(Adds details on flu vaccine companies, background)

WASHINGTON, July 12 (Reuters) - The United States will spend another $1 billion on ingredients for an H1N1 vaccine, U.S. Health and Human Services Secretary Kathleen Sebelius said on Sunday.

“There’ll be another $1 billion worth of orders placed to get the bulk ingredients for an H1N1 vaccination. Congress has agreed with the president that this is the number one priority, keeping Americans safe and secure,” Sebelius said on CNN.

Sebelius has said plans were on track for a mid-October vaccination program, although it was not certain Americans would be offered the vaccine for the so-called swine flu.

“We are aggressively working on, first of all, testing the virus strains to get a vaccination ready. It needs to be safe so testing and clinical trials will start this month. We’ll know a lot more by the end of the summer and it needs to be effective,” she said.

The World Health Organization may issue guidance as soon as Monday on whether an H1N1 swine flu vaccine will be offered alongside the seasonal flu vaccine.

Vaccine makers Sanofi-Aventis SASY.PA, Novartis NOVN.VX, Baxter BAX.N, GlaxoSmithKline GSK.L, Solvay SOLB.BR and AstraZeneca's AZN.L MedImmune subsidiary have finished making seasonal flu vaccines for this year.

The U.S. Food and Drug Administration has scheduled a July 23 advisory panel meeting to discuss clinical trials of the vaccines against the H1N1 influenza virus and the U.S. Advisory Committee on Immunization Practice wills meet July 29.

“FDA is working with the scientists at NIH (National Institutes of Health) to make sure that we have a safe and effective strain and then we’re getting ready to make sure that we have a vaccination program,” Sebelius said.

Health experts estimate at least 1 million people have been infected with H1N1 in the United States, and the U.S. Centers for Disease Control and Prevention has confirmed 211 deaths. It often takes weeks or months to collect data on flu deaths.

About 36,000 people die each year from the seasonal flu in the United States alone, and 250,000 to 500,000 die globally. (Editing by Maggie Fox)