Costs and return to scale analysis of extending the offer of pre-exposure prophylaxis (PrEP) to key populations aged 15–17 years old in two Brazilian cities - Andreia Costa Santos; Joilson Nascimento Paim; Patricia Coelho de Soarez; Natacha Cerchiari; Laio Magno; Ines Dourado; Alexandre Grangeiro (PLoS ONE, 2025)