Smallpox vaccination immunity in relation to number of insertions.

The influence of the number of insertions of vaccine on smallpox vaccination immunity has been the subject of considerable controversy. In this study the problem was investigated in children of comparable age, sex, and smallpox vaccination history. The incidence of post-vaccinal fever and the antibody response of subjects given single and double insertions were compared. While single insertions caused fewer febrile reactions than double insertions, the difference was not significant. Successful vaccination invariably elicited a marked neutralizing antibody response. However, subjects who received two insertions developed significantly higher antibody titres than subjects with a single insertion. In the revaccinated group the difference in titre was still demonstrable 16 months after vaccination. It is concluded that the number of insertions may play a role in the duration of vaccination immunity.