first three injections and subsequent inoculations on schedule (personal communication, S. A. Plotkin, consultant to the Institute of Medicine Committee to Assess the Safety and Efficacy of the Anthrax Vaccine, January 29, 2001). Otherwise, the inoculees were referred to as “incomplete.” There were 379 complete vaccine recipients and 414 complete placebo recipients. Only data for those designated complete inoculees were included in the calculation of efficacy. Routine visits and environmental sampling were conducted throughout the study to confirm exposure and identify cases of anthrax.
Over the course of the study, 26 cases of anthrax occurred, including an outbreak of 9 cases over a 10-week period at one of the mills (Mill A). Twenty-one of the 26 cases were cutaneous anthrax and 5 were cases of inhalational anthrax, all of which occurred during the outbreak at Mill A. Of the 26 cases, 3 occurred among vaccine recipients (1 complete and 2 incomplete inoculees), 17 occurred among individuals in the placebo group (15 complete and 2 incomplete inoculees), and 6 cases occurred among individuals who were not inoculated with the vaccine or the placebo. None of the cases of inhalational anthrax occurred in persons who had received the vaccine.
The overall effectiveness of the vaccine against anthrax infection generally was 92.5 percent (lower 95 percent confidence interval = 65 percent): 91.4 percent in the high-risk group of workers and 100 percent in the low-risk group of workers. It was not possible to evaluate the efficacy of the vaccine against inhalational anthrax separately because of the small number of cases. (Given the definition of “complete” vaccination in the published paper, one of the cases counted in the incomplete vaccination group should have been included in the complete vaccination group. Doing so would have reduced the reported effectiveness somewhat. It is not possible to calculate the effectiveness after the addition of this case to the complete vaccination group since the necessary information is not provided in the paper.)
As part of another effort, the Centers for Disease Control and Prevention (CDC) collected observational data on the occurrence of anthrax in industrial settings like textile mills between 1962 and 1974 (FDA, 1985). During this period, both the Merck vaccine and AVA, produced by the Michigan Department of Public Health, were used and 27 cases of cutaneous anthrax were identified. No cases occurred in those who had received the full course of immunizations. Three cases occurred in persons who worked in or near mills but who were not mill employees and who were not vaccinated. The remaining 24 cases occurred among mill employees; 3 of these had received one or two doses of the vaccine, and the remaining 21 persons were unvaccinated. Thus, of the 27 cases observed, 2 occurred in persons who had received two doses of vaccine, 1 occurred in a person who