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Technical Examples
The present invention is concerned with isolated infectious bursitis (or bursal) disease virus(es) and a vaccine containing said virus(es) which is capable of protecting poultry against disease caused by infectious bursitis virus, characterized in that the vaccine virus(es) has/have the combined properties of, upon administration to a chicken, causing a reduction in the bursal size, expressed as bursa/body weight ratio, of less than 55%, and the capability to protect poultry having an ELISA antibody titer of at least about 500.