South Africa, on cervical cancer and the potential for achieving elimination before 2120, considering faster HPV disease progression and higher cervical cancer risk among women living with HIV(WLHIV) and HIV interventions.MethodsThree independent transmission-dynamic models simulating HIV and HPV infections and disease progression were used to predict the impact on cervical cancer incidence of three scenarios for all women: 1) girls' vaccination (9–14 years old), 2) girls' vaccination plus 1 lifetime cervical screen (at 35 years), and 3) girls’ vaccination plus 2 lifetime cervical screens (at 35 and 45 years) and three enhanced scenarios for WLHIV: 4) vaccination of young WLHIV aged 15–24 years, 5) three-yearly cervical screening of WLHIV aged 15–49 years, or 6) both. Vaccination assumed 90% coverage and 100% lifetime protection with the nonavalent vaccine (against HPV-16/18/31/33/45/52/58). Cervical cancer screening assumed HPV testing with uptake increasing from 45% (2023), 70% (2030) to 90% (2045+). We also assumed that UNAIDS 90-90-90 HIV treatment and 70% male circumcision targets are reached by 2030. We examined three elimination thresholds: age-standardised cervical cancer incidence rates below 4 or 10 per 100,000 women-years, and >85% reduction in cervical cancer incidence rate. We conducted sensitivity analyses and presented the median age-standardised predictions of outcomes of the three models (minimum–maximum across models).FindingsGirls' vaccination could reduce age-standardised cervical cancer incidence from a median of 47.6 (40.9–79.2) in 2020 to 4.5 (3.2–6.3) per 100,000 women-years

Journal article

Fotso AS, Johnson C, Vautier A, Kouame KB, Diop PM, Silhol R, Maheu-Giroux M, Boily M-C, Rouveau N, Doumenc-Aidara C, Baggaley R, Ehui E, Larmarange Jet al., 2022, Routine programmatic data show a positive population-level impact of HIV self-testing: the case of Cote d'Ivoire and implications for implementation, AIDS, Vol: 36, Pages: 1871-1879, ISSN: 0269-9370

Objectives: We estimate the effects of ATLAS's HIV self-testing (HIVST) kit distribution on conventional HIV testing, diagnoses, and antiretroviral treatment (ART) initiations in Côte d’Ivoire.Design: Ecological study using routinely collected HIV testing services program data.Methods: We used the ATLAS's programmatic data recorded between the third quarter of 2019 and the first quarter of 2021, in addition to data from the President's Emergency Plan for AIDS Relief dashboard. We performed ecological time series regression using linear mixed models. Results are presented per 1000 HIVST kits distributed through ATLAS.Results: We found a negative but nonsignificant effect of the number of ATLAS’ distributed HIVST kits on conventional testing uptake (−190 conventional tests; 95% confidence interval [CI]: −427 to 37). The relationship between the number of HIVST kits and HIV diagnoses was significant and positive (+8 diagnosis; 95% CI: 0 to 15). No effect was observed on ART initiation (−2 ART initiations; 95% CI: −8 to 5).Conclusions: ATLAS’ HIVST kit distribution had a positive impact on HIV diagnoses. Despite the negative signal on conventional testing, even if only 20% of distributed kits are used, HIVST would increase access to testing. The methodology used in this paper offers a promising way to leverage routinely collected programmatic data to estimate the effects of HIVST kit distribution in real-world programs.

Journal article

Ky-Zerbo O, Desclaux A, Boye S, Vautier A, Rouveau N, Kouadio BA, Fotso AS, Pourette D, Maheu-Giroux M, Sow S, Camara CS, Doumenc-Aidara C, Keita A, Boily MC, Silhol R, D'Elbee M, Bekelynck A, Gueye PA, Diop PM, Geoffroy O, Kamemba OK, Diallo S, Ehui E, Ndour CT, Larmarange Jet al., 2022, Willingness to use and distribute HIV self-test kits to clients and partners: a qualitative analysis of female sex workers' collective opinion and attitude in Cote d'Ivoire, Mali, and Senegal, Women's Health, Vol: 18, ISSN: 1745-5057

Background:In West Africa, female sex workers are at increased risk of HIV acquisition and transmission. HIV self-testing could be an effective tool to improve access to and frequency of HIV testing to female sex workers, their clients and partners. This article explores their perceptions regarding HIV self-testing use and the redistribution of HIV self-testing kits to their partners and clients.Methods:Embedded within ATLAS, a qualitative study was conducted in Côte-d’Ivoire, Mali, and Senegal in 2020. Nine focus group discussions were conducted. A thematic analysis was performed.Results:A total of 87 participants expressed both positive attitudes toward HIV self-testing and their willingness to use or reuse HIV self-testing. HIV self-testing was perceived to be discreet, confidential, and convenient. HIV self-testing provides autonomy from testing by providers and reduces stigma. Some perceived HIV self-testing as a valuable tool for testing their clients who are willing to offer a premium for condomless sex. While highlighting some potential issues, overall, female sex workers were optimistic about linkage to confirmatory testing following a reactive HIV self-testing. Female sex workers expressed positive attitudes toward secondary distribution to their partners and clients, although it depended on relationship types. They seemed more enthusiastic about secondary distribution to their regular/emotional partners and regular clients with whom they had difficulty using condoms, and whom they knew enough to discuss HIV self-testing. However, they expressed that it could be more difficult with casual clients; the duration of the interaction being too short to discuss HIV self-testing, and they fear violence and/or losing them.Conclusion:Overall, female sex workers have positive attitudes toward HIV self-testing use and are willing to redistribute to their regular partners and clients. However, they are reluctant to promote such use with their casual client

Journal article

Fotso AS, Johnson C, Vautier A, Kouamé KB, Diop PM, Silhol R, Maheu-Giroux M, Boily M-C, Rouveau N, Doumenc-Aïdara C, Baggaley R, Ehui E, Larmarange Jet al., 2022, Using routine programmatic data to estimate the population-level impacts of HIV self-testing: The example of the ATLAS program in Cote d’Ivoire

