Tracheal ring across neutralize test suggested that A2 and 4/91 were the same serotype,and the 4/91 vaccine could protect the chickens challenged by A2.

• 气管环交叉中和试验表明A2株与 4/ 91病毒为同一血清型 ; 临床的动物免疫试验结果表明 ,4/ 91疫苗有较好的保护作用