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Despite the widespread use of Bluetongue serotype 8 (BTV-8) vaccines across Europe, there remain unanswered questions on the effects of maternally derived antibodies (MDA) on vaccination. This study investigated the persistence of MDA in lambs and their interference on vaccine efficacy. Lambs born from hyperimmune ewes-. i.e. ewes naturally infected with BTV-8 in 2007 and repeatedly vaccinated thereafter-were used. They were allocated to five groups (n=7/group), four receiving a BTV-8 vaccine at different ages (3, 5, 7 and 9 months) and one left unvaccinated. Vaccine protection was assessed upon a BTV-8 challenge performed at 10 months of age. The neutralising antibody titres were also measured throughout the study. Vaccination at 5, 7 and 9 months led to seroconversion and full clinical and virological protection in all animals. Among lambs vaccinated at 3 months, three (3/7) did not respond to vaccination and were not protected from challenge infection. Others animals (4/7) were fully protected from clinical disease and viraemia in accordance with their seropositive status at time of challenge. In our very stringent conditions, MDA hampered response to BTV-8 vaccination in lambs for at least 3 months after birth. Accordingly, the optimal age of vaccination needs to be adjusted to the immune status of the sheep population.
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