Last month officials said 120 million doses would be available to vaccinate those considered most at risk from the A(H1N1) influenza virus, which has claimed nearly 500 lives in the United States.
"Our latest information from the manufacturers tells us that we now expect to have about 45 million doses by October 15 with approximately 20 million doses being delivered each week thereafter, up to the 195 million doses that we have purchased," said health service spokesman Bill Hall.
Clinical trials of experimental vaccines began at the end of July on thousands of volunteers to determine if one or two doses would be required per patient.
The government was blaming the vaccination shortfall on delays in manufacturing and packaging the new vaccines, The Washington Times daily reported Tuesday.
It has ordered vaccines from five drug makers -- Switzerland's Novartis, Britain's GlaxoSmithKline, Sanofi in France, Australia's CSL Limited (Commonwealth Serum Laboratories) and local Maryland company Medimmune.
The Centers for Disease Control and Prevention, a government agency, recommended at a July meeting that around 160 million people, considered high risk, should be the initial focus of the immunization.
Officials at the meeting also came up with a smaller target population, numbering about 41 million, who would be vaccinated before others if supplies of A(H1N1) vaccine were very limited.
They would include: pregnant women and those in contact with children under six months, because very young infants cannot be vaccinated against flu; along with a subset of health care workers, children aged six months to four years of age, and children aged five to 18 with underlying health conditions.
The US toll from swine flu was 477 as of August 13, the Centers for Disease Control and Prevention told AFP on Tuesday.
The World Health Organization said that as of August 6, there are 1,462 reported deaths from the virus, but noted that this figure likely underreports the true number of fatalities.
Reports this month said the US government is planning for a worst-case scenario in which up to 40 percent of the American workforce was affected by swine flu -- either directly contracting the virus or having to stay home to look after an affected relative.