Salmonella-based vaccines.

There continues to be considerable interest in the development of a safe, effective, live, oral vaccine to combat typhoid fever of humans. Such a vaccine may be a derivative of the causative agent of the disease, Salmonella typhi. The prototype of such a vaccine, Ty21a, is not ideal, but no replacement for Ty21a is yet obvious. The construction and trial of bivalent vaccines, in which an attenuated Salmonella strain expresses determinants from another pathogen, awaits the development of a suitably attenuated derivative. In parallel with vaccine development programmes, a variety of techniques have been designed to effect stable association between Salmonella carrier and introduced cloned DNA.