The US Department of Health and Human Services (HHS) contract is the second antigen contract for the UK-based pharma giant, with the company awarded a contract last year for the supply of five million doses of H5N1 clade 2 bulk antigen at 15ug HA/dose.

The new deal, the dollar value of which is undisclosed, would see 22.5 million doses of 15ug H5N1 bulk antigen being manufactured at GSK's site in Quebec, Canada, and stored at the company's site in Marietta, Pennsylvania, US.

The terms of the contract state that the HHS may direct GSK to formulate the antigen into a vaccine in the future, though the HHS may manufacture the vaccine itself.

Additionally, the HHS has the option to purchase pre-pandemic vaccines together with one of GSK's proprietary adjuvant systems.

"We applaud the US Government's proactive steps in protecting American citizens against this potential health threat. We are committed to helping government's and pubic health officials around the world effectively respond to the global threat of a flu pandemic and we are pleased to be working with the US Government in its preparedness efforts," GSK president of pharmaceutical operations David Stout said in a statement.

The antigen is not approved in the US. Approval would be dependent on the vaccine formulation involving the antigen.

Meanwhile, the clinical trials for GSK's proprietary adjuvanted H5N1 vaccine have started in North America.

The Phase l/II studies are supported by a research contract awarded to GSK in January from the Biomedical Advanced Research and Development Authority (BARDA), the Assistant Secretary for Preparedness and Response and the HHS, which looked at the advanced development of an antigen-sparing pandemic influenza vaccine with a commitment to produce 150 million doses of vaccine within six months of a pandemic onset.

The adjuvant used in the vaccine is undisclosed.

However, a GSK spokeswoman told in-PharmaTechnologist.com that early studies had shown the vaccine to illicit a higher immune response than vaccines without the adjuvant.

Earlier studies have also showed that the influenza candidate vaccine may be able to induce immune responses that might protect against different strains of H5N1.

"From our perspective this is important," the spokeswoman said.

"It allows us to target different variants, and until a pandemic, we don't know which variant it will be, so having cross reactivity is an important aspect of the vaccine."

GSK is actively pursuing research into ways to make vaccines more effective through the use of adjuvant systems, where compounds are added to the vaccine to illicit a greater immune response.

Sanofi-Aventis is the only pharmaceutical company to have an H5N1 vaccine approved in the US, but the French company's vaccine does not contain an adjuvant. The GSK spokeswoman said it was too early to compare Sanofi's vaccine with GSK's.

The Phase I/II study, conducted in seven US states and two Canadian provinces, would evaluate the H5N1 antigen alone and in combination with one of its proprietary adjuvant systems in 675 subjects, and is expected to lead into a larger, multi-centre, Phase III trial later this year.

Data has been submitted for EU approval, with the hope this will be granted next year, while a Biological License Application is planned to be submitted in the US next year.

In 2005, GSK invested about $2bn (€1.4bn) into vaccine manufacturing in North America, primarily with the acquisition of Canadian company ID Biomedical.

The company is also modernizing its Pennsylvanian facility to develop and manufacture cell-culture-based seasonal and pandemic influenza vaccines.

About 200,000 people are hospitalized in the US every year with influenza. An additional 36,000 people die annually from the disease.

If a pandemic occurred without a vaccine, the WHO estimates that at least 50 million people could die. In the 1918-1919 Spanish flu outbreak, 50 million people died.

Novartis, which manufactures seasonal influenza vaccines for the US, is also working on an H5N1 vaccine.