The SENTINEL study of differentiated service delivery models for HIV treatment in Malawi, South Africa, and Zambia: research protocol for a prospective cohort study oleh Sophie Pascoe, Amy Huber, Idah Mokhele, Nkgomeleng Lekodeba, Vinolia Ntjikelane, Linda Sande, Timothy Tchereni, Prudence Haimbe, AMBIT SENTINEL study team, Sydney Rosen