The invention provides an attenuated polio virus having a 5' non-coding regionconsisting of the 5' non-coding regionof 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 basepairs in stems (a) and (b) are U-A or A-U base pairs; and a capsid proteinfrom the Sabin 1, Mahoney, MEF or Saukett strain.