VACCINES IN PROPHYLAXIS OF DISEASE 183 



careful bacteriologic study. (2) In order to 

 establish the potency and purity of the finished 

 product, the backs of white rabbits are shaved 

 and Vaccine Virus rubbed on the shaved areas 

 without scarification. Active virus will grow 

 and cause characteristic vesicles in 3 or 4 days 

 and the animals recover without serious conse- 

 quences. (3) The purity and safety of the fin- 

 ished product are further established by bac- 

 teriologic study and injections of the Glycerin- 

 ated Vaccine Virus into guinea-pigs and white 

 mice. 



Deterioration of Virus Vaccine Virus rap- 

 idly deteriorates unless kept cold below 50 F. 

 and there is no way of preventing it from be- 

 coming inactive if exposed for any length of 

 time to higher temperatures. Careful experi- 

 ments show that Vaccine Virus kept at 98 F. 

 for three days is killed : vaccine kept at 70 F. 

 for one week becomes inactive: vaccine kept at 

 50 F. (ordinary refrigerator temperature) for 

 three to six months remains active; while vac- 

 cine kept at 10 F. for four years is still active. 



Hence, the important factor is the tempera- 

 ture at which Vaccine Virus is kept and not the 

 age of the virus. Therefore, the expiration 



