**Background:** Key populations—including sex workers, men who have sex with men, people who inject drugs, people in prisons, and trans and gender diverse individuals—are disproportionately affected by HIV, sexually transmitted infections (STIs), and viral hepatitis (VH). In 2020, 65% of the 1.5 million new HIV infections globally occurred among key populations and their sexual partners. Counselling behavioural interventions, defined as interventions where information is exchanged and support is provided so an individual can make decisions and take action to reduce risk, are widely used in key population programmes. However, their impact on HIV/STI/VH acquisition had not been previously synthesized. This systematic review was conducted to inform updated WHO consolidated guidelines on HIV, STI, and VH prevention, diagnosis, treatment, and care for key populations.
**Methods:** The review followed PRISMA guidelines and included three components: effectiveness, values and preferences, and cost studies. For the effectiveness review, the authors searched CINAHL, PsycINFO, PubMed, and EMBASE for studies published between January 2010 and December 2022. Inclusion criteria were RCTs comparing counselling behavioural interventions versus comparators among key populations, measuring HIV/STI/VH incidence (primary outcomes) or unprotected sex, needle/syringe sharing, and mortality (secondary outcomes, captured only if studies also reported primary outcomes). Two independent reviewers screened abstracts, extracted data, and assessed risk of bias using the Cochrane Collaboration tool. Pooled risk ratios were generated through random effects meta-analysis, and findings were summarized in GRADE evidence profiles. Values and preferences and cost data were summarized descriptively.
**Key Results:** From 6400 unique citations, nine articles met inclusion criteria for the effectiveness review, two for values and preferences, and two for costs. All nine effectiveness studies focused on HIV/STIs; one also covered HCV. Five were conducted in the United States, and one each in Kazakhstan, China, Kenya, and Mexico. Each of the five key populations was represented in at least one study. Meta-analysis of six RCTs (1280 participants) showed no statistically significant effect on HIV incidence (combined RR: 0.70, 95% CI: 0.41–1.20; no significant heterogeneity: Q=1.92, p=0.86, I²=0.00). Meta-analysis of six RCTs (3783 participants) showed no effect on STI incidence (RR: 0.99, 95% CI: 0.74–1.31; no significant heterogeneity: Q=2.12, p=0.83, I²=0.00). One RCT (139 participants) showed no statistically significant effect on HCV incidence when calculated as an unadjusted RR (RR: 0.45, 95% CI: 0.16–1.27), but a statistically significant reduction when presented as a rate ratio adjusting for baseline unsafe injection (rate ratio: 0.31, 95% CI: 0.10–0.90). For secondary outcomes, meta-analysis of seven RCTs (1811 participants) showed no effect on unprotected sex (RR: 0.82, 95% CI: 0.66–1.02; significant heterogeneity: Q=22.06, p=0.001, I²=72.75), and meta-analysis of two RCTs (564 participants) showed no effect on needle/syringe sharing (RR: 0.72, 95% CI: 0.32–1.63; no significant heterogeneity: Q=1.14, p=0.29, I²=11.91). Certainty of evidence was moderate for HIV and STI incidence outcomes, low for HCV and needle/syringe sharing, and very low for unprotected sex. Two values and preferences studies (one among men who have sex with men, one among trans and gender diverse youth, both in the US) found high satisfaction with specific counselling interventions. Two cost studies (one with sex workers on the US/Mexico border, one with young men recently released from US prisons) found reasonable intervention costs.
**Clinical Implications:** This review found no evidence that counselling behavioural interventions reduce HIV, STI, or viral hepatitis incidence among key populations. The findings contrast with earlier reviews showing moderate effectiveness in general populations and broader psychosocial interventions. The authors suggest that these interventions may be insufficient on their own to address the structural factors shaping risk among key populations. Based on this evidence, the WHO guideline development group issued a good practice statement noting that counselling behavioural interventions aimed at behaviour change have not been shown to affect HIV, VH, or STI incidence, while acknowledging that counselling and information-sharing not aimed at behaviour change can be a key component of engagement with key populations when provided non-judgmentally alongside other prevention interventions. The authors recommend prioritizing interventions with proven effectiveness and addressing structural barriers, rather than investing in counselling behavioural interventions for incidence reduction.