Ratios of vaccinia virus particles to virus infectious units; studies of ratio changes during growth and adaptation in eggs, guinea pigs, and rabbits.

Total virus particle counts, infectivity titrations and the ratios between particles and infective units have been determined for vaccinia virus infected tissues. Growth curves of vaccinia in the chorioallantoic membrane are characterized by relatively low ratios from 1 to 4 days after inoculation and a marked rise in the ratio at more prolonged intervals. Ratio determinations of vaccinia virus passages in the egg, rabbit skin, and guinea pig skin have been made to study the phenomenon of adaptation in different hosts. The embryonated egg chorioallantoic membrane shows no variation in the ratio of particles to infectious units during passage and it is concluded that this host is completely susceptible to vaccinia. During adaptive passages on the skin of rabbits and guinea pigs relatively large amounts of non-infective virus appear as indicated by a rise in the particle-infectivity ratios. The extent of ratio increase appears related to the general resistance of the host to the virus. Finally, treatment of crude tissue extracts with sonic vibration is described as an aid in dispersing the virus particles for quantitative particle counts.