The Project Bioshield Act was an act passed by the United States Congress in 2004 calling for $5 billion for purchasing vaccines that would be used in the event of a bioterrorist attack. This was a ten-year program to acquire medical countermeasures to biological, chemical, radiological, and nuclear agents for civilian use. A key element of the Act was to allow stockpiling and distribution of vaccines which had not been tested for safety or efficacy in humans, due to ethical concerns. Efficacy of such agents cannot be directly tested in humans without also exposing humans to the chemical, biological, or radioactive threat being treated, so testing follows the FDA Animal Rule for pivotal animal efficacy.