Assessing the impact of the COVID-19 pandemic among Venezuelan refugees and migrants in Colombia using respondent-driven sampling (RDS).

OBJECTIVES: To determine factors associated with adherence to COVID-19 mitigation measures, related symptoms and testing, as well as pandemic-related income loss among Venezuelan refugee and migrant adults in urban and border areas of Colombia. DESIGN: Phone-based respondent-driven sampling SETTING: Bogotá and Norte de Santander, Colombia. PARTICIPANTS: 605 adult Venezuelan refugees and migrants residing in Bogotá (n=305) and Norte de Santander (n=300), who arrived in Colombia after 2014 and completed the survey in August and September 2020. PRIMARY AND SECONDARY OUTCOME MEASURES: Full COVID-19 compliance (vs incomplete or no compliance), any COVID-19-related symptoms (vs none) and income loss due to isolation measures in Colombia (vs no income change or increase in income). RESULTS: Older age was associated with lower odds of compliance with physical distancing measures (0.94, 0.90-0.99; p=0.01) for those in Bogotá. Nearly 15% of refugees and migrants in both locations (81 of 605) experienced at least one symptom consistent with COVID-19. Having a health condition was associated with higher odds of experiencing COVID-19-related symptoms in Bogotá (4.00, 1.22-13.06; p=0.02) and Norte de Santander (6.99, 1.95-24.99; p=0.003). Around 8% in both locations (48 of 605) were tested for COVID-19. Around 90% in both locations (537 of 605) had trouble earning an income after the introduction of isolation measures, and the median reported monthly income decreased by half in Bogotá and by 30% in Norte de Santander. A higher level of education (3.46, 1.02-11.75; p=0.05) was associated with higher odds of income loss among participants in Norte de Santander. CONCLUSIONS: Results indicate high compliance with COVID-19 mitigation measures, low testing rates and high pandemic-related income loss among Venezuelan refugees and migrants in Colombia. This study provides insights into a hard-to-reach refugee and migrant population in Colombia; additional study on the effects of the pandemic on hidden populations is warranted.

Deriving and interpreting population size estimates for adolescent and young key populations at higher risk of HIV transmission: Men who have sex with men and females who sell sex.

Population sizes of adolescent (15- to 19-years) and young (20 to 24-years) key populations at risk for HIV transmission are essential for developing effective national HIV control strategies. We present new population size estimates of adolescent and young men who have sex with men and females who sell sex from 184 countries in nine UNICEF regions using UNAIDS published population size estimations submitted by national governments to derive 15-24-year-old population proportions based on the size of equivalent adult general populations. Imputed sizes based on regional estimates were used for countries or regions where adult proportion estimates were unavailable. Proportions were apportioned to adolescents and young adults based on age at sexual debut, by adjusting for the cumulative percentage of the sexually active population at each age for sex. Among roughly 69.5 million men who have sex with men, 12 million are under the age of 24 years, of whom 3 million are adolescents. There are an estimated 1.4 million adolescent and 3.7 million young females who sell sex. Roughly four and a half million adolescent men who have sex with men and females who sell sex would benefit from early HIV interventions. These population size estimates suggest there are roughly 17 million adolescent and young men who have sex with men and females who sell sex who need HIV prevention services and social support. These data provide evidence for national and international programs to determine how many adolescent and young key populations need essential health services and are living with HIV and other infections. Age disaggregated population sizes inform epidemic models, which increasingly use age-sex structures and are often used to obtain and allocate resources and human capacity and to plan critical prevention, treatment, and infection control programs.

Populations size estimations using SS-PSE among MSM in four European cities: how many MSM are living with HIV?

BACKGROUND: Although men who have sex with men (MSM) are considered at high risk for transmission of sexually transmitted infections, including HIV, there are few studies estimating the population size of MSM in Europe. We used network data from a survey of MSM in four cities to perform successive sampling-population size estimations (SS-PSE) to estimate MSM population sizes. METHODS: Data were collected in 2013-14 in Bratislava, Bucharest, Verona and Vilnius using respondent-driven sampling (RDS). SS-PSE uses a Bayesian framework to approximate the RDS sampling structure via a successive sampling model and uses the selection order of the sample to provide information about the distribution of network sizes over the population members of MSM. RESULTS: We estimate roughly 4600 MSM in Bratislava, 25 300 MSM in Bucharest, 7200 in Verona and 2900 in Vilnius. This represents 2.9% of the estimated adult male population in Bratislava, 2.3% in Bucharest, 2.7% in Verona and 1.5% in Vilnius. The number of MSM living with HIV would roughly be 200 in Bratislava, 4554 in Bucharest, 690 in Verona and 100 in Vilnius. CONCLUSIONS: Benefits of this method are that no additional information from an RDS survey needs to be collected, that the sizes can be calculated ex post facto a survey and that there is a software programme that can run the SS-PSE models. However, this method relies on having reliable priors. Although many countries are estimating the sizes of their vulnerable populations, European countries have yet to incorporate similar and novel methods.

