On May 27, 2004, the National Institute of Allergy and Infectious Diseases (NIAID) announced it had awarded contracts to two companies to develop vaccines based on the H5N1 avian influenza virus in an effort to limit the risk of a human influenza pandemic.

Contracts were awarded to Aventis Pasteur of Swiftwater, Pa., and Chiron Corp., Emeryville, Calif., both of which make inactivated flu virus vaccines that are used each flu season. Each company produced 8,000 to 10,000 doses of vaccine, which the NIAID then tested in clinical trials.

Widespread outbreaks of H5N1 avian flu occurred in eight Asian countries beginning in December 2003, triggering the loss of more than 100 million poultry from illness or culling. The outbreaks led to 34 documented human cases, including 23 deaths. Disease experts feared that the avian virus would combine with a human flu virus, possibly producing a new strain that could trigger a pandemic because people would have no immunity to it.