High passage in cells or eggs. When a virulent virus is isolated in cells or egg culture and then passaged into fresh cultures about 30-40 times it often becomes avirulent. This is because it has become culture adapted. The virus which grows fastest in culture has multiplied at the expense of the original virulent virus which replicated best in the target organs of the host. During adaptation the virus has changed genetically. The attenuated virus should now be cloned three times by plaque picking or by limiting dilution so that no virulent wild-type virus remains in the stock. This genetic change of virus occurs because viruses mutate at a rapid rate e.g. 10^-5 per neutralisation site per replication cycle as a result of errors during their replication.

The downside of this is that a backmutation with reversion to virulence may occur. For this reason vaccines are best tested for reversion to virulence by 6 passages in the original host.

Temperature-sensitive respiratory vaccines which have been selected for growth only in the upper respiratory tract but not in the lungs have been developed e.g. for parainfluenza virus 3 of calves.