Ensuring Inclusion of Adolescent Key Populations at Higher Risk of HIV Exposure: Recommendations for Conducting Biological Behavioral Surveillance Surveys.

Ending acquired immune deficiency syndrome (AIDS) depends on greater efforts to reduce new human immunodeficiency virus (HIV) infections and prevent AIDS-related deaths among key populations at highest HIV risk, including males who have sex with males, sex workers, and people who inject drugs. Although adolescent key populations (AKP) are disproportionately affected by HIV, they have been largely ignored in HIV biological behavioral surveillance survey (BBSS) activities to date. This paper reviews current ethical and sampling challenges and provides suggestions to ensure AKP are included in surveillance activities, with the aim being to enhance evidence-informed, strategic, and targeted funding allocations and programs toward ending AIDS among AKP. HIV BBSS, conducted every few years worldwide among adult key populations, provide information on HIV and other infections' prevalence, HIV testing, risk behaviors, program coverage, and when at least three of these surveys are conducted, trend data with which to evaluate progress. We provide suggestions and recommendations on how to make the case to ethical review boards to involve AKP in surveillance while assuring that AKP are properly protected. We also describe two widely used probability sampling methods, time location sampling and respondent driven sampling, and offer considerations of feature modifications when sampling AKP. Effectively responding to AKP's HIV and sexual risks requires the inclusion of AKP in HIV BBSS activities. The implementation of strategies to overcome barriers to including AKP in HIV BBSS will result in more effective and targeted prevention and intervention programs directly suited to the needs of AKP.

Sampling Males Who Inject Drugs in Haiphong, Vietnam: Comparison of Time-Location and Respondent-Driven Sampling Methods.

Accurate measurements of HIV prevalence and associated risk factors among hidden and high-risk groups are vital for program planning and implementation. However, only two sampling methods are purported to provide representative estimates for populations without sampling frames: time-location sampling (TLS) and respondent-driven sampling (RDS). Each method is subject to potential biases and questionable reliability. In this paper, we evaluate surveys designed to estimate HIV prevalence and associated risk factors among people who inject drugs (PWID) sampled through TLS versus RDS. In 2012, males aged ≥16 years who reported injecting drugs in the previous month and living in Haiphong, Vietnam, were sampled using TLS or RDS. Data from each survey were analyzed to compare HIV prevalence, related risk factors, socio-demographic characteristics, refusal estimates, and time and expenditures for field implementation. TLS (n = 432) and RDS (n = 415) produced similarly high estimates for HIV prevalence. Significantly lower proportions of PWID sampled through RDS received methadone treatment or met an outreach worker. Refusal estimates were lower for TLS than for RDS. Total expenditures per sample collected and number of person-days of staff effort were higher for TLS than for RDS. Both survey methods were successful in recruiting a diverse sample of PWID in Haiphong. In Vietnam, surveys of PWID are conducted throughout the country; although the refusal estimate was calculated to be much higher for RDS than TLS, RDS in Haiphong appeared to sample PWID with less exposure to services and required fewer financial and staff resources compared with TLS.

Chlamydia prevalence and associated behaviours among female sex workers in Vanuatu: results from an integrated bio-behavioural survey, 2011.

There is insufficient data on sexually transmitted infections (STI) and related behaviours among key populations, including female sex workers (FSW), in the Pacific region. Using respondent driven sampling, we conducted an integrated bio-behavioural survey with FSW in Vanuatu (aged ≥18 years) to investigate risk behaviours associations with Chlamydia trachomatis (CT). Weighted population estimates and correlates of CT infection were calculated. Among 149 FSW, prevalence of CT was 36 % (95 % CI 26-48 %). Few FSW reported consistent condom use with recent transactional sex partners (TSP) (8 %; 95 % CI 2-13 %). CT infection was positively associated with increasing number of TSP (adjusted odds ratio [AOR] 1.1; 95 % CI 1.0-1.2) and group sex (AOR 2.9; 95 % CI 1.1-8.2). CT was negatively associated with increasing age of first sex (AOR 0.6; 95 % CI 0.5-0.9) and previous STI treatment (AOR 0.1; 95 % CI 0.0-0.4). A comprehensive public health strategy for prevention and treatment of STI among FSW, incorporating community empowerment strategies, FSW-targeted health services and periodic presumptive treatment, is urgently needed in Vanuatu.

