Using an adjuvant - a substance with an oil and water base that is added to the vaccine - could mean having four times as much vaccine supply, Chen said. But adjuvants, used with flu vaccines in Europe, are not licensed for flu vaccines here. Federal approval would require an emergency declaration by the Food and Drug Administration.
Still, Chen thinks studying adjuvants now could prove helpful later in the flu season or in future years.
"Depending how bad H1N1 gets, we may want to stretch out our vaccine supply even further," he said. "This study still addresses a critical issue that is important for public health planning as we get into the flu season."
In addition, no one knows if the vaccine will have adverse effects on people in the long term. So far, the trials done at academic centers nationwide have not shown any immediate problems, other than minor swelling at the site of the shot and arm soreness, Fauci said.
Federal officials said they have stepped up monitoring for possible adverse reactions down the road, specifically, a rare neurological disorder called Guillain-Barre syndrome. After a 1976 outbreak of a different swine flu, the government launched a vaccination effort but some reports found the vaccine increased the risk of Guillain-Barre, which can be fatal.
"It's on people's minds," said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases. "It's a very rare condition, but it's something we want to be ready for. We might see more Guillain-Barre this fall, because we'll expect to see more flu this fall. We want to know if the vaccine has any kind of link to that. We don't expect that. But we want to be on the lookout."
A stronger vaccine for swine flu
* Eighty percent to 97 percent of healthy adults exhibited immune response after one dose of swine flu vaccine in three separate studies, preliminary data showed.
* The vaccines were not as effective among the elderly, with about 60 percent showing an immune response.
* Testing continues of children and pregnant women to see whether they will require one or two doses.
