GENETIC STABILITY OF SABIN STRAIN OF TYPE 3 POLIOVIRUS IN THE SENSITIVITY TO INHIBITORY BOVINE SERUM I. SELECTION OF INHIBITORY BOVINE SERUM AGAINST TYPE 3 SABIN STRAIN AND SENSITIVITY TO THE SERUM OF VACCINE PROGENIES

Investigations were carried out on the identification and genetic stability of type 3 Sabin vaccine (Leon 12 a1b) strain using a bovine inhibitor marker. Sera with strong inhibitory activity to the virus were rarely found in calves, while with high frequencies in heifers, and found to be reduced slightly in number at advanced ages. Of 495 sera tested, two (BS-19 and BE-19) specifically neutralized 3 laboratory strains of type 3 poliovirus including Leon 12 a1b, and left wild virus strains of the same type entirely unaffected. The strain-specific reactivity of the inhibitory sera was clearly demonstrated in the kinetic neutralization test. In vitro or human passage progenies of the vaccine virus were investigated for their sensitivity to the two bovine sera. While both the vaccine virus and its progenies were equally sensitive to BS-19, the progenies were less sensitive than the original vaccine virus to the inhibition by BE-19. On the basis of these results, the practical utility of bovine inhibitor marker for the identification of vaccine-derived viruses was discussed.