*4.2. Future Studies*

The limited studies available and reviewed in this review point to the need for more evidence generation and documentation, including documenting and sharing tested innovations within the urban context. These are not only essential but also have the potential to be incredibly innovative and informative. Many countries included in the search have not performed or reported studies on immunization performance in urban contexts during or after the pandemic. Future studies could consider examining the efficacy and effectiveness of innovations, urban settlement micro plans and micro tracking mechanisms, GIS mapping, the intertwining of conflicts, displacements and urban immunization, digital registry systems, and how urban immunization plans can be combined with other social sector innovations.

### **5. Limitations**

The literature search was limited to the top 20 zero-dose countries based on 2021 WUENIC data; these, however, may not be fully representative of other countries. Only 15 papers were eligible after screening, and some of the 20 countries were not represented at the conclusion of the literature review, indicating a limitation of existing literature on the subject. Grey literature and other non-published reports were not included in this review. Some of the studies did not fully elucidate the nuances of challenges faced in urban or peri-urban settings.
