NEW YORK - The United States could authorize emergency use of some currently unapproved immune system boosters called adjuvants to make a swine flu vaccine more effective, an official at the U.S. Centers for Disease Control and Prevention said on Thursday.

A big challenge facing manufacturers of a vaccine for the new swine flu story href=/news/globalcoverage/swineflu>H1N1 flu will be making sufficient quantities of vaccine from a limited supply of active ingredient, or antigen.

One option to extend supply is to use an adjuvant to increase the body's immune response and reduce the amount of antigen needed in each shot. Several companies are working on this approach, including GlaxoSmithKline and Switzerland's Novartis AG.

Michael Shaw, associate director of the CDC's Laboratory Science Influenza Division, told Reuters on the sidelines of a conference on swine flu in New York that a vaccine could be stretched to make more doses by using an adjuvant.

There are a couple of adjuvants that are being looked at now, he said. There's nothing that's been approved for use in the United States, but there are several that could possibly get emergency use authorization if it appeared it was called for.

Philip Dormitzer, senior director of viral vaccine research at Novartis Vaccines and Diagnostics, said at the conference that the company had submitted to the U.S. Food and Drug Administration a large file on its MF59 adjuvant, which is already licensed in Europe.

The CDC said earlier on Thursday companies were starting preliminary work on a vaccine for the new swine flu story href=/news/globalcoverage/swineflu>H1N1 flu and should begin clinical trials this summer, though the new vaccine would not be ready for widespread use until October.

So far, there have been 8,585 probable and confirmed U.S. cases of the new swine flu story href=/news/globalcoverage/swineflu>H1N1 strain, commonly called swine flu, with 12 deaths and 507 hospitalizations, the CDC said.

Dr. Anne Schuchat of CDC said in a telephone briefing the CDC had shipped virus samples for making vaccines to different manufacturers several days ago.

Clinical trials will need to answer a number of questions, including whether one or two injections will be needed, whether vaccinating different age groups yields different results and whether an adjuvant would be needed to get a good response.