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>HIV self-testing (HIVST) is recommended by the World Health Organization as an additional HIV testing approach. Since 2019, it has been implemented in Côte d’Ivoire through the ATLAS project, including primary and secondary distribution channels. While the discreet and flexible nature of HIVST makes it appealing for users, it also makes the monitoring and estimation of the population-level programmatic impact of HIVST programs challenging. We used routinely collected data to estimate the effects of ATLAS’ HIVST distribution on access to testing, conventional testing (self-testing excluded), diagnoses, and antiretroviral treatment (ART) initiations in Côte d’Ivoire.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We used the ATLAS project’s programmatic data between the third quarter (Q) of 2019 (Q3 2019) and Q1 2021, in addition to routine HIV testing services program data obtained from the President’s Emergency Plan for AIDS Relief dashboard. We performed ecological time series regression using linear mixed models.</jats:p></jats:sec><jats:sec><jats:title>Findings</jats:title><jats:p>The results are presented for 1000 HIVST kits distributed through ATLAS. They show a negative but nonsignificant effect of the number of ATLAS HIVST on conventional testing uptake (−190 conventional tests [95% CI: −427 to 37, p=0·10]). We estimated that for 1000 additional HIVST distributed through ATLAS, +590 [95% CI: 357 to 821, p&lt;0·001] additional individuals have accessed HIV testing, assuming an 80% HIVST utilization rate (UR) and +390 [95% CI: 161 to 625, p&lt;0·001] assuming a 60% UR. The statistical relationship between the number of HIVST and HIV diagnoses was significant and posit

Journal article

Silhol R, Coupland H, Baggaley R, Miller L, Staadegaard L, Gottlieb S, Stannah J, Turner K, Vickerman P, Hayes R, Mayaud P, Looker K, Boily M-Cet al., 2021, What is the burden of heterosexually-acquired HIV due to HSV-2? Global and regional model-based estimates of the proportion and number of HIV infections attributable to HSV-2 infection, JAIDS: Journal of Acquired Immune Deficiency Syndromes, Vol: 88, Pages: 19-30, ISSN: 1525-4135

Background: Biological and epidemiological evidence suggest that herpes simplex virus type 2 (HSV-2) elevates HIV acquisition and transmission risk. We improved previous estimates of the contribution of HSV-2 to HIV infections by using a dynamic-transmission model.Setting: WHO regions.Methods: We developed a mathematical model of HSV-2/HIV transmission among 15-49-year-old heterosexual, non-drug-injecting populations, calibrated using region-specific demographic and HSV-2/HIV epidemiological data. We derived global and regional estimates of the contribution of HSV-2 to HIV infection over ten years (the transmission population-attributable fraction, tPAF) under three additive scenarios, assuming: (1) HSV-2 only increases HIV acquisition (“conservative”); (2) HSV-2 also increases HIV transmission (“liberal”); (3) HIV/ART (antiretroviral therapy) also modifies HSV-2 transmission and HSV-2 decreases ART effect on HIV transmission ("fully liberal”).Results: Under the conservative scenario, the predicted tPAF was 37.3% (95% uncertainty interval 33.4-43.2%) and an estimated 5.6 (4.5-7.0) million incident heterosexual HIV infections were due to HSV-2 globally over 2009-2018. The contribution of HSV-2 to HIV infections was largest for the African region (tPAF=42.6% (38.0-51.2%)), and lowest for the European region (tPAF=11.2% (7.9-13.8%)). The tPAF was higher among female sex-workers, their clients, and older populations, reflecting their higher HSV-2 prevalence. The tPAF was ∼50% and 1.3-2.4-fold higher for the liberal/fully liberal than the conservative scenario across regions.Conclusion: HSV-2 may have contributed to at least 37% of incident HIV infections in the last decade worldwide, and even more in Africa, and may continue to do so despite increased ART access unless future improved HSV-2 control measures, such as vaccines, become available.

Journal article

Silhol R, Geidelberg L, Mitchell K, Mishra S, Dimitrov D, Bowring A, Behanzin L, Guedou F, Diabate S, Schwartz S, Billong S, Mfochive Njindam I, Levitt D, Mukandavire C, Maheu-Giroux M, Rönn M, Dalal S, Vickerman P, Baral S, Alary M, Boily M-Cet al., 2021, Assessing the potential impact of disruptions due to COVID-19 on HIV among key and lower-risk populations in the largest cities of Cameroon and Benin, JAIDS: Journal of Acquired Immune Deficiency Syndromes, Vol: 87, Pages: 899-911, ISSN: 1525-4135

Background: The COVID-19 pandemic indirectly impacts HIV epidemiology in Central/West Africa. We estimated the potential impact of COVID-19-related disruptions to HIV prevention/treatment services and sexual partnerships on HIV incidence and HIV-related deaths among key populations including female sex workers (FSW), their clients, men who have sex with men (MSM), and overall.Setting: Yaoundé (Cameroon) and Cotonou (Benin).Methods: We used mathematical models of HIV calibrated to city- and risk-population-specific demographic/behavioural/epidemic data. We estimated the relative change in 1-year HIV incidence and HIV-related deaths for various disruption scenarios of HIV prevention/treatment services and decreased casual/commercial partnerships, compared to a scenario without COVID-19.Results: A 50% reduction in condom use in all partnerships over 6 months would increase 1-year HIV incidence by 39%, 42%, 31% and 23% among MSM, FSW, clients, and overall in Yaoundé respectively, and 69%, 49% and 23% among FSW, clients and overall respectively in Cotonou. Combining a 6-month interruption of ART initiation and 50% reduction in HIV prevention/treatment use would increase HIV incidence by 50% and HIV-related deaths by 20%. This increase in HIV infections would be halved by a simultaneous 50% reduction in casual and commercial partnerships.Conclusions: Reductions in condom use following COVID-19 would increase infections among key populations disproportionately, particularly FSW in Cotonou, who need uninterrupted condom provision. Disruptions in HIV prevention/treatment services have the biggest impacts on HIV infections and deaths overall, only partially mitigated by equal reductions in casual/commercial sexual partnerships. Maintaining ART provision must be prioritised to minimise short-term excess HIV-related deaths.

