Titration of Smallpox Vaccine by Pock-counting Method

Abstract

Titration of the smallpox vaccines released in 1959, 1960 and 1961 was carried out by pock-counting method and the reduction in virus titer during the storage period by the regulation of the Japanese Government was investigated. Correlation between virus titer in terms of PFU/ml and the rate of positive take in human vaccination was also examined, and the following conclusion was obtained.
1) Hundred per cent positive take in the primary vaccination of infants can be expected rby the multiple puncture method with a vaccine having a potency of more than 107.24 PFU/ml.
2) A reduction in potency on the date of expiration was less than 100.75 PFU/ml, if the vaccine was stored under the conditions indicated by the minimum requirement of biological products.
3) From the above it can safely be said that a vaccine having a potency of more than 108.00: PFU/ml at the time of the national assay will give 100 per cent positive take in the primary vaccination on the date of expiration of the vaccine.