The invention provides an attenuated polio virus having a 5' non-coding region consisting of the 5' non-coding region of Sabin 3, modified so that it does not have a base pair mismatch in stem (a) or (b) of domain V, wherein seven or eight of the base pairs in stems (a) and (b) are U-A or A-U base pairs； and a capsid protein from the Sabin 1, Mahoney, MEF or Saukett strain.