Journal article

Mishra S, Silhol R, Knight J, PhaswanaMafuya R, Diouf D, Wang L, Schwartz S, Boily M, Baral Set al., 2021, Estimating the epidemic consequences of HIV prevention gaps among key populations, Journal of the International AIDS Society, Vol: 24, Pages: 1-6, ISSN: 1758-2652

IntroductionHIV epidemic appraisals are used to characterize heterogeneity and inequities in the context of the HIV pandemic and the response. However, classic measures used in appraisals have been shown to underestimate disproportionate risks of onward transmission, particularly among key populations. In response, a growing number of modelling studies have quantified the consequences of unmet prevention and treatment needs (prevention gaps) among key populations as a transmission population attributable fraction over time (tPAFt). To aid its interpretation and use by programme implementers and policy makers, we outline and discuss a conceptual framework for understanding and estimating the tPAFt via transmission modelling as a measure of onward transmission risk from HIV prevention gaps; and discuss properties of the tPAFt.DiscussionThe distribution of onward transmission risks may be defined by who is at disproportionate risk of onward transmission, and under which conditions. The latter reflects prevention gaps, including secondary prevention via treatment: the epidemic consequences of which may be quantified by the tPAFt. Steps to estimating the tPAFt include parameterizing the acquisition and onward transmission risks experienced by the subgroup of interest, defining the most relevant counterfactual scenario, and articulating the time-horizon of analyses and population among whom to estimate the relative difference in cumulative transmissions; such steps could reflect programme-relevant questions about onward transmission risks. Key properties of the tPAFt include larger onward transmission risks over longer time-horizons; seemingly mutually exclusive tPAFt measures summing to greater than 100%; an opportunity to quantify the magnitude of disproportionate onward transmission risks with a per-capita tPAFt; and that estimates are conditional on what has been achieved so far in reducing prevention gaps and maintaining those conditions moving forward as the status

Journal article

Boye S, Bouaré S, Ky-Zerbo O, Rouveau N, Simo Fotso A, d'Elbée M, Silhol R, Maheu-Giroux M, Vautier A, Breton G, Keita A, Bekelynck A, Desclaux A, Larmarange J, Pourette Det al., 2021, Challenges of HIV self-test distribution for index testing when HIV status disclosure is low: preliminary results of a qualitative study in Bamako (Mali) as part of the ATLAS project, Frontiers in Public Health, Vol: 9, Pages: 1-11, ISSN: 2296-2565

Context: The rate of HIV status disclosure to partners is low in Mali, a West African country with a national HIV prevalence of 1.2%. HIV self-testing (HIVST) could increase testing coverage among partners of people living with HIV (PLHIV). The AutoTest-VIH, Libre d'accéder à la connaissance de son Statut (ATLAS) program was launched in West Africa with the objective of distributing nearly half a million HIV self-tests from 2019 to 2021 in Côte d'Ivoire, Mali, and Senegal. The ATLAS program integrates several research activities. This article presents the preliminary results of the qualitative study of the ATLAS program in Mali. This study aims to improve our understanding of the practices, limitations and issues related to the distribution of HIV self-tests to PLHIV so that they can offer the tests to their sexual partners.Methods: This qualitative study was conducted in 2019 in an HIV care clinic in Bamako. It consisted of (i) individual interviews with eight health professionals involved in the distribution of HIV self-tests; (ii) 591 observations of medical consultations, including social service consultations, with PLHIV; (iii) seven observations of peer educator-led PLHIV group discussions. The interviews with health professionals and the observations notes have been subject to content analysis.Results: HIVST was discussed in only 9% of the observed consultations (51/591). When HIVST was discussed, the discussion was almost always initiated by the health professional rather than PLHIV. HIVST was discussed infrequently because, in most of the consultations, it was not appropriate to propose partner HIVST (e.g., when PLHIV were widowed, did not have partners, or had delegated someone to renew their prescriptions). Some PLHIV had not disclosed their HIV status to their partners. Dispensing HIV self-tests was time-consuming, and medical consultations were very short. Three main barriers to HIVST distribution when HIV status had not been disclos

Journal article

Silhol R, Baral S, Bowring A, Mukandavire C, Mfochive Njindam I, Rao A, Schwartz S, Tamoufe U, Billong S, Njoya O, Zoung-Kanyi Bissek A-C, Garcia Calleja JM, Vickerman P, Mishra S, Boily M-Cet al., 2021, Quantifying the evolving contribution of HIV interventions and key populations to the HIV epidemic in Yaoundé, Cameroon, JAIDS: Journal of Acquired Immune Deficiency Syndromes, Vol: 86, Pages: 396-405, ISSN: 1525-4135

Background: Key populations (KP) including men who have sex with men (MSM), female sex workers (FSW), and their clients are disproportionately affected by HIV in Sub-Saharan Africa. We estimated the evolving impact of past interventions and contribution of unmet HIV prevention/treatment needs of KP and lower-risk groups to HIV transmission.Setting: Yaoundé, Cameroon.Methods: We parametrised and fitted a deterministic of HIV transmission model to Yaoundé-specific demographic, behavioural, HIV and intervention coverage data in a Bayesian framework. We estimated the fraction of incident HIV infections averted by condoms and antiretroviral therapy (ART) and the fraction of all infections over 10-year periods directly and indirectly attributable to unprotected sex within and between each risk group.Results: Condom use and ART together may have averted 43% (95% uncertainty interval: 31-54) of incident infections over 1980-2018 and 72% (66-79) over 2009-2018. Most onward transmissions over 2009-2018 stemmed from unprotected sex between lower-risk individuals (47% (32-61)), clients (37% (23-51)), and MSM (35% (20-54)) with all their partners. The contribution of commercial sex decreased from 25% (8-49) over 1989-1998 to 8% (3-22) over 2009-2018, due to higher intervention coverage among FSW.Conclusion: Condom use and recent ART scale-up mitigated the HIV epidemic in Yaoundé and changed the contribution of different partnerships to onward transmission over time. Findings highlight the importance of prioritizing HIV prevention and treatment for MSM and clients of FSW whose unmet needs now contribute most to onward transmission, while maintaining services which successfully reduced transmissions in the context of commercial sex.

