Vaccine may permit shorter antibiotic course for anthrax

May 9, 2006 (CIDRAP News) – Combining vaccination with 14 days of antibiotic therapy after exposure to airborne anthrax may be an alternative to the current recommendation of 60 days of antibiotics alone, according to an animal study reported in the Proceedings of the National Academy of Sciences.

The study, by researchers at the US Army Medical Research Institute of Infectious Diseases, the National Institute of Allergy and Infectious Diseases, and Bayer Pharmaceutical Corp., involved 24 adult rhesus macaques. Rhesus macaques have shown a response to inhalational anthrax most closely mimicking that of humans, according to the report.

All the animals were exposed to Bacillus anthracis spores by aerosol. Beginning 1 to 2 hours after exposure, 10 macaques received 14 days of ciprofloxacin treatment twice daily. Another group of 10 received the same antibiotic regimen plus three doses of anthrax vaccine adsorbed (AVA), the only vaccine licensed for humans. Four macaques received no therapy.

All 4 control animals died 4 to 5 days after exposure, but all 20 treated animals survived during the treatment window. Once the antibiotic therapy was discontinued, however, only 4 of 9 ciprofloxacin-only macaques (44%) survived (with the 10th dying of undetermined causes and therefore excluded from the study). The other five died 19 to 24 days postexposure.

Among the vaccine-plus-ciprofloxacin group, all animals survived until the study’s conclusion at 150 days—a statistically significant increase in survival compared with the antibiotic-only group (P = .011).

“These data,” the report says, “provide evidence that postexposure vaccination can shorten the duration of antibiotic prophylaxis to protect against inhalation anthrax and may impact public health management of a bioterrorism event.”

In an addendum, the authors note that the surviving ciprofloxacin-only macaques survived a repeat aerosol challenge 8 to 11 months after the antibiotic was discontinued, suggesting that they had developed an immune response to anthrax. In a May 1 Reuters news article, senior author Arthur Friedlander, MD, was quoted as saying that this antibody response may help determine when antibiotics can safely be discontinued.

Vietri NJ, Purcell BK, Lawler JV, et al. Short-course postexposure antibiotic prophylaxis combined with vaccination protects against experimental inhalataional anthrax. Proc Nat Acad Sci 2006 (early online publication May 3) [Abstract]

See also:

CIDRAP information on postexposure prophylaxis for anthrax

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