HIV, syphilis and sexual risk behaviours among men who have sex with men in Agadir and Marrakesh, Morocco.

OBJECTIVE: To collect baseline measurements of HIV and syphilis prevalence and sexual risk behaviours among men who have sex with men (MSM) in Agadir and Marrakech, Morocco, and provide strategic information to improve outreach programmes. METHODS AND DESIGN: Respondent-driven sampling was used to recruit men who reported having anal sex with another man in the last 6 months, aged 18 years and older and living in either Agadir or Marrakech for the past 6 months, regardless of nationality. Data were analysed with the multiplicity estimator using respondent-driven sampling analysis tool V.6.0. RESULTS: 323 MSM in Agadir and 346 in Marrakech were recruited into the survey. Most MSM in both cities reported being < 25 years, being unemployed, bisexual and in a couple with both a man and a woman. Most reported selling sex and having sex with women. HIV prevalence was 5.6% in Agadir and 2.8% in Marrakesh; syphilis was 7.0% in Agadir and 10.8% in Marrakesh. Among MSM who tested positive for HIV, 31.6% in Agadir and 56.4% in Marrakesh were co-infected with syphilis. CONCLUSIONS: HIV and syphilis findings coupled with high risk activities indicate the need for expanding programmes targeting MSM throughout Morocco. Selling sex and sex with women may be a strategy to cope with extreme stigma towards MSM. Criminalisation and discrimination of MSM in Morocco underscores the urgent need for long-term and sustainable risk reduction through legal reforms and promotion and protection of human rights.

Determinants of HIV infection among female sex workers in two cities in the Republic of Moldova: the role of injection drug use and sexual risk.

In 2009-2010, females who reported having vaginal, anal or oral sex in exchange for money in the previous year, ≥16 years, and living in either Chisinau (n = 299) or Balti (n = 359), Moldova, were recruited into a respondent driven sampling survey. One fifth reported ever injecting drugs and over 30 % ever had sexual intercourse with men who inject drugs. In both cities, condom use with permanent and casual partners was much lower than condom use with commercial partners. In Chisinau, 6.9 % and in Balti, 24.7 % tested positive for HIV; 18 and 23.7 % had antibodies to hepatitis C; 9.1 and 8.9 % had antibodies to HBV; and, 8.4 and 6.1 % tested positive for syphilis. HIV seropositive FSW in either city were more likely to have ever injected drugs and to be infected with HCV. Limited government initiative and drastic reductions in international funding will likely impact urgently needed HIV prevention and harm reduction services targeting FSW in Moldova.

Unexpectedly high injection drug use, HIV and hepatitis C prevalence among female sex workers in the Republic of Mauritius.

Female sex workers (FSW) often have a disproportionately high prevalence of HIV infection and they, along with their clients, are considered a core group contributing to the transmission of HIV in many countries. In 2010, females who reported having vaginal/anal/oral sex in the last 6 months with a male in exchange for money or gifts, aged ≥15 years, and living in Mauritius were recruited into a survey using respondent driven sampling. Consenting females (n = 299) completed a behavioral questionnaire and provided venous blood for HIV, HCV and HBV testing. HIV seroprevalence among FSW was 28.9 % and 43.8 % were infected with HCV; among HIV seropositive FSW, 88.2 % were also infected with HCV. Almost 40 % of FSW reported injecting drugs sometime in their lives and 30.5 % of all FSW reported doing so in the previous 3 months. Among those who ever injected drugs, 82.5 % did so in the past 3 months and among those 60 % reported injecting drugs at least once a day. Among FSW who ever injected drugs, 17.5 % reported sharing a needle at last injection. Regression analyses found injection drug use behaviors to be positively associated with HIV seroprevalence. These findings indicate that FSW, especially those who inject drugs, are at high risk for HIV and HCV infection and transmission and illustrates the need for gender responsive HIV and injection drug use prevention and treatment models that respond to the unique situations that affect this population.

Prevalence of HIV and sexually transmitted infections and patterns of recent HIV testing among men who have sex with men in Zagreb, Croatia.

