Among them may be mentioned the variable degree of attenuation of different tubes of the vaccine and the varying susceptibility of the animals to be inoculated.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
The immunity which such vaccination confers upon the human subject has led many writers to assert that cowpox is simply a modified form of smallpox, whose harmless attack upon the human system is owing to a certain attenuation derived during its passages through the system of the cow or horse.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
Jericho lent the money, in the hope of climbing into the House of Lords with the assistance of the Duke; but this last drain upon his resources, with its penalty of attenuation, had left very little of him to go anywhere.
"John Leech, His Life and Work. Vol. 1"
William Powell Frith