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.

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