OBJECTIVE: To determine the prevalence of HIV and other sexually transmitted infections (STIs) among men who have sex with men (MSM) in Zagreb, Croatia, and assess correlates of testing for HIV in the past 12 months. METHODS: The authors carried out a bio-behavioural survey using respondent-driven sampling (RDS) from September 2010 to February 2011. Participants completed a questionnaire and were asked to provide blood, urine, oropharyngeal and rectal swabs for the detection of infections. Data were analysed using RDS Analysis Tool 6.0.1 and STATA V.8.0. RESULTS: A total of 387 MSM were recruited at the University Hospital for Infectious Diseases. The age range of recruited men was 18-57 years. HIV prevalence was 2.8% (95% CI 1.1% to 5.1%) (3.6%, unadjusted), lower than that found in the first RDS survey carried out in 2006 (4.5%, 95% 2.2% to 7.3%) (4.9%, unadjusted). The seroprevalence of herpes virus type 2 was 5.9% (6.9, unadjusted) and that of syphilis measured by Treponema pallidum haemagglutination assay was 7.6% (6.7%, unadjusted). The authors found urethral and/or rectal infections with Chlamydia trachomatis in 7.2% (8.5%, unadjusted) of men and gonoccocal in 2.7% (2.1%, unadjusted). HIV testing in the past 12 months was reported by 32.7% (38.9%, unadjusted). In the multivariate analysis, significant correlates of recent HIV testing were having more than three partners in the past 12 months and the knowledge of HIV status of a regular partner. CONCLUSIONS: The results indicate that there might have not been a progression of an HIV and STI epidemic in the past 5 years among MSM in Croatia. Prevention should expand by providing better uptake of HIV and STI testing services, thus enabling timely treatment.

HIV risk behaviours and their relationship to intimate partner violence (IPV) among men who have multiple female sexual partners in Cape Town, South Africa.

HIV/AIDS and intimate partner violence (IPV) are growing public health concerns in South Africa. Knowledge about adult men's perpetration of IPV and links between HIV risk behaviours and IPV is limited. Respondent driven sampling was used to recruit men who have multiple concurrent female sexual partners. Forty-one percent of the 428 recruited men had perpetrated IPV. Inconsistent condom use was associated with physical IPV; experiencing a symptom of a sexually transmitted infection and engaging in transactional sex were associated with physical and sexual IPV; problem alcohol use was associated with physical, and any IPV, but not sexual IPV; having five or more partners was associated with sexual IPV; perceptions of partners' infidelity were associated with physical and any IPV. HIV risk reduction interventions among men, especially those with multiple female sex partners, should incorporate strategies to change the underlying construction of masculinity that combines the anti-social and risky behaviours of IPV perpetration, inconsistent condom use, transactional sex and heavy alcohol consumption.

"Taking care of business": alcohol as currency in transactional sexual relationships among players in Cape Town, South Africa.

In this article we examine the dynamics of social relationships in which alcohol use and risky sexual behaviors cooccur. As part of a larger biological and behavioral HIV surveillance survey, 20 men who lived in an urban, informal settlement on the outskirts of Cape Town, South Africa participated in in-depth interviews. Interview transcripts were analyzed according to a latent content analysis. Findings highlight the latent association between alcohol and transactional sex, and enable an in-depth examination of the normative role that alcohol plays in the formation of casual sexual partnerships characterized by exchange. We build on an existing conceptual model that traces the potential pathways by which alcohol use and transactional sex are linked to sexual risk behaviors. The study findings point to the need for multilevel HIV risk-reduction interventions among men to reduce excessive alcohol use, risky sexual behaviors, and underlying perceptions of ideal masculinity.

Formative research to optimize respondent-driven sampling surveys among hard-to-reach populations in HIV behavioral and biological surveillance: lessons learned from four case studies.

Respondent-driven sampling (RDS) is widely adopted as a method to assess HIV and other sexually transmitted infection prevalence and risk factors among hard-to-reach populations. Failures to properly implement RDS in several settings could potentially have been avoided, had formative research been conducted. However, to date there is no published literature addressing the use of formative research in preparing for RDS studies. This paper uses examples from Banja Luka, Bosnia and Herzegovina; Bangkok, Thailand; Podgorica, Montenegro; and St Vincent's and Grenadine Islands, Eastern Caribbean; among populations of men who have sex with men, female sex workers, and injecting drug users to describe how formative research was used to plan, implement, and predict outcomes of RDS surveys and to provide a template of RDS-specific questions for conducting formative research in preparation for RDS surveys. We outline case studies to illustrate how formative research may help researchers to determine whether RDS methodology is appropriate for a particular population and sociocultural context, and to decide on implementation details that lead to successful study outcomes.

