The fact that "vaccination protects against smallpox and against itself" has been backed by such a mass of evidence that no practitioner of experience would attempt to question it. In the hundred and thirty years since Jenner's classic experiment, the value of vaccinia as a prophylaxis against smallpox has been demonstrated by clinical observation to the point at which we can count it scientifically proved. The fact that vaccination protects against itself has been recognized for practically as long a time owing to the inability of a person recently vaccinated to get a typical "take" on subsequent vaccination. It was not until Pirquet, 1 in 1906, demonstrated the neglected fact that this presumed nonsusceptibility was in reality the prompt reaction of an immunized individual that there was a fundamental observation on which to base a judgment as to the immune status of such an individual.
It is hard to say,
LEAKE JP, THOMAS S. THE VACCINATION SCAR AS AN INDEX OF IMMUNITY. JAMA. 1926;87(14):1125–1126. doi:10.1001/jama.1926.02680140043011
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