Journal article

Mitchell KM, Dimitrov D, Silhol R, Geidelberg L, Moore M, Liu A, Beyrer C, Mayer KH, Baral S, Boily M-Cet al., 2021, The potential effect of COVID-19-related disruptions on HIV incidence and HIV-related mortality among men who have sex with men in the USA: a modelling study, The Lancet HIV, Vol: 8, Pages: e206-e215, ISSN: 2405-4704

BACKGROUND: During the COVID-19 pandemic, men who have sex with men (MSM) in the USA have reported similar or fewer sexual partners and reduced HIV testing and care access compared with before the pandemic. Pre-exposure prophylaxis (PrEP) use has also declined. We aimed to quantify the potential effect of COVID-19 on HIV incidence and HIV-related mortality among US MSM. METHODS: We used a calibrated, deterministic, compartmental HIV transmission model for MSM in Baltimore (MD, USA) and available data on COVID-19-related disruptions to HIV services to predict effects of reductions in sexual partners (0%, 25%, 50%), condom use (5%), HIV testing (20%), viral suppression (10%), PrEP initiations (72%), PrEP adherence (9%), and antiretroviral therapy (ART) initiations (50%). In our main analysis, we modelled disruptions due to COVID-19 starting Jan 1, 2020, and lasting 6 months. We estimated the median change in cumulative new HIV infections and HIV-related deaths among MSM over 1 and 5 years, compared with a base case scenario without COVID-19-related disruptions. FINDINGS: A 25% reduction in sexual partners for 6 months among MSM in Baltimore, without HIV service changes, could reduce new HIV infections by median 12·2% (95% credible interval 11·7 to 12·8) over 1 year and median 3·0% (2·6 to 3·4) over 5 years. In the absence of changes in sexual behaviour, the 6-month estimated reductions in condom use, HIV testing, viral suppression, PrEP initiations, PrEP adherence, and ART initiations combined are predicted to increase new HIV infections by median 10·5% (5·8 to 16·5) over 1 year, and by median 3·5% (2·1 to 5·4) over 5 years. Disruptions to ART initiations and viral suppression are estimated to substantially increase HIV-related deaths (ART initiations by median 1·7% [0·8 to 3·2], viral suppression by median 9·5% [5·2 to 15·9]) over 1 year, with

Journal article

Geidelberg L, Mitchell KM, Alary M, Mboup A, Béhanzin L, Guédou F, Geraldo N, Goma-Matsétsé E, Giguère K, Aza-Gnandji M, Kessou L, Diallo M, Kêkê RK, Bachabi M, Dramane K, Lafrance C, Affolabi D, Diabaté S, Gagnon M-P, Zannou DM, Gangbo F, Silhol R, Cianci F, Vickerman P, Boily M-Cet al., 2021, A mathematical model impact analysis of a real-life pre-exposure prophylaxis and treatment-as-prevention study among female sex workers in Cotonou, Benin, JAIDS: Journal of Acquired Immune Deficiency Syndromes, Vol: 86, Pages: e28-e42, ISSN: 1525-4135

BACKGROUND: Daily pre-exposure prophylaxis (PrEP) and treatment-as-prevention (TasP) reduce HIV acquisition and transmission risk, respectively. A demonstration study (2015-2017) assessed TasP and PrEP feasibility among female sex workers (FSW) in Cotonou, Benin. SETTING: Cotonou, Benin METHODS:: We developed a compartmental HIV transmission model, featuring PrEP, and ART among the high-risk (FSW, clients) and low-risk populations, calibrated to historical epidemiological and demonstration study data, reflecting observed lower PrEP uptake, adherence and retention compared to TasP. We estimated the population-level impact of the two-year study and several twenty-year intervention scenarios, varying coverage and adherence independently and together. We report the percentage (median, 2.5th-97.5th percentile uncertainty interval (95%UI)) of HIV infections prevented comparing the intervention and counterfactual (2017 coverages: 0% PrEP, 49% ART) scenarios. RESULTS: The two-year study (2017 coverages: 9% PrEP, 83% ART) prevented an estimated 8% (95%UI 6-12) and 6% (3-10) infections among FSW over two and twenty years, respectively, compared to 7% (3-11) and 5% (2-9) overall. The PrEP and TasP arms prevented 0.4% (0.2-0.8) and 4.6% (2.2-8.7) infections overall over 20 years, respectively. Twenty-year PrEP and TasP scale-ups (2035 coverages: 47% PrEP, 88% ART) prevented 21% (17-26) and 17% (10-27) infections among FSW respectively, and 5% (3-10) and 17% (10-27) overall. Compared to TasP scale-up alone, PrEP and TasP combined scale-up prevented 1.9x and 1.2x more infections among FSW and overall, respectively. CONCLUSIONS: The modest demonstration study impact was modest, and mostly from TasP. Increasing PrEP adherence and coverage improves impact substantially among FSW, but little overall. We recommend TasP in prevention packages.

