A dermal strain of vaccine virus has been passed through 99 successive culture passages. This procedure led to a diminution in the pathogenicity of the active agent for the rabbit. By repeated testicular passages in rabbits, however, the virus regained its pathogenicity for that host. New cultures were initiated with the revived virus. A culture strain of virus that has been twice revived in this manner has remained fairly stable for the rabbit through 60 culture passages and it produces mild, yet effective vaccinal reactions in man.
Virus in early cultures was not attenuated for man, but later cultures of the original strain and cultures of the 2nd and 3rd revived strains produced mild reactions without fever and discomfort to the patients. Intradermal vaccinations with the culture virus are safe and satisfactory.
With the culture virus 118 infants and children have been inoculated and in 100 of them a positive reaction occurred. The culture virus produced a refractory state to a standard dermal strain of calf lymph and vice versa.
Culture virus stored in 50 per cent neutral glycerol at –10°C. or at +3°C. maintained a considerable amount of its activity for at least 1 year. Desiccated culture virus sealed in tubes maintained some of its activity when stored at 37°C. for 5 weeks.
Fresh cultures can be initiated without difficulty from desiccated virus or from virus that has been stored with or without glycerol.