After a strain of mumps virus was serially passaged in the amniotic cavity of chick embryo, it was tested for the grade of attenuation and reactogenicity as a live mumps virus vaccine in volunteered children who had been seronegative to mumps virus.
The vaccines prepared at the 4th, 6th, 9th, 15th, 21st, and 26th passage levels respectively were inoculated into different groups of those children. No clinical manifestations were observed after vaccination and the vaccines from the 4th, 6th, 9th, 15th passage level elicited an antibody response in all the vaccinees. However, the seroconversion rate decreased gradually as the passage level increased, those of 21st and 26th levels being 86% and 75%, respectively.
Then the plaque cloning was carried out using the 6th passage level (U-1005) virus: six clones were selected by the different plaque size on the chick embryo fibroblast cells. When six vaccine preparations prepared from those clones were compared, the seroconversion rates of U-1005-1, U-1005-2, and U-1005-9 clone vaccines were 97.4%, 95.8% and 97.5% respectively and they gave the geometric mean neutralizing antibody titer of 8.1, 9.9 and 6.3 respectively.
In order to test influences of the futher passage on antibody response, U-1005-9 strain was passaged up to 10th in chick amniotic cavity. When examined at 5th and 10th passage levels, it was proved stable concerning clinical reaction as well as antibody response.
