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IOM Deems Anthrax Vaccine Safe, Effective

Kate Traynor

A report issued recently by the Institute of Medicine (IOM) declares the currently licensed anthrax vaccine to be acceptably safe and effective for the prevention of the disease. Nevertheless, IOM concludes that the vaccine should one day be replaced by a product requiring few doses and an easy-to-tolerate route of administration.

IOM prepared the report "The Anthrax Vaccine: Is It Safe? Does It Work?" at the request of the Department of Defense (DoD), which had been directed by Congress to oversee an independent analysis of the vaccine's safety and efficacy. In a March 2000 "letter report" to DoD, IOM had declined to study the vaccine's efficacy and said that insufficient peer-reviewed literature existed to support conclusions about the vaccine's safety. The report urged DoD to submit its "large body of results" on the vaccine to peer-reviewed journals so that the data could be used for IOM's analysis.

The project assumed new urgency last fall after thousands of Americans were exposed to Bacillus anthracis spores in bioterrorist attacks that killed five people. The number of medical articles that address anthrax has surged in recent months, and the IOM report refers to several newly published studies. Of note, IOM cautioned that findings from studies involving military personnel may not hold true for civilians or for people with chronic medical conditions. Virtually all anthrax vaccine recipients in the United States have been military personnel.

According to IOM, the results of the studies involving animals and humans suggest that the U.S.-licensed anthrax vaccine adsorbed, known since January by the brand name Biothrax (BioPort Corp.), can prevent all forms of the disease, including inhalational anthrax, in humans. The report also concludes that the vaccine is likely to provide protection from both naturally occurring and genetically engineered strains of B. anthracis.

IOM found sufficient evidence in the animal studies to state that the anthrax vaccine, when used immediately after exposure and in combination with antiinfective therapy, provides "excellent" protection against inhalational anthrax. The report drew no conclusions about the effectiveness of the vaccine when first administered after a long course of antimicrobial prophylaxis, a treatment regimen followed by some people who were exposed to the pathogen last fall. IOM noted that this issue is worthy of study.

For the analysis of the anthrax vaccine's safety profile, IOM examined data from the U.S. military, the federal Vaccine Adverse Event Reporting System, and epidemiologic studies. In general, IOM found that the anthrax vaccine is no more likely than other vaccines to cause adverse events.

But the anthrax vaccine, which is administered just beneath the skin, often causes localized swelling, redness, or nodules at the injection site. To lessen the frequency of such reactions, IOM suggested that studies be conducted to see if the vaccine can be delivered safely and effectively through other than the s.c. route. Because women seem to be more prone than men to localized reactions, IOM called for studies to examine response and adverse event differences between the sexes.

IOM also said that the anthrax vaccination schedule should be simplified. Proper vaccination with the current product requires six s.c. injections delivered over 18 months and annual boosters thereafter.

Despite its overall endorsement of the anthrax vaccine, which was originally licensed in 1970, IOM said there is a clear need to develop a new product that is based on "more modern principles of vaccinology." According to IOM, an improved anthrax vaccine would be administered in two or three doses, induce immunity for at least a year, and not cause severe local or systemic reactions. IOM also noted that a new vaccine should have a long shelf life so the product can be stockpiled.


Civilian Anthrax Cases Could Have Been Worse

The 60-day antimicrobial prophylaxis regimen used by people who were exposed to Bacillus anthracis spores last fall may have cut the number of inhalational anthrax cases in half, say biostatisticians at the Johns Hopkins Bloomberg School of Public Health in Baltimore.1

The researchers used a statistical model to examine data from about 5000 people who were exposed to B. anthracis spores, including 8 patients who were diagnosed with inhalational anthrax. On the basis of the analysis, the researchers predicted that postexposure antimicrobial prophylaxis prevented 9 additional cases of inhalational anthrax.

The statistical model incorporated data on exposure dates, date of symptom onset, the disease's incubation period, and the period between exposure and the beginning of prophylaxis. A key assumption used in the model is that the inhalational disease occurs when the incubation period is shorter than the interval between exposure and the start of prophylaxis.

  1. Brookmeyer R, Blades N. Prevention of inhalational anthrax in the U.S. outbreak. Science. 2002; 295:1861.