Journal article

Rouveau N, Ky-Zerbo O, Boye S, Fotso AS, d'Elbée M, Maheu-Giroux M, Silhol R, Kouassi AK, Vautier A, Doumenc-Aïdara C, Breton G, Keita A, Ehui E, Ndour CT, Boilly M-C, Terris-Prestholt F, Pourette D, Desclaux A, Larmarange J, ATLAS Teamet al., 2021, Describing, analysing and understanding the effects of the introduction of HIV self-testing in West Africa through the ATLAS programme in Côte d'Ivoire, Mali and Senegal., BMC Public Health, Vol: 21, Pages: 1-14, ISSN: 1471-2458

BACKGROUND: The ATLAS programme aims to promote and implement HIV self-testing (HIVST) in three West African countries: Côte d'Ivoire, Mali, and Senegal. During 2019-2021, in close collaboration with the national AIDS implementing partners and communities, ATLAS plans to distribute 500,000 HIVST kits through eight delivery channels, combining facility-based, community-based strategies, primary and secondary distribution of HIVST. Considering the characteristics of West African HIV epidemics, the targets of the ATLAS programme are hard-to-reach populations: key populations (female sex workers, men who have sex with men, and drug users), their clients or sexual partners, partners of people living with HIV and patients diagnosed with sexually transmitted infections and their partners. The ATLAS programme includes research support implementation to generate evidence for HIVST scale-up in West Africa. The main objective is to describe, analyse and understand the social, health, epidemiological effects and cost-effectiveness of HIVST introduction in Côte d'Ivoire, Mali and Senegal to improve the overall HIV testing strategy (accessibility, efficacy, ethics). METHODS: ATLAS research is organised into five multidisciplinary workpackages (WPs): Key Populations WP: qualitative surveys (individual in-depth interviews, focus group discussions) conducted with key actors, key populations, and HIVST users. Index testing WP: ethnographic observation of three HIV care services introducing HIVST for partner testing. Coupons survey WP: an anonymous telephone survey of HIVST users. Cost study WP: incremental economic cost analysis of each delivery model using a top-down costing with programmatic data, complemented by a bottom-up costing of a representative sample of HIVST distribution sites, and a time-motion study for health professionals providing HIVST. Modelling WP: Adaptation, parameterisation and calibration of a dynamic compartmental model that considers the varied p

Journal article

Mitchell KM, Dimitrov D, Silhol R, Geidelberg L, Moore M, Liu A, Beyrer C, Mayer KH, Baral S, Boily M-Cet al., 2020, Estimating the potential impact of COVID-19-related disruptions on HIV incidence and mortality among men who have sex with men in the United States: a modelling study., Publisher: Cold Spring Harbor Laboratory

Background: During the COVID-19 pandemic, gay and other men who have sex with men (MSM) in the United States (US) report similar or fewer sexual partners and reduced HIV testing and care access. Pre-exposure prophylaxis (PrEP) use has declined. We estimated the potential impact of COVID-19 on HIV incidence and mortality among US MSM. Methods: We used a calibrated HIV transmission model for MSM in Baltimore, Maryland, and available data on COVID-19-related disruptions to predict impacts of data-driven reductions in sexual partners(0%,25%,50%), condom use(5%), HIV testing(20%), viral suppression(10%), PrEP initiations(72%), PrEP use(9%) and ART initiations(50%), exploring different disruption durations and magnitudes. We estimated the median (95% credible interval) change in cumulative new HIV infections and deaths among MSM over one and five years, compared with a scenario without COVID-19-related disruptions. Findings: A six-month 25% reduction in sexual partners among Baltimore MSM, without HIV service changes, could reduce new HIV infections by 12·2%(11·7,12·8%) and 3·0%(2·6,3·4%) over one and five years, respectively. In the absence of changes in sexual behaviour, the six-month data-driven disruptions to condom use, testing, viral suppression, PrEP initiations, PrEP use and ART initiations combined were predicted to increase new HIV infections by 10·5%(5·8,16·5%) over one year, and by 3·5%(2·1,5·4%) over five years. A 25% reduction in partnerships offsets the negative impact of these combined service disruptions on new HIV infections (overall reduction 3·9%(-1·0,7·4%), 0·0%(-1·4,0·9%) over one, five years, respectively), but not on HIV deaths (corresponding increases 11·0%(6·2,17·7%), 2·6%(1·5,4·3%)). The predicted impacts of reductions in partnerships or viral suppression doubled if they lasted 12

Working paper

Silhol R, Boily M-C, Dimitrov D, German D, Flynn C, Farley JE, Gelman M, Hughes JP, Donnell D, Adeyeye A, Remien RH, Beyrer C, Paz-Bailey G, Wejnert C, Mitchell KMet al., 2020, Understanding the HIV epidemic among MSM in Baltimore: a modelling study estimating the impact of past HIV interventions and who acquired and contributed to infections., JAIDS: Journal of Acquired Immune Deficiency Syndromes, Vol: 84, Pages: 253-262, ISSN: 1525-4135

INTRODUCTION: Men who have sex with men (MSM) in the United States (US) are disproportionately affected by HIV. We estimated the impact of past interventions and contribution of different population groups to incident MSM HIV infections. SETTING: Baltimore, US METHODS:: We used a deterministic model, parameterised and calibrated to demographic and epidemic Baltimore MSM data, to estimate the fraction of HIV infections among MSM averted by condoms and antiretroviral therapy (ART) over 1984-2017 and the fraction of infections acquired and transmission contributed by MSM from different demographic groups and disease and care continuum stages over 10-year periods from 1988 to 2017, using population attributable fractions (PAFs). RESULTS: Condom use and ART averted 19% (95% uncertainty interval: 14-25%) and 23% (15-31%) of HIV infections that would have occurred since 1984 and 1996, respectively. Over 2008-2017, 46% (41-52%) of incident infections were acquired by, and 35% (27-49%) of transmissions contributed by MSM aged 18-24 years old (who constitute 27% of all MSM, 19% of HIV+ MSM). MSM with undiagnosed HIV infection, those with diagnosed infection but not in care, and those on ART contributed to 41% (31-54%), 46% (25-56%), and 14% (7-28%) of transmissions, respectively. CONCLUSION: Condoms and ART have modestly impacted the HIV epidemic among Baltimore MSM to date. Interventions reaching MSM with diagnosed infection who are not in care should be implemented since the largest percentage of HIV transmissions among Baltimore MSM are attributed to this group.

