WASHINGTON -- A single dose of the swine flu vaccine works well for almost all pregnant women, but young children will still need two doses for best results, federal health officials said Monday.

Twenty-one days after receiving a single 15-microgram dose of the vaccine, 92 percent of pregnant women showed a robust immune response, Dr. Anthony Fauci reported.

A larger, 30-microgram dose produced a strong response in 96 percent of pregnant women, said Fauci, director of the National Institute of Allergy and Infectious Disease.

"This should be reassuring news to those women who have received the vaccine," Fauci said.

While a single dose is recommended for healthy adults and pregnant women, officials have said younger children will need two doses.

That has been confirmed by continuing studies, Fauci said.

He said among children aged 6 months to 35 months just one in four had a strong immune response after 21 days. For those 3 years to 9 years old it was 55 percent. There was a sharp improvement after a second dose, he said, raising the fully protected rate to 100 percent in those 6 months to 35 months and 94 percent in those aged 3 years to 9 years.

Giving the larger 30 microgram dose gave no added benefit, Fauci said.

Also at the briefing at the Department of Health and Human Services:

-- Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Disease at the Centers for Disease Control and Prevention, said the flu is currently widespread in 48 states and hospitalizations and deaths continue to increase.

She said 30 million doses of the vaccine are available as of Monday. There will be continued challenges, she said, but the supply is expected to catch up with demand.

--Dr. Bruce Gellin, director of the National Vaccine Program Office, reported that a program to monitor any side effects is in place with a new safety advisory working group meeting Monday.