Egg-attenuated duck hepatitis type (Rispens H55) was exhaustively tested as a potential vaccine under controlled conditions in ducklings fully susceptible to the disease at day 2 after hatching. Data are presented which indicate that this vaccine fulfils essential criteria of efficacy in terms of (a) the optimal age at which successful vaccination is practicable (day 2 or earlier), (b) the rapidity of onset of immunity (in 48 to 72 hours), (c) the high level of immunity induced (88.0 to 94.0 per cent), (d) the persistence of this degree of immunity in the individual bird throughout the period when it would otherwise be at risk (until the end of the fourth week of life) and (e) the consistency of the effects of the vaccine in successive groups of ducklings hatched over a four year period. Employed as a vaccine. H55 was completely innocuous to the vaccinated ducklings under laboratory conditions.
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