Journal article

Elmes J, Silhol R, Hess KL, Gedge LM, Nordsletten A, Staunton R, Anton P, Shacklett B, McGowan I, Dang Q, Adimora AA, Dimitrov DT, Aral S, Handanagic S, PazBailey G, Boily Met al., 2020, Receptive anal sex contributes substantially to heterosexually‐acquired HIV infections among at‐risk women in Twenty US Cities: results from a modelling analysis, American Journal of Reproductive Immunology, Vol: 84, Pages: 1-14, ISSN: 1046-7408

ProblemReceptive anal intercourse (RAI) is more efficient than receptive vaginal intercourse (RVI) at transmitting HIV, but its contribution to heterosexually‐acquired HIV infections among at‐risk women in the US is unclear.Method of studyWe analysed sexual behaviour data from surveys of 9,152 low‐income heterosexual women living in 20 cities with high rates of HIV conducted in 2010 and 2013 as part of US National HIV Behavioral Surveillance. We estimated RAI prevalence (past‐year RAI) and RAI fraction (fraction of all sex acts (RVI and RAI) at the last sexual episode that were RAI among those reporting past‐year RAI) overall and by key demographic characteristics. These results and HIV incidence were used to calibrate a risk‐equation model to estimate the population attributable fraction of new HIV infections due to RAI (PAFRAI) accounting for uncertainty in parameter assumptions.ResultsRAI prevalence (overall: 32%, city range: 19‐60%) and RAI fraction (overall: 27%, city‐range: 18‐34%) were high overall and across cities, and positively associated with exchange sex. RAI accounted for an estimated 41% (uncertainty range: 18‐55%) of new infections overall (city range: 21‐57%). Variability in PAFRAI estimates was most influenced by uncertainty in the estimate of the per‐act increased risk of RAI relative to RVI and the number of sex acts.ConclusionsRAI may contribute disproportionately to new heterosexually‐acquired HIV infections among at‐risk low‐income women in the US, meaning that tools to prevent HIV transmission during RAI are warranted. Number of RVI and RAI acts should also be collected to monitor heterosexually‐acquired HIV infections.

Journal article

Owen BN, Baggaley RF, Elmes J, Harvey A, Shubber Z, Butler AR, Silhol R, Anton P, Shacklett B, van der Straten A, Boily MCet al., 2020, What proportion of female sex workers practise anal intercourse and how frequently? A systematic review and meta-analysis, AIDS and Behavior, Vol: 24, Pages: 697-713, ISSN: 1090-7165

HIV is more efficiently acquired during receptive anal intercourse (AI) compared to vaginal intercourse (VI) and may contribute substantially to female sex workers’ (FSW) high HIV burden. We aim to determine how common and frequent AI is among FSW globally. We searched PubMed, Embase and PsycINFO for studies reporting the proportion of FSW practising AI (prevalence) and/or the number of AI acts (frequency) worldwide from 01/1980 to 10/2018. We assessed the influence of participant and study characteristics on AI prevalence (e.g. continent, study year and interview method) through sub-group analysis. Of 15,830 identified studies, 131 were included. Nearly all (N = 128) reported AI prevalence and few frequency (N = 13), over various recall periods. Most studies used face-to-face interviews (N = 111). Pooled prevalences varied little by recall period (lifetime: 15.7% 95%CI 12.2–19.3%, N = 30, I 2 = 99%; past month: 16.2% 95%CI 10.8–21.6%, N = 18, I 2 = 99%). The pooled proportion of FSW reporting < 100% condom use tended to be non-significantly higher during AI compared to during VI (e.g. any unprotected VI: 19.1% 95%CI 1.7–36.4, N = 5 and any unprotected AI: 46.4% 95%CI 9.1–83.6, N = 5 in the past week). Across all study participants, between 2.4 and 15.9% (N = 6) of all intercourse acts (AI and VI) were anal. Neither AI prevalence nor frequency varied substantially by any participant or study characteristics. Although varied, AI among FSW is generally common, inconsistently protected with condoms and practiced sufficiently frequently to contribute substantially to HIV acquisition in this risk group. Interventions to address barriers to condom use are needed.

Journal article

Stannah J, Silhol R, Elmes J, Owen B, Shacklett BL, Anton P, McGowan I, van der Straten A, Dimitrov D, Baggaley RF, Boily M-Cet al., 2019, Increases in HIV incidence following receptive anal intercourse among women: A systematic review and meta-analysis., AIDS and Behavior, Vol: 24, Pages: 667-681, ISSN: 1090-7165

Receptive anal intercourse (RAI) carries a greater per-act risk of HIV acquisition than receptive vaginal intercourse (RVI) and may influence HIV epidemics driven by heterosexual sex. This systematic review explores the association between RAI and incident HIV among women, globally. We searched Embase and Medline through September 2018 for longitudinal studies reporting crude (cRR) or adjusted (aRR) relative risks of HIV acquisition by RAI practice among women. Of 27,563 articles identified, 17 eligible studies were included. We pooled independent study estimates using random-effects models. Women reporting RAI were more likely to acquire HIV than women not reporting RAI (pooled cRR = 1.56 95% CI 1.03-2.38, N = 18, I2 = 72%; pooled aRR = 2.23, 1.01-4.92, N = 5, I2 = 70%). In subgroup analyses the association was lower for women in Africa (pooled cRR = 1.16, N = 13, I2 = 21%) than outside Africa (pooled cRR = 4.10, N = 5, I2 = 79%) and for high-risk (pooled aRR = 1.69, N = 4, I2 = 63%) than general-risk women (pooled aRR = 8.50, N = 1). Interview method slightly influenced cRR estimates (p value = 0.04). In leave-one-out sensitivity analyses pooled estimates were generally robust to removing individual study estimates. Main limitations included poor exposure definition, incomplete adjustment for confounders, particularly condom use, and use of non-confidential interview methods. More and better data are needed to explain differences in risk by world region and risk population. Women require better counselling and greater choice in prevention modalities that are effective during RVI and RAI.

