Purpose:
To examine the question of when to open multi-dose vials of vaccines, given that partially used vials have a limited shelf life which results in significant open-vial wastage.
Method:
Because many vaccines are manufactured in large, multi-dose vials which once opened, must be used within hours, clinicians (especially those in remote locations) face difficult tradeoffs between opening a vial to satisfy a small immediate demand, versus retaining the vial to satisfy a potentially large future demand. Hence, we formulate a Markov decision process model that determines when to cease vaccine administration as a function of time of day, the current vial inventory and the remaining number of clinic-days until the next replenishment. The objective is to maximize the expected number of vaccinations administered between two consecutive stock replenishments, or equivalently, to minimize open-vial waste. We compare the resulting policy performance to that of the current “greedy” policy that is to never close the clinic early.
Result:
We analytically prove that the optimal policy is of the form in Figure 1, i.e., there exists an optimal time-of-day threshold at which point it is optimal to discontinue service immediately if no vial is open or to discontinue service when the vial currently open is emptied. We also evaluate a practical heuristic policy that performs well (Figure 2) and is easier to implement.
Conclusion:
We conduct extensive sensitivity analysis and evaluate optimal and heuristic policy performance as a function of the number of clinic-days between successive replenishments, buffer stock and vial size. The heuristic policy is consistently competitive with the optimal policy and outperforms the greedy policy by more than 15% in many cases. These analyses lend insight on the best combination of problem parameters (e.g., clinic days per week, vial size) to minimize vaccine wastage.
See more of: The 34th Annual Meeting of the Society for Medical Decision Making