of the pathogen, and the host response. Even a vaccine considered highly effective may fail to protect some individuals under some circumstances.


The data used to evaluate the efficacy of AVA come from three sources. Studies with textile mill workers tested the efficacy of AVA and a related vaccine against occupational exposures to anthrax spores. Serological studies with humans tested the ability of AVA to elicit antibodies to protective antigen (PA), an indication of an immune response to the vaccine. Studies with animals tested the efficacy of the vaccine in protecting the animals from inhalational exposure to anthrax spores.

Human Efficacy Trials

Brachman and colleagues (1962) conducted the only randomized, placebo-controlled trial of the efficacy of a PA-containing anthrax vaccine. Although the safety information that it provides is reported separately in Chapter 6, here the committee describes the information on vaccine efficacy provided in that study. The vaccine studied was not AVA but was an earlier formulation produced from the R1-NP mutant of the Vollum strain of anthrax manufactured by Merck (see Chapter 7 for more details). The study was carried out from January 1955 through March 1959 in four textile mills in the northeastern United States that processed raw, imported goat hair for production into the interlinings of suit coats. The goat hair was typically contaminated with anthrax spores, and workers were exposed during handling of this material. Before receiving the vaccine, the average annual incidence of cutaneous anthrax among workers at these four mills ranged from 0.6 to 1.8 cases per 100 workers.

The worker population eligible for the study included 1,249 men and women with no history of prior anthrax infection. Approximately 47 percent of employees worked in high-risk areas within the mills, and about one-half of the eligible study subjects came from one of the four mills (Mill A). Rates of refusal to participate in the study among the four mills ranged from <1 to 45 percent, and refusals were approximately equally distributed between the placebo and vaccine groups.

Participating workers were randomly allocated by length of employment, age, department, and job to receive either vaccine or placebo. Inoculations of 0.5 milliliters (ml) of either vaccine or placebo (0.1 percent alum) were given; the first three inoculations were administered at 2-week intervals, followed by three injections at 6-month intervals and annual boosters thereafter. Those referred to as “complete” inoculees received at least the

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