ATTENUATED VIRUSES An attenuated poliovirus in which the 5' non-coding region of the genome is: (a) the 5' non-coding region of poliovirus type 3 Leon strain modified by the provision of the bases uracil and adenine at positions 472 and 537 thereof respectively, or (b) the 5' non-coding region of another poliovirus modified by the provision of the bases uracil and adenine at respective positions corresponding to position 472 and 537 of the 5' non-coding region of poliovirus type 3 Leon strain.
ATTENUATED VIRUSES