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The advisory board of the Department of Health of Pennsylvania has ruled that three faithfully performed vaccinations at intervals of two successive weeks without taking are sufficient evidence that the individual is for the time being (estimated as one year) immune from smallpox. This is probably correct enough for most sanitary purposes, and the chances of a smallpox epidemic with such a rule in force may be practically negligible. Many physicians, however, have seen cases in which typical vaccinia has been produced by a fourth or fifth attempt at vaccination, hence it is not an altogether safe rule to follow with patients who have never been successfully vaccinated, that is, so far as absolute immunity is concerned.
VACCINE IMMUNITY.. JAMA. 1907;XLIX(9):780. doi:10.1001/jama.1907.02530090056011