Many vaccines are manufactured in large, multi-dose vials that once opened, must be used within a matter of hours. As a result, clinicians (especially those in remote locations) face difficult tradeoffs between opening a vial to satisfy a potentially small immediate demand versus retaining the vial to satisfy a potentially large future demand. This article formulates a Markov decision process model that determines when to conserve vials as a function of time of day, the current vial inventory, and the remaining clinic-days until the next replenishment. The objective is to minimize open-vial waste while administering as many vaccinations as possible. It is analytically established that the optimal policy is of a threshold type. Furthermore, an extensive sensitivity analysis is conducted that speaks to the benefits of consolidating demand, investing in buffer stock, and adopting different vial sizes. Lastly, a practical heuristic is evaluated and shown to perform competitively with the optimal policy.Keywords: Markov decision process, perishable inventory model, multi-dose vial, vaccine wastage * Corresponding author gitis, etc. A key step in achieving this goal is to reduce the large amount of vaccine "wastage", i.e., doses that are manufactured and shipped but then not administered while still viable, in developing countries. WHO (2005) estimates the overall global vaccine wastage rate to be a staggering 50% which varies with vial size, vaccine type, location, etc. More specifically, a recent study conducted in Bangladesh (Guichard et al., 2010) reports average wastage rates of 85.1% for BCG (tuberculosis), 71.2% for measles, 36.6% for TT (tetanus), and 44.2% for DPT (diphtheria, pertussis, and tetanus combination). Reducing wastage has been mandated by WHO (2005) and identified as a key factor in maintaining the financial sustainability of immunization programs in the world's poorest countries (Kamara et al., 2008) and in increasing the delivery of vaccines to patients. In the overall effort to improve vaccine coverage rates, it is critical that clinics manage vaccine supplies efficiently in the field to minimize wastage and maximize the number of immunizations administered.The primary driver of vaccine wastage is the fact that many vaccines are manufactured in large, multi-dose vials, which are more economical to produce than single-dose vials (Lee et al., 2010) but result in so-called open-vial waste (Guichard et al., 2010). Although unopened vials can be stored in reliable temperature-controlled environments for relatively long periods of time, for many vaccine types once a vial is reconstituted (i.e., "opened"), all of its doses must be used within a relatively short period of time