Published: Thursday, 23 Jul 2009 | 11:20 AM ET

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The new pandemic H1N1 is causing a "brisk" flu season in the Southern Hemisphere and U.S. health officials said Thursday they want to move swiftly to approve a vaccine to battle it.

AP

Getting a vaccine approved as soon as possible is important, the U.S. Food and Drug Administration's Dr. Wellington Sun told a committee of experts who advise the FDA on vaccine policy.

"Time is really not on our side," Sun said. "The 2009 H1N1 pandemic is a declared national emergency.

The severity of disease seems comparable to seasonal" influenza, Sun added. But it could change, he said, and "We have to prepare for the worst."

He noted that the companies making H1N1 swine flu vaccine had a proven track record of safety and had been making seasonal flu vaccines for the U.S. market for years.

These vaccine makers are struggling to make the swine flu vaccine because they can produce less than a third of the usual amount, compared to seasonal flu vaccine strains, another expert told the meeting.

"Development work has indicated the existing reference strains have an expected yield of around 30 percent of H1N1 seasonal vaccine strains," said Dr. Jerry Weir, the FDA's director of the division of viral products.

This means makers may end up with fewer doses than expected of vaccine against the pandemic strain.

Earlier this month the World Health Organization reported that vaccine makers were not getting the expected yield from swine flu virus samples they were sent, which they have been growing in eggs to then purify and use to make vaccines.

AstraZeneca's MedImmune unit, Australia's CSL Ltd, Baxter International, GlaxoSmithKline Plc, Novartis AG and Sanofi-Aventis SA all are making H1N1 vaccines.

Spreading the Virus

They will tell the FDA committee about what they have learned as they work with the virus, which spread globally in less than two months.

"This is clearly a brisk flu season," Dr. Anthony Fiore of the U.S. Centers for Disease Control and Prevention told the meeting.

H1N1 is the dominant strain, and while it is about as bad as a moderate seasonal flu, it is attacking younger people than seasonal flu usually does. Seasonal influenza kills between 250,000 and 500,000 people globally in an average year.

Global health officials are gearing up to vaccinate against both the seasonal flu, using a traditional cocktail that protects against the three most common strains, as well as against the pandemic H1N1 virus.

The U.S. National Institutes of Health reported Wednesday it had assigned human trials of CSL's and Sanofi's vaccines to start in August at various academic centers and clinics. But the FDA must approve these before they can start.

Vaccine experts will test various doses of the H1N1 vaccine to see how large a dose people need to be protected, and also deliver it along with the seasonal flu vaccine to see if that will work.

The CDC's Dr. Nancy Cox said tests had confirmed that people who have received seasonal flu vaccines in the past have little or no immunity to the new H1N1 virus, including people in Europe who have received vaccines containing materials called adjuvants to boost the immune response to the immunizations.

The FDA committee will consider letting companies test the adjuvants in the United States in the hope of making limited supplies of the new H1N1 vaccine stretch further.

The CDC has a committee of vaccine advisers meeting next week to recommend who should get an H1N1 vaccine.

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