A U.S. clinical study suggests that just half a dose of the Novartis AG H1N1 vaccine may be enough to generate a protective immune response, the Swiss drugmaker said on Tuesday.

Novartis said data from about 4,000 individuals suggested that half a dose of its U.S.-approved unadjuvanted H1N1vaccine met immune criteria in adults and the elderly.

The data also showed that its adjuvant was well tolerated, which could potentially quadruple the Novartis vaccine supply.

Adjuvanted vaccines contain an additive to boost the immune system response and need less of the active ingredient than the unadjuvanted types approved by the U.S. Food and Drug Administration.

U.S. health officials have been scrambling to get the swine flu vaccine to market after the virus emerged too late to be included in this year's regular seasonal flu version.

Initially, officials said there would be enough for everyone, but the public quickly grew frustrated when supplies ran short.

The FDA has approved H1N1 vaccine from five makers who already produce seasonal flu vaccine -- GlaxoSmithKline Plc, AstraZeneca Plc's MedImmune unit, Novartis, Sanofi-Aventis, and CSL Limited.