| These previous outbreaks were due to an endemic avirulent strain of Newcastle disease virus that had become virulent due to mutation. |
| The avirulent live ND vaccines such as I-2 and NDV4-HR are unusual in that it is not possible to overdose with them. |
| Some avirulent isolates also became virulent after their inoculation in resistant rice varieties. |
| Nonrice isolates were generally avirulent or weakly pathogenic toward rice. |
| Forty-five isolates of avirulent ND virus were examined for antigenicity, safety and ability to spread. |
| Cross-protection between virulent isolates of serologically different strains is usually complete, but avirulent isolates do not afford protection. |
