DURATION OF IMMUNITY FOLLOWING VACCINATION AGAINST SMALLPOX

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Abstract
It is still the traditional belief that the protective effects of vaccination last about seven years. The basis of this dictum lies in the observation that immunity to smallpox wears off in some individuals, and the magic number seven has been taken as the time to revaccinate. In the earlier studies on revaccination, results were recorded as successful, "a take," or unsuccessful, "no take." In the latter case the inference was drawn that the person was immune. This does not necessarily follow. In the light of present knowledge, the reaction on revaccination is classed as immediate, accelerated or primary, depending on the period of incubation, the height of the reaction and the course of events. The immediate reaction indicates immunity, the accelerated reaction partial immunity, and the primary reaction that immunity has worn off. Jenner knew of the reaction now called immediate and described it as occurring when variolous matter

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