Journal article

Baggaley R, Owen B, Silhol R, Elmes J, Anton P, McGowan I, van der Straten A, Shacklett B, Dang Q, Swann EM, Bolton DL, Boily MCet al., 2018, Does per-act HIV-1 transmission risk through anal sex vary by gender? An updated systematic review and meta-analysis, American Journal of Reproductive Immunology, Vol: 80, ISSN: 1046-7408

Quantifying HIV‐1 transmission risk per‐act of anal intercourse (AI) is important for HIV‐1 prevention. We updated previous reviews by searching Medline and Embase to 02/2018. We derived pooled estimates of receptive AI (URAI) and insertive AI (UIAI) risk unprotected by condoms using random‐effects models. Subgroup analyses were conducted by gender, study design, and whether antiretroviral treatment (ART) had been introduced by the time of the study. Two new relevant studies were identified, one of which met inclusion criteria, adding three new cohorts and increasing number of individuals/partnerships included from 1869 to 14 277. Four studies, all from high‐income countries, were included. Pooled HIV‐1 risk was higher for URAI (1.25%, 95% CI 0.55%‐2.23%, N = 5, I2 = 87%) than UIAI (0.17%, 95 % CI 0.09%‐0.26%, N = 3, I2 = 0%). The sole heterosexual URAI estimate (3.38%, 95% CI 1.85%‐4.91%), from a study of 72 women published in a peer‐reviewed journal, was significantly higher than the men‐who‐have‐sex‐with‐men (MSM) pooled estimate (0.75%, 95% CI 0.56%‐0.98%, N = 4, P < 0.0001) and higher than the only other heterosexual estimate identified (0.4%, 95% CI 0.08%‐2.0%, based on 59 women, excluded for being a pre‐2013 abstract). Pooled per‐act URAI risk varied by study design (retrospective‐partner studies: 2.56%, 95% CI 1.20%‐4.42%, N = 2 (one MSM, one heterosexual); prospective studies: 0.71%, 95% CI 0.51%‐0.93%, N = 3 MSM, P < 0.0001). URAI risk was lower for studies conducted in the ART era (0.75%, 95% CI 0.52%‐1.03%) than pre‐ART (1.67%, 95% CI 0.44%‐3.67%) but not significantly so (P = 0.537). Prevention messages must emphasize that HIV‐1 infectiousness through AI remains high, even in the ART era. Further studies, particularly among heterosexual populations and in resource‐limited settings, are required to elucidate whether AI risk differs by gender, region and following population‐level ART scale‐up.

Journal article

Silhol R, Gregson S, Nyamukapa C, Mhangara M, Dzangare J, Gonese E, Eaton JW, Case KK, Mahy M, Stover J, Mugurungi Oet al., 2017, Empirical validation of the UNAIDS Spectrum model for subnational HIV estimates: case-study of children and adults in Manicaland, Zimbabwe, AIDS, Vol: 31, Pages: S41-S50, ISSN: 0269-9370

Background: More cost-effective HIV control may be achieved by targeting geographical areas with high infection rates. The AIDS Impact model of Spectrum – used routinely to produce national HIV estimates – could provide the required subnational estimates but is rarely validated with empirical data, even at a national level.Design: The validity of the Spectrum model estimates were compared to empirical estimates.Methods: Antenatal surveillance and population survey data from a population HIV cohort study in Manicaland, east Zimbabwe, were input into Spectrum 5.441 to create a simulation representative of the cohort population. Model and empirical estimates were compared for key demographic and epidemiological outcomes. Alternative scenarios for data availability were examined and sensitivity analyses were conducted for model assumptions considered important for subnational estimates.Results: Spectrum estimates generally agreed with observed data but HIV incidence estimates were higher than empirical estimates while estimates of early age all-cause adult mortality were lower. Child HIV prevalence estimates matched well with the survey prevalence among children. Estimated paternal orphanhood was lower than empirical estimates. Including observations from earlier in the epidemic did not improve the HIV incidence model fit. Migration had little effect on observed discrepancies - possibly because the model ignores differences in HIV prevalence between migrants and residents.Conclusions: The Spectrum model, using subnational surveillance and population data, provided reasonable subnational estimates although some discrepancies were noted. Differences in HIV prevalence between migrants and residents may need to be captured in the model if applied to subnational epidemics.

Journal article

Owen B, Elmes J, Silhol R, Dang Q, McGowan I, Shacklett B, Swann EM, van der Straten A, Baggaley RF, Boily MCet al., 2017, How common and frequent is heterosexual anal intercourse among South Africans? A systematic review and meta-analysis, Journal of International AIDS Society, Vol: 20, ISSN: 1758-2652

Background: HIV is transmitted more effectively during anal intercourse (AI) than vaginal intercourse (VI). However, patterns of heterosexual AI practice and its contribution to South Africa’s generalized epidemic remain unclear. We aimed to determine how common and frequent heterosexual AI is in South Africa.Methods: We searched for studies reporting the proportion practising heterosexual AI (prevalence) and/or the number of AI and unprotected AI (UAI) acts (frequency) in South Africa from 1990 to 2015. Stratified random-effects meta-analysis by subgroups was used to produce pooled estimates and assess the influence of participant and study characteristics on AI prevalence. We also estimated the fraction of all sex acts which were AI or UAI and compared condom use during VI and AI.Results: Of 41 included studies, 31 reported on AI prevalence and 14 on frequency, over various recall periods. AI prevalence was high across different recall periods for sexually active general-risk populations (e.g. lifetime = 18.4% [95%CI:9.4–27.5%], three-month = 20.3% [6.1–34.7%]), but tended to be even higher in higher-risk populations such as STI patients and female sex workers (e.g. lifetime = 23.2% [0.0–47.4%], recall period not stated = 40.1% [36.2–44.0%]). Prevalence was higher in studies using more confidential interview methods. Among general and higher-risk populations, 1.2–40.0% and 0.7–21.0% of all unprotected sex acts were UAI, respectively. AI acts were as likely to be condom protected as vaginal acts.Discussion: Reported heterosexual AI is common but variable among South Africans. Nationally and regionally representative sexual behaviour studies that use standardized recall periods and confidential interview methods, to aid comparison across studies and minimize reporting bias, are needed. Such data could be used to estimate the extent to which AI contributes to South Africa’s HIV epidemic.

