Most vaccines consist of viruses that have been attenuated, disabled, weakened or killed in some way so that their virulent properties are no longer effective. A simple
genetically modified vaccine, based on a thymidine kinase deficient mutant of pseudorabies virus was reportedly available as early as 2001 as a commercial vaccine to control Aujeszky’s disease in Europe, North America and Japan.^[1]

This article relies largely or entirely on a single source. (November 2020)