THE INTERNATIONAL REFERENCE PREPARATION OF SMALLPOX VACCINE. AN INTERNATIONAL COLLABORATIVE ASSAY.

A purified, concentrated sheep vaccine, prepared from a vaccinia strain in use in the United Kingdom for more than 60 years, has been established as the International Reference Preparation of Smallpox Vaccine, intended to permit comparative assay with national reference vaccines.Before its establishment as the International Reference Preparation, the proposed international reference vaccine was tested in seven laboratories in as many countries together with four other distributed vaccines and one local vaccine in each country. All laboratories used the scarification test on rabbits; four used the pock count method; the intracutaneous test in rabbits, the plaque count in tissue culture and the LD(50) test in eggs, cultures and newborn mice were used by one to three of the participants.The proposed international reference preparation met all the requirements laid down by the WHO Study Group on Requirements for Smallpox Vaccine in six laboratories when the scarification test was used and in all laboratories when the pock count test was used.