The immediate reaction to smallpox vaccination is widely interpreted as evidence of immunity to the disease.1 This response, however, is a manifestation of sensitization to virus fractions, and immunity and sensitivity do not necessarily go hand in hand. Dead virus produces the same reaction in persons previously vaccinated but no longer immune as does live potent vaccine in highly immune persons.2 The danger inherent in relying on an immediate reaction after vaccination is emphasized by the occurrence of smallpox among soldiers whose vaccination records indicate that they had shown an "immune" response as recently as two to three months before the onset of the disease.3 It is possible that in some of these cases the "immune" reaction was administrative in nature4; it is probable that in others an inert vaccine was used.
Recent workers, recognizing the part of sensitivity in the immediate reaction, have relied on
BENENSON AS. IMMEDIATE (SO-CALLED "IMMUNE") REACTION TO SMALLPOX VACCINATION. JAMA. 1950;143(14):1238–1240. doi:10.1001/jama.1950.02910490016006