The Barbervax® vaccine, directed against Haemonchus contortus, has been commercially available in Australia since 2014. However, to date the optimal timing for booster vaccination of lambing ewes has not been identified. Therefore the aim of this experiment was to compare the efficacy of Barbervax® when administered to ewes one week prior to the commencement of lambing versus at lamb marking 7 weeks later. A total of 400 single bearing Merino ewes, run on a commercial sheep property in the New England region of New South Wales, previously primed with Barbervax® were enrolled in the project. The experiment incorporated a cross-over design with two treatments imposed pre-lambing; Barbervax® booster vaccination with anthelmintic (n = 200) or anthelmintic alone (n = 200). At lamb marking, there were four treatment groups (n = 100) and anthelmintic was administered to all treatment groups. The four treatment combinations were Barbervax® booster vaccination at pre-lambing and lamb marking (V-V), booster vaccination only at pre-lambing (V-A), booster vaccination only at lamb marking (A-V) or anthelmintic only at both at pre-lambing and lamb marking (A-A). Pre-lambing Barbervax® booster vaccination of ewes induced an antibody response and, when used in conjunction with anthelmintic, significantly reduced worm egg counts (WEC) assessed at 7 weeks (lamb marking) and 13 weeks later, relative to treatment with anthelmintic alone. Booster vaccination plus anthelmintic treatment at lamb marking was not effective at reducing WEC compared to anthelmintic treatment alone. Body weight and packed cell volume did not differ between the treatment groups throughout the trial. In conclusion Barbervax® booster vaccination at pre-lambing provides additional protection for reproducing ewes given an effective short acting anthelmintic at this time.Copyright © 2020 Elsevier B.V. All rights reserved.