Federal health officials said Friday they are planning to test H1N1 influenza vaccines that contain additional material known as adjuvants in order to boost vaccine effectiveness amid reports from vaccine makers about potential production problems.

On Thursday top officials from Baxter International Inc. (BAX) and Novartis AG (NVS) said so far that the viruses used to make the vaccine isn't yielding as much active ingredient typically seen for seasonal influenza vaccines. Novartis said the yield of active ingredient - or antigen - so far is about 30% to 50% of what the company normally sees for seasonal flu vaccines. If similar scenarios hold true for all vaccine makers, that could limit the amount of H1N1 influenza vaccine.

However, the addition of other materials known as adjuvants designed to boost the body's response to a vaccine, could potentially reduce the amount of antigen necessary for the body to recognize and fight a virus. The antigen is the active ingredient in a vaccine that causes the human body's immune system to develop antibodies that help fight an invading virus.

Anne Schuchat, the director of the Centers for Disease Control and Prevention's Immunization and Respiratory Diseases Center, said reports of low yields so far hasn't "changed our expectation for vaccine ability in the fall" because officials expected possible production problems.

Indeed, Jesse Goodman, the Food and Drug Administration's acting chief scientist, said there are plans to test H1N1 influenza vaccines that are made in the same manner as seasonal vaccines along side vaccines that contain adjuvants.

In a document posted on FDA's Web site, the agency said it was considering adjuvants made by GlaxoSmithKline PLC (GSK) and Novartis in clinical studies. The FDA is holding a meeting next week to discuss the details of such studies. The studies are expected to start next month and the first doses of a vaccine could be available by mid-October.

Goodman explained that the FDA views the H1N1 vaccines being developed as essentially a strain change typically seen in seasonal vaccines from year-to-year, so there's already an established pathway to approving the vaccines. However, there aren't any licensed adjuvants in the U.S., so if those are used they would have to be approved under an emergency-use authorization that allows for a fast-track approval process.

Last week, the government signed contracts worth $884 million with U.S. units of Sanofi Aventis (SNY) and AstraZeneca PLC (AZN) along with GlaxoSmithKline and Novartis to purchase ingredients used to make H1N1 vaccines. A unit of CSL Ltd (CSL.AU) is also expected to supply some vaccine to the U.S. market.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com