Efficacy of convenience sampling through the internet versus respondent driven sampling among males who have sex with males in Tallinn and Harju County, Estonia: challenges reaching a hidden population.

This paper examines challenges obtaining representative samples of males who have sex with males (MSM) in Estonia and provides descriptive HIV behavioral data gathered from four cross-sectional surveys; three using the internet, and one using respondent driven sampling (RDS) to recruit MSM in Tallinn and Harju County. Estonian MSM were sampled between March and May in 2004 (n=193), August and November in 2005 (n=146) and September and December in 2007 (n=238) using internet websites. MSM in Tallinn and Harju County were sampled between April and June in 2007 (n=59) using RDS. Recruitment of MSM using RDS did not acquire the calculated sample size. The RDS study reached a less diverse group of MSM than did the internet studies which recruited a larger proportion of MSM who were older, bisexual, having female sexual partners during the last six months, and unlikely to have been tested for HIV. The findings and observations presented in this paper could inform researchers in Estonia, and the region, about the efficacy of and socio-cultural challenges to sampling MSM to collect HIV biological and/or behavioral data.

Prevalence of sexually transmitted infections among men who have sex with men in Zagreb, Croatia.

We used respondent-driven sampling among men who have sex with men (MSM) in Zagreb, Croatia in 2006 to investigate the prevalence of HIV, other sexually transmitted infections and sexual behaviours. We recruited 360 MSM. HIV infection was diagnosed in 4.5%. The seroprevalence of antibodies to viral pathogens was: herpes simplex virus type-2, 9.4%; hepatitis A, 14.2%; hepatitis C, 3.0%. Eighty percent of participants were susceptible to HBV infection (HBs antigen negative, and no antibodies to HBs and HBc antigen). Syphilis seroprevalence was 10.6%. Prevalence of Chlamydia and gonorrhoea was 9.0%, and 13.2%, respectively. Results indicate the need for interventions to diagnose, treat and prevent sexually transmitted infections among this population.

Implementation challenges to using respondent-driven sampling methodology for HIV biological and behavioral surveillance: field experiences in international settings.

Using respondent-driven sampling (RDS), we gathered data from 128 HIV surveillance studies conducted outside the United States through October 1, 2007. We examined predictors of poor study outcomes, reviewed operational, design and analytical challenges associated with conducting RDS in international settings and offer recommendations to improve HIV surveillance. We explored factors for poor study outcomes using differences in mean sample size ratios (recruited/calculated sample size) as the outcome variable. Ninety-two percent of studies reported both calculated and recruited sample sizes. Studies of injecting drug users had a higher sample size ratio compared with other risk groups. Study challenges included appropriately defining eligibility criteria, structuring social network size questions, selecting design effects and conducting statistical analysis. As RDS is increasingly used for HIV surveillance, it is important to learn from past practical, theoretical and analytical challenges to maximize the utility of this method.

Using respondent-driven sampling methodology for HIV biological and behavioral surveillance in international settings: a systematic review.

To determine operational and analytical characteristics of respondent-driven sampling (RDS) in international settings and to explore factors that may affect recruitment of most-at-risk populations using RDS, we reviewed HIV biological and behavioral surveillance studies that used this method outside of the United States. We identified 123 eligible studies, 59 from Europe, 40 from Asia and the Pacific, 14 from Latin America, seven from Africa and three from Oceania. Studies collectively recruited 32,298 participants between 2003 and 2007; 53% of studies were conducted among injecting drug users, which generally had faster recruitment compared with studies among sex workers. All but 13 studies reached > or = 90% of their intended sample size, and six studies failed to reach equilibrium for key variables. This review has shown that RDS is an effective technique, when designed and implemented appropriately, to sample most-at-risk populations for HIV biological and behavioral surveys.

The effectiveness of respondent driven sampling for recruiting males who have sex with males in Dhaka, Bangladesh.

This paper evaluates the effectiveness of respondent driven sampling (RDS) to sample males who have sex with males (MSM) in Dhaka, Bangladesh. A major objective for conducting this survey was to determine whether RDS can be a viable sampling method for future routine serologic and behavioral surveillance of MSM as well as other socially networked, hard to reach populations in Bangladesh. We assessed the feasibility of RDS (survey duration; MSM social network properties; number and types of initial recruits) to recruit a diverse group of MSM, the efficacy of an innovative technique (systematic coupon reduction) to manage the implementation and completion of the RDS recruitment process and reasons why MSM participated or did not participate. The findings provide useful information for improving RDS field techniques and demonstrate that RDS is an effective sampling method for recruiting diverse groups of MSM to participate in HIV related serologic and behavioral surveys in Dhaka.

