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A 45-year-old anthrax vaccine gained approval for a new use Monday through a regulatory path never before applied by the Food and Drug Administration for this kind of product.

The vaccine is BioThrax, which hit the market in 1970 to protect against anthrax infections for people at high risk of exposure to the deadly bacterium. The FDA says the vaccine, which is made by Emergent BioSolutions of Gaithersburg, Md., can now also be used for what is called post-exposure prophylaxis in people aged 18 to 65. If a person is suspected or confirmed to have come into contact with Bacillus anthracis, BioThrax can be given in conjunction with the antibiotics that have made up the standard treatment regimen.

The FDA gave the green light using the so-called Animal Rule, a tool devised in 2002, shortly after the 9/11 attacks, to help bring to market medical countermeasures in instances where efficacy cannot be ethically or feasibly tested in humans.


Anthrax fits that bill. The pathogen is high on the list of feared bioterror weapons and was the cause of the worst biological attack in US history. In the autumn of 2001, a number of letters laced with anthrax spores were sent to several congressional offices and media outlets. Seventeen people were infected and five died.

Dr. Amesh Adalja, a senior associate at the UPMC Center for Health Security in Baltimore, Md., welcomed the approval of the new indication for BioThrax, though he noted it won’t change clinical practice. The vaccine was used on an off-label basis in 2001 on the belief it would help those already exposed to anthrax.


Emergent BioSolutions conducted safety testing in people but turned to rabbits to provide the FDA with evidence that giving the vaccine after exposure to anthrax increased the chances of survival. The survival rate for rabbits treated with both antibiotics and BioThrax was 70 percent to 100 percent, depending on the vaccine dose used; rabbits only treated with antibiotics had a far lower survival rate — 44 percent and 23 percent in two different studies — according to the FDA. The company also showed in humans and monkeys that the vaccine produced an immune response against anthrax.

The Animal Rule was established to help the manufacturers find a way to provide the FDA with evidence that drugs and vaccines might be effective against diseases that are among the most lethal for people. A company making a vaccine to protect against respiratory syncytial virus — which causes a cold-like illness — can give people the vaccine then expose them to the virus to see if they are protected. But that kind of testing will never take place for deadly threats like Ebola, anthrax, or SARS.

Companies developing vaccines against other dangerous pathogens will likely be heartened by the news that a vaccine has finally made it through the FDA’s review process using the Animal Rule. (Nine other therapies to combat bacterial, chemical, and radiological threats have been approved using the regulatory pathway, but never before a vaccine.) Every time the Animal Rule is used, Adalja said, the path becomes clearer for others.