Observations on the classification and interpretation of reactions to smallpox vaccination.

Over the last ten years extensive trials have been carried out on various smallpox vaccines prepared by the Lister Institute of Preventive Medicine in England, and the opportunity has been taken to make careful records of the reactions encountered, to investigate the significance of the immediate reaction and to determine the relationship of techniques to vaccination failures.In this paper the various classifications of vaccination reactions are reviewed, and a modified system of classification is suggested, based on the experience of a large number of vaccinations performed with a uniform technique.The two main vaccination techniques (linear scratch and multiple pressure) are considered and evidence is adduced to suggest that the depth of insertion of the virus is the most important single factor contributing to successful vaccination. A standard scratch technique is recommended, which, by its speed and simplicity, is well suited to mass vaccination as well as to general use for individual patients.The problem of duration of immunity following successful vaccination is also discussed, experimental results suggesting that it is unusual to get a "revaccination vaccinia" reaction on revaccination within three years, providing the previous vaccination was successfully performed.