WASHINGTON (Reuters)—U.S. officials on Monday said they had slashed their estimate of how many H1N1 flu vaccine doses will be available for the start of a mass vaccination campaign in the fall.

Citing delays in manufacturing and packaging the vaccines, the Department of Health and Human Services said only 45 million doses of the new H1N1 vaccine would be on hand in mid-October, instead of the 120 million previously forecast.

The revised delivery guidelines would push back a U.S. government estimate that all those requiring vaccinations be immunized by the first week of December.

"Our latest information from the manufacturers tells us that we now expect to have about 45 million doses by Oct. 15 with approximately 20 million doses being delivered each week thereafter, up to the 195 million doses that we have purchased," Bill Hall, an HHS spokesman, said in an e-mail.

The Geneva-based World Health Organization declared H1N1 a full pandemic in June, and the virus has spread to about 180 countries. World health officials have said people should receive the two-dose swine flu vaccination as well as the single-dose seasonal flu vaccination this year.

In July, U.S. advisers said about half the U.S. population, or 160 million people, should get vaccinated against the new pandemic influenza strain, with pregnant women and health care workers at the front of the line.

Clinical trials of the H1N1 vaccine are under way, and five companies are making it for the U.S. market—AstraZeneca's MedImmune unit, CSL Ltd., GlaxoSmithKline P.L.C., Novartis A.G. and Sanofi-Aventis S.A.

Dr. Robin Robinson of HHS said in a teleconference on Friday that Australia's CSL Biotherapies must produce swine flu vaccine first for Australia, where it is winter and both swine flu and seasonal flu are circulating, according to a news service run by the Center for Infectious Disease Research & Policy at the University of Minnesota.

In the National Biodefense Safety Board teleconference, Dr. Robinson also said one of the companies due to make the new vaccine was still struggling to finish its production of seasonal flu vaccine.

The news service also quoted Dr. Robinson as saying 84 million courses of antiviral drugs Tamiflu and Relenza, which are used to treat swine flu, are in federal and state stockpiles and 3 million additional doses are expected soon.

He projected that a total of 100 million treatment courses will be available in the autumn.

The United States would need 600 million doses to immunize all those needing the H1N1 vaccine, as people will need two doses for full immunity.

Baxter International Inc. said earlier this month it completed its first commercial batches of H1N1 vaccine made using a cell culture process designed to be faster than traditional vaccine production methods that use chicken eggs.

Baxter has supply contracts with five countries, including Britain, Ireland and New Zealand, but not the United States.