Characteristics of the attenuated oral vaccine strain "S. typhi" Ty 21a.

The lack of a reliable in vitro test or animal model that can predict the potency of typhoid vaccines in man makes it extremely difficult to develop guidelines for the quality control of these vaccines. Attenuated S. typhi strain Ty 21a that has been shown to be safe and efficacious as live oral vaccine in clinical studies and in a controlled field trial, has been extensively characterized by genetical, biochemical and biological data. Production of live oral typhoid vaccine Ty 21a is based on a parent and working seed lot system. Characteristics of the seed lot cultures have to correspond with the original strain. Characteristics of strain Ty 21a relevant to its avirulence and potency are discussed.