New vaccines and immunotherapies will only achieve their potential if they are designed and tested with a deep understanding of the early human host response to infection and vaccination. This talk will highlight how small, immunology‑rich clinical trials and controlled human infection studies serve as a “microscope” to characterise that response, optimise new modalities development and de‑risk larger efficacy trials. Using examples from platform vaccines (such as live-attenuated and mRNA approaches), experience from novel anti‑infective monoclonal antibodies, and host‑directed or metabolic interventions, the session will illustrate how dense longitudinal sampling and multi‑omics readouts in the first days after vaccination or pathogen exposure inform key decisions on dose, schedule, target populations, and go/no‑go. Particular emphasis will be placed on heterogeneity in responses in older adults and people with comorbidities, and on how early‑phase data can be translated into practical clinical and policy choices. The talk will also discuss the potential role the region can play in accelerating the development of regionally relevant vaccines and immunotherapies. Together, these perspectives hope to illuminate the importance of early‑phase, host‑response‑focused trials as central to the rational development and deployment of new therapeutic modalities in infectious diseases.