Adequate surveillance of hard-to-reach and 'hidden' subpopulations is crucial to containing the HIV epidemic in low prevalence settings and in slowing the rate of transmission in high prevalence settings. For a variety of reasons, however, conventional facility and survey-based surveillance data collection strategies are ineffective for a number of key subpopulations, particularly those whose behaviors are illegal or illicit. This paper critically reviews alternative sampling strategies for undertaking behavioral or biological surveillance surveys of such groups. Non-probability sampling approaches such as facility-based sentinel surveillance and snowball sampling are the simplest to carry out, but are subject to a high risk of sampling/selection bias. Most of the probability sampling methods considered are limited in that they are adequate only under certain circumstances and for some groups. One relatively new method, respondent-driven sampling, an adaptation of chain-referral sampling, appears to be the most promising for general applications. However, as its applicability to HIV surveillance in resource-poor settings has yet to be established, further field trials are needed before a firm conclusion can be reached.

Because the modified sampling strategy maintains the scientific rigor of conventional cluster sampling methods while retaining many of the desirable features of the EPI survey methodology, the methodology is likely to be a preferred 'middle ground' survey design, relevant for many applications, particularly surveys designed to monitor multiple health indicators over time. The fieldwork burden in the modified design is only marginally higher than in EPI cluster surveys, and considerably lower than in conventional cluster surveys.

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There is concern that orphans may be at particular risk of HIV infection due to earlier age of sexual onset and higher likelihood of sexual exploitation or abuse; however, there is limited empirical evidence examining this phenomenon. Utilizing data from 1,694 Black South African youth aged 14-18, of whom 31% are classified as orphaned, this analysis explores the relationship between orphan status and sexual risk. The analysis found both male and female orphans significantly more likely to have engaged in sex as compared to non-orphans (49% vs. 39%). After adjusting for socio-demographic variables, orphans were nearly one and half times more likely than non-orphans to have had sex. Among sexually active youth, orphans reported younger age of sexual intercourse with 23% of orphans having had sex by age 13 or younger compared to 15% of non-orphans. Programmatic implications of these findings for the care and protection of orphans are discussed.

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The findings provide further justification for interventions targeting key contextual factors that influence youth behaviors in addition to providing youth with necessary communication, negotiation and other life skills.

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