suggests that this intervention might address these dynamics and might eventually be combined with vaccine strategies. Viral suppression within high-density networks of MSM might also be an approach fi t to the molecular epidemiology of MSM transmission.
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However, the public health importance of our new understanding might be aff ected by several important factors. First, the available data for HIV prevalence and incidence in MSM remain incomplete, with only one incidence estimate from Africa, two from Asia, and very few prospective cohorts of MSM in any setting. More than 30 years after the discovery of a viral infection in this population, 93 countries have no available reports on MSM in the past 5 years. What data we do have are hampered by the lack of population-based measures of the prevalence of same-sex behaviours in men, the size of the populations at risk, and the great diversity of these populations in diff ering social, cultural, and political contexts. Social response biases against reporting some behaviours, including receptive anal sex, are likely to aff ect risk factor assessment.
85Improved size estimation approaches are urgently needed. Encouragingly, these limitations have led to a range of innovations in epidemiology and are proving of use to the description of other hidden, stigmatised, or otherwise hard-to-reach populations.
103,104
Other biases in the global evidence base include the disproportionate urban sampling of MSM populations, the relative lack of data from the Middle East and north African region, and the lack of data for adolescents in sexual and gender minorities, who are systematically undersampled in most contexts. What data are available on younger MSM suggest that they are high-incidence groups in need of targeted interventions, and that waiting until they are aged 18 years, or older, is, as with girls in many high-risk transmission contexts, inadequate to protect them against early acquisition of HIV.