Journal article

Obadia T, Silhol R, Opatowski L, Temime L, Legrand J, Thiebaut ACM, Herrmann J-L, Fleury E, Guillemot D, Boelle P-Yet al., 2015, Detailed contact data and the dissemination of staphylococcus aureus in hospitals, PLoS Computational Biology, Vol: 11, Pages: 1-16, ISSN: 1553-734X

Close proximity interactions (CPIs) measured by wireless electronic devices are increasingly used in epidemiological models. However, no evidence supports that electronically collected CPIs inform on the contacts leading to transmission. Here, we analyzed Staphylococcus aureus carriage and CPIs recorded simultaneously in a long-term care facility for 4 months in 329 patients and 261 healthcare workers to test this hypothesis. In the broad diversity of isolated S. aureus strains, 173 transmission events were observed between participants. The joint analysis of carriage and CPIs showed that CPI paths linking incident cases to other individuals carrying the same strain (i.e. possible infectors) had fewer intermediaries than predicted by chance (P < 0.001), a feature that simulations showed to be the signature of transmission along CPIs. Additional analyses revealed a higher dissemination risk between patients via healthcare workers than via other patients. In conclusion, S. aureus transmission was consistent with contacts defined by electronically collected CPIs, illustrating their potential as a tool to control hospital-acquired infections and help direct surveillance.

Journal article

Silhol R, Boelle P-Y, 2011, Modelling the effects of population structure on childhood disease: the case of varicella, PLoS Computational Biology, Vol: 7, Pages: 1-9, ISSN: 1553-734X

Realistic, individual-based models based on detailed census data are increasingly used to study disease transmission. Whether the rich structure of such models improves predictions is debated. This is studied here for the spread of varicella, a childhood disease, in a realistic population of children where infection occurs in the household, at school, or in the community at large. A methodology is first presented for simulating households with births and aging. Transmission probabilities were fitted for schools and community, which reproduced the overall cumulative incidence of varicella over the age range of 0–11 years old.Moreover, the individual-based model structure allowed us to reproduce several observed features of VZV epidemiology which were not included as hypotheses in the model: the age at varicella in first-born children was older than in other children, in accordance with observation; the same was true for children residing in rural areas. Model predicted incidence was comparable to observed incidence over time. These results show that models based on detailed census data on a small scale provide valid small scale prediction. By simulating several scenarios, we evaluate how varicella epidemiology is shaped by policies, such as age at first school enrolment, and school eviction. This supports the use of such models for investigating outcomes of public health measures.

Journal article

Silhol R, Zins M, Chauvin P, Chaix Bet al., 2010, Investigating the spatial variability in incidence of coronary heart disease in the Gazel Cohort: the impact of area socioeconomic position and mediating role of risk factors, Journal of Epidemiology & Community Health, Vol: 65, Pages: 137-143, ISSN: 0143-005X

Study objective The aim of the study was to improve understanding of the relationships between contextual socioeconomic characteristics and coronary heart disease (CHD) incidence in France. Several authors have suggested that CHD risk factors (diabetes, hypertension, cholesterol, overweight, tobacco consumption) may partly mediate associations between socioeconomic environmental variables and CHD. Studies have assessed the overall mediating role of CHD risk factors, but have never investigated the specific mediating role of each risk factor, not allowing their specific contribution to the area socioeconomic position–CHD association to be disentangled.Design After assessing geographical variations in CHD incidence and socioeconomic environmental effects on CHD using a multilevel Cox model, the extent to which this contextual effect was mediated by each of the CHD risk factors was assessed.Participants Data of the French GAZEL cohort (n=19 808) were used.Main results After adjustment for several individual socioeconomic indicators, it was found, in men from highly urbanised environments, that CHD incidence increased with decreasing socioeconomic position of the residential environment. After individual-level adjustment, a higher risk of obesity, smoking and cholesterol was observed in the most deprived residential environments. When risk factors were introduced into the model, a modest decrease was observed in the magnitude of the association between the socioeconomic contextual variable and CHD. Risk factors that contributed most to the decrease of the association were smoking and cholesterol.Conclusions Classic risk factors, although some of them more than others, mediated a modest part of the association between area socioeconomic position and CHD.

Journal article

SILHOL R, ALVAREZ FP, ARENA C, AMOROS JP, FLAHAULT A, HANSLIK T, BOËLLE PYet al., 2010, Micro and macro population effects in disease transmission: the case of varicella, Epidemiology and Infection, Vol: 138, Pages: 482-490, ISSN: 0950-2688

Profiles of varicella-zoster virus (VZV) seroprevalence have shown large variability in European countries in which vaccination has not been implemented. Differences in micro and macro population structures (e.g. household and municipality, respectively) may explain such variability, which is the focus of a population-based study of varicella in 12 000 children from 7800 French households in Corsica. The cumulative incidence was 89% at age 11 years, as the median age at infection was 5 years in first-born children, but decreased to 3·9 years in younger siblings. Hazard of infection in households increased as the first-born child initially enrolled in primary school. Age at infection was higher in less populated areas. Household attack rates increased with age (55% in <6 months, >90% in >3 years) and household secondary attack rate was ∼70%. Several levels of population structure independently affect age at varicella infection.

Journal article

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