The preceding series of papers has furnished information pertaining to the number of infectious units of attenuated measles virus needed to immunize susceptible children. These presentations have also touched on the question of the optimum virus content of the live vaccine, both as it leaves the hands of the manufacturer and at the actual moment of human inoculation.
Several general remarks can be made as regards potency estimations of live virus vaccines. These products confer protection through actual infection of the recipient. In the case of the well-established vaccines, i.e., smallpox, yellow fever, and poliomyelitis, extensive experience has shown that a given number of infectious units of the attenuated virus will successfully immunize man. Thus, potency tests for these vaccines have become merely a matter of infectivity titrations in a suitable animal or cell culture system.
The total clinical experience with measles vaccine has now brought us to this same