Dictionary.net

The clear liquid contained in the pock of vaccinia, holding in suspension the microorganisms peculiar to the disease. In the human subject it exudes abundantly on multiple puncture of the pock on or before the eighth day after vaccination; in the cow its spontaneous flow is very sparing, but much of it may be squeezed out with forceps. Human vaccine l. is viscid, but clear; that from the cow is often straw-colored. Inoculated directly from the pock or in the form of stored l. (preserved in the liquid form in sealed glass tubes or dried be tween plates of glass or on a non-absorbent surface, and moistened at the time of use), vaccine l. is employed in vaccination.