OBJECTIVES: To estimate the cost impact to the NHS of switching from vial and syringe (V&S) to pre-filled syringe (PFS) administration of the ‘5-in-1’ vaccine for diphtheria, tetanus, pertussis, polio and Haemophilus influenzae type b. METHODS: A model was developed to estimate the cost impact of the switch in children less than 2 years old. Vaccines supplied were assumed to reach three destinations: administered (an estimate of 785,320 primary courses); opened and not administered (assumed to account for 7% of V&S and 0% of PFS supplied); or not opened by the expiry date (assumed to account for 0% of supply). All vaccines accrued acquisition and storage costs. PFS has bulkier packaging, leading to higher storage costs. Administered vaccines incurred costs for staff time, consumables (only required for V&S) and potential needlestick injuries (assumed not to occur with PFS).  As prices paid for vaccines by the NHS are not disclosed, the cost of a single dose was assumed equal for PFS and V&S.  Appointment times were obtained from a survey of 200 nurses, which estimated that PFS saved on average 4 minutes 47 seconds relative to V&S across three doses. All unit costs were sourced from the literature. RESULTS: The introduction of PFS was estimated to save £7.91 per 3-dose primary course and £6,214,562 per year for the NHS. Reductions in wastage and staff time contributed the greatest savings. Varying the wastage rate from 1-10% resulted in total cost savings of £3-8million; investigation into better estimates of vial wastage would strengthen the results. All other sensitivity analyses had a minimal impact on results. CONCLUSIONS: The switch to PFS administration is estimated to have generated substantial cost-savings to the NHS.  PFS has the potential to improve the efficiency of immunisation programmes by simplifying vaccine delivery and reducing the risk of handling errors.