International collaborative laboratory studies on the International Reference Preparations of
Typhoid Vaccine have so far failed to provide data on which international units for these
vaccines can be based. Further assays carried out using the active mouse protection test, with immunization by the subcutaneous, intraperitoneal or intravenous route, confirmed the findings by some workers that the International Reference Preparation of
Typhoid Vaccine (
Acetone-
Inactivated) (vaccine K) was more effective than the International Reference Preparation of
Typhoid Vaccine (Heat-
Phenol-
Inactivated) (vaccine L), and indicated that intraperitoneal immunization was the most promising method.
Vaccine K, together with the material extracted by the
acetone in the preparation of the
vaccine, had a significantly lower effectiveness (at the 5% probability level) only when intraperitoneal immunization was used. The reasons for the differences found between the various
vaccines and routes of immunization are discussed at length.It is suggested that challenge with a strain of Salmonella moscow instead of the strain of Salm. typhi used until now gives a true
infection and forms the basis of a reliable method for the potency assay of
typhoid vaccines.