Antigenic variants in a plaque-isolate of foot-and-mouth disease virus: implications for vaccine production

The occurrence of many subtypes within a serotype of foot-and-mouth disease virus (FMDV) makes it difficult to control the disease by vaccination. Although inactivated vaccines are used successfully in many countries, the appearance in the field of antigenic variants against which the vaccines do not confer protection is a constant problem in vaccine manufacture. We had found previously a mixture of antigenic variants in a field isolate of serotype A12. In this report we demonstrate the presence of two variants in a plaque-isolate from this mixture. The second variant was detected only when the growth conditions were altered. Our observation points to the problems which may be encountered in the large scale growth of a virus for vaccine production.