Exploring barriers to 'respondent driven sampling' in sex worker and drug-injecting sex worker populations in Eastern Europe.

Respondent driven sampling (RDS) has been used in several counties to sample injecting drug users, sex workers (SWs) and men who have sex with men and as a means of collecting behavioural and biological health data. We report on the use of RDS in three separate studies conducted among SWs between 2004 and 2005 in the Russian Federation, Serbia, and Montenegro. Findings suggest that there are limitations associated with the use of RDS in SW populations in these regions. Findings highlight three main factors that merit further investigation as a means of assessing the feasibility and appropriateness of RDS in this high risk population: the network characteristics of SWs; the appropriate level of participant incentives; and lack of service contact. The highly controlled and hidden nature of SW organizations and weak SW social networks in the region can combine to undermine assumptions underpinning the feasibility of RDS approaches and potentially severely limit recruitment. We discuss the implications of these findings for recruitment and the use of monetary and non-monetary incentives in future RDS studies of SW populations in Eastern Europe.

Assessment of respondent driven sampling for recruiting female sex workers in two Vietnamese cities: reaching the unseen sex worker.

Respondent driven sampling (RDS) is a relatively new method to sample hard-to-reach populations. Until this study, female sex workers (FSWs) in Vietnam were sampled using a variety of methods, including time location sampling (TLS), which may not access the more hidden types of FSWs. This paper presents an analysis from an HIV biological and behavioral surveillance survey to assess the feasibility and effectiveness of RDS to sample FSWs, to determine if RDS can reach otherwise inaccessible FSWs in Vietnam and to compare RDS findings of HIV risk factors with a theoretical TLS. Through face-to-face interviews with FSWs in Ho Chi Minh City (HCMC) and Hai Phong (HP), data were collected about the venues where they most often solicit their clients. These data were used to create three variables to assess whether FSWs solicit their clients in locations that are visible, semi-visible and non-visible. For this analysis, the visible group simulates a sample captured using TLS. Survey results in HIV prevalence and related risk factors and service utilization, adjusted for sampling methodology, were compared across each of the three FSW visibility groups to assess potential bias in TLS relative to RDS. The number of self-reported visible FSWs (HCMC: n=311; HP: n=162) was much larger than those of the semi-visible (HCMC: n=65; HP: n=43) and non-visible (HCMC: n=37; HP: n=10) FSWs in HCMC and HP. Non-visible FSWs in both cities were just as likely as visible and semi-visible FSWs to be HIV positive (HCMC: visible 14.5%, semi-visible 13.8%, non-visible 13.5%, p value = 0.982; HP: visible 35.2%, semi-visible 30.2%, non-visible 30.0%, p value = 0.801), to practice behaviors that put them at risk for contracting and transmitting HIV (injecting drug use-HCMC: visible 13.8%, semi-visible 12.3%, non-visible 5.4%, p value = 0.347; HP: visible 38.9%, semi-visible 23.3%, non-visible 30.0%, p value = 0.378, to have no condom use in the past month -HCMC only: visible 52.7%, semi-visible 63.1%, non-visible 48.6%, p value = 0.249) and to have symptoms of a sexually transmitted infection (STI) in the past year (HCMC: visible 16.1%, semi-visible 12.3%, non-visible 16.2%, p value = 0.742; HP: visible 13.6%, semi-visible 18.6%, non-visible 20.0%, p value = 0.640). There was a difference found among the visible, semi-visible and non-visible groups in HP for no past month condom use (visible 53.1%, semi-visible 79.1%, non-visible 60.0%, p value = 0.009). This study found that RDS was successful at recruiting hidden types of FSWs in Vietnam. Past reports of FSWs in Vietnam have assessed the more visible FSWs as being the most vulnerable and at risk for HIV. Although the number of visible FSWs is much higher than those of the semi and non-visible groups, this study found that the non-visible FSWs are very vulnerable to HIV infection. If prevention programs are targeting and responding to those who are most likely to be assessed (e.g., more visible types of FSWs) then this analysis indicates that a significant proportion of the FSW population at risk for HIV may not be receiving optimal HIV information and services.