Introduction
Open vial vaccine wastage in multi-dose vials is a major contributor to vaccine wastage. Although switching from 10-dose vials to 5-dose vials could reduce wastage, a higher total cost could be triggered because smaller vials cost more to purchase and store.

Methods
This study drew field data of daily session sizes in local vaccination facilities from Bangladesh, India (Uttar Pradesh), Mozambique, and Uganda, and used Akaike Information Criteria to determine the best fit statistical distribution across various clinic types. These distributions were input to estimate the vaccine wastage using Lee's (2010) model. Inactivated polio vaccine (IPV) immunization was simulated to compare the costs over ten years with 10-dose vials versus 5-dose vials.

Results
By switching from 10- to 5-dose vials, the observed open vial wastage rate due to vial size preference and session size for IPV was reduced from 0.25 to 0.11 in Bangladesh, 0.17 to 0.08 in India (Uttar Pradesh), 0.13 to 0.06 in Mozambique, and 0.09 to 0.04 in Uganda, respectively. The cost savings realized from lower IPV wastage did not offset the higher costs of procurement and storage costs associated with smaller dose presentation.

Conclusion
While our model showed that switching from 10-dose vials to 5-dose vials of IPV reduced open vial wastage, it was not cost-saving.