Up to 30 million doses of vaccine against the pandemic H1N1 flu have been delivered to the U.S. government and production is now picking up, officials said on Monday.
But they said more studies confirm that children under the age of 9 will need two doses to be fully protected.
And studies in pregnant women, one of the groups most vulnerable to swine flu, show no indication of side effects from the vaccine.
The U.S. government is working to make vaccines and drugs available to fight the pandemic while countering fears about safety and criticisms that officials were too optimistic in predicting how quickly the vaccine would be ready.
Original predictions suggested that at least 80 million doses should have been delivered to state health departments, clinics and retailers by now and a few politicians have complained.
Lines have formed as people seek the vaccine for themselves and their children to protect against the virus, which has killed at least 1,000 Americans and infected an estimated 5 million.
Over time, we expect that supply will start to increase and eventually catch up with the tremendous demand that we are seeing now, Dr Anne Schuchat of the U.S. Centers for Disease Control and Prevention told a news briefing.
As of today, 30 million doses of the H1N1 vaccine are available for the states to order. That is a cumulative amount -- CDC had 26.6 million doses of vaccine available on Friday.
We know that about half the vaccine that has been administered so far has been to children under 18, Schuchat said. Unlike seasonal flu, which is most dangerous to the elderly, H1N1 is hitting younger adults and children especially hard.
Clinical trials being run by the government confirm that children under age 9 need two doses of the swine flu vaccine -- optimally four weeks apart -- to be fully protected.
Last week the World Health Organization said governments might consider giving a single dose to as many children as possible, but Dr Anthony Fauci of the U.S. National Institute of Allergy and Infectious Diseases said the scientific data showed it is important to get children vaccinated twice.
Children 6 months to 9 years had a less robust immune response, Fauci said. Initially, children were tested a week to 10 days after getting the first dose. Follow-up for three weeks confirms they need a second boost, Fauci said.
Fauci said results from pregnant women also showed the vaccine worked well -- not unexpected because seasonal vaccine also works well -- and caused no serious side effects.
He said 28 pregnant women have died in the United States from swine flu so far.
A special team of non-government experts has been assembled and was holding its first meeting on Monday to look at reports on the vaccine's safety, said Dr Bruce Gellin of the National Vaccine Program Office at the Health and Human Services Department.
Fauci disputed critics who say the vaccine is being distributed too slowly to be of use against the virus, which is active across the entire country.
You cannot assume that this is going to disappear, he said. I don't think you can make the assumption that anything is going to be too little, too late.
And Schuchat said the CDC was working to make an experimental antiviral drug available to hospitals that might need it for the most seriously ill patients.
Biocryst Pharmaceuticals Inc's peramivir has been cleared for experimental intravenous use in the sickest patients. Schuchat said the company donated 1,200 courses of the drug but that works out to just 600 treatments in reality.