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.