Scalability can be understood as the ability to expand without changing. Yet, expanding an intervention to a global scale, we suggest, is a significant and difficult accomplishment. In this paper we propose to explore the kind of evidential exigencies that this accomplishment involves. To do so, we focus on the field of global health and examine how child immunization against the pneumococcus bacterium has been scaled up in low-income countries. The paper first attends to initial epidemiological scrutiny that revealed the existence of a large-scale public health problem and the possibility of an expandable solution (vaccination). It then describes the set-up of a funding arrangement using overseas aid to purchase vaccine doses manufactured by pharmaceutical companies, before paying attention to various frictions that affect the widespread use of pneumococcal vaccines. In these different moments through which scalability is accomplished, always partially and temporarily, we show that a dual activity can be witnessed, a pivoting between referential work and forward projection. To conclude, we suggest that scalability is more usefully approached as a form of expansion that is always attentive to the possibilities of change
