ATLANTA, Georgia (CNN) -- The Food and Drug Administration has approved applications from four manufacturers to make H1N1 flu vaccine, the secretary of the Department of Health and Human Services said Tuesday.
The vaccine is expected to be available in about three weeks, according to the CDC director.
"We will have enough vaccine available for everyone," Kathleen Sebelius told the House Energy and Commerce Committee.
The plan is to begin the large-scale vaccination program in mid-October in as many as 90,000 sites, though limited amounts of the vaccine are expected to be available a week to 10 days earlier, she said.
Monday, the director of the nation's top disease agency told CNN the vaccine will be available earlier than had been expected.
"We think the first doses of some of the vaccine forms should be available in about three weeks," said Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention.
Previously, the CDC had been predicting the vaccine would not be available before middle or late October.
Frieden said that the vaccines appear to confer protection from the virus eight to 10 days after they are administered.
The news about the vaccine against H1N1, also called swine flu, comes a week after researchers concluded that a single injection would suffice to protect against the virus.
Health officials are urging that pregnant women, school-age children and anyone with underlying health conditions, like diabetes, heart disease or lung disease, get the swine flu shot. See which states are showing the greatest swine flu activity »
Frieden said it appears that health workers will be able to administer the H1N1 vaccine at the same time that they administer the shot against seasonal flu.
The symptoms of seasonal flu are similar to those of swine flu, and patients and their caregivers need not know which one they have, he said.
"The key messages are the same in either case: If you're sick, stay home," he said. "If you're severely ill -- and that means you have trouble breathing, you have severe illness, your fever comes back or you have one of those underlying conditions like diabetes or people with special health care problems, like children with disabilities, that make it difficult for them to breathe -- then see your doctor right away."
The timing is important because 11 states already are reporting widespread flu activity. "We wish we had the vaccine today," Frieden said.
He said flu vaccines have a good safety record. "Literally, hundreds of millions of people have gotten the flu vaccine, and certainly my kids will be getting the H1N1 vaccine when it becomes available for everybody."
Frieden's two children are ages 5 and 15.