The companies' tenders were selected among five for best meeting "all the required criteria including technical and scientific requirements as well as providing the best value for money."
The vaccines are scheduled for delivery in May and October 2006 and will be offered as a possible first line of defence for National Health Service (NHS) workers whilst the exact vaccine to match the pandemic flu strain is manufactured.
Baxter will produce 2m doses of candidate vaccine based on a wild-type avian strain using its vero cell platform.
Chiron, on the other hand, will use its MF59 adjuvant to make 1.5m doses of the vaccine.
Baxter's proprietary vero cell technology enables it to exclude any added proteins or raw materials derived from human or animal sources in the manufacture, purification and formulation of its vaccines.
This mean that this vaccine platform has the potential to offer such advantages as a protein- and serum-free production medium, high yield and purity, and no antibiotics, egg proteins, or mercury-containing preservatives.
Crucially, by growing wild-type virus in its vero-cell culture in BioSafety Level 3 facilities, Baxter can begin vaccine production straight away without having to wait two or three months for high-growth or attenuated virus reassortants normally used when vaccine is produced in more traditional, chicken egg-based systems.
"We will be producing the vaccine at our manufacturing sites in Austria and the Czech Republic," Baxter spokeswoman Deborah Spak told In-PharmaTechnologist.com.
"There is R&D and clinical trials to be carried out of course but we will deliver the vaccines before the end of 2006."
Chiron, which started working on avian flu soon after the H5N1 outbreak in Hong Kong first affected humans in 1997, will be producing vaccine formulations containing the adjuvant MF59 which has proven highly immunogenic, inducing antibody levels believed to confer protection against the influenza strain.
Studies have found that, without the adjuvant, a range of doses of vaccine did not induce protection.
However, with the adjuvant the vaccine induced protective antibody levels against the original H5N1 strain, including when used at low doses (7.5 mg).
This 'dose-sparing', using less vaccine per person, allows more people to be immunised.
Furthermore, people immunised with the adjuvanted vaccine showed protective antibody titers not just against the original H5N1 strain, but also against drifted strains of H5N1 that had changed over time.
Chiron will produce vaccine for this stockpile at its manufacturing facilities in Italy.
As part of its strategy, it expects that it can manufacture avian influenza vaccines during the traditional break between seasonal influenza vaccine campaigns, finishing these activities in time for the seasonal influenza vaccine production to avoid interruption of the normal manufacturing cycle.
Like other influenza manufacturers participating in stockpiling efforts, Chiron has also submitted a 'mock-up' file for pandemic influenza vaccine containing its MF59 adjuvant to the European Medicines Agency (EMEA), while Baxter is working with the US National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), to develop a cell culture-based H5N1 candidate pandemic influenza vaccine.