(CNN) -- At least three of the four makers of H1N1 vaccine have begun shipping their products, their representatives told CNN Tuesday.
A single dose induces a strong immune response in healthy adults and children as young as 9.
Sanofi Pasteur said Tuesday it shipped the first batch of H1N1 flu vaccine from its plant in Swiftwater, Pennsylvania, several days earlier than expected.
Further shipments will follow, with a total of 75.3 million doses expected through December, said Donna Cary, spokeswoman for the vaccine maker.
Citing security concerns, she would not divulge which of the distribution centers set up by the Department of Health and Human Services will get the first doses.
Another vaccine maker, MedImmune, shipped its first batch of 5 million doses to regional distribution centers last Tuesday, company spokeswoman Karen Lancaster said. HHS bought 40 million doses from the company, she said.
Novartis began its shipments on Sunday, said spokesman Eric Althoff, who said he did not know the size of the order.
"This is the first of many shipments," he said.
The fourth maker, GlaxoSmithKline, did not immediately respond to a telephone call.
"We will have enough vaccine available for everyone," Kathleen Sebelius told the House Energy and Commerce Committee this month.
Tuesday's shipment comes a few days before health officials had anticipated. This month, Sebelius had predicted that the large-scale vaccination program against H1N1 -- also called swine flu -- would begin in mid-October at as many as 90,000 sites, and that limited amounts of the vaccine were expected to be available a week to 10 days earlier.
A single dose induces a strong immune response in healthy adults and children as young as 9, though children younger than that may need two doses, she said.
Clinical trials are under way among pregnant women, who appear to be at heightened risk of dying from the disease.
Though researchers had originally expected it would take 21 days from the time of inoculation for the vaccine to induce an immune response robust enough to confer protection, they were pleasantly surprised when the first trials found that protection occurred in eight to 10 days for most people older than 9 years of age.
The two types of vaccine that have been approved -- a flu shot made from inactivated or dead virus and a nasal spray made from live, weakened virus -- will be available free of charge, though some providers may charge an "administration fee," Sebelius said.
The last attempt to inoculate the U.S. population against a type of swine flu occurred in 1976 after some 200 soldiers from Fort Dix, New Jersey, became infected.
The flu never spread, but some 40 million Americans got the vaccine, which was blamed for hundreds of cases of Guillain-Barre syndrome, an autoimmune disorder that causes severe muscle weakness.