BETHESDA, US - More safety data would be needed before a new type of influenza vaccine made in insect cells should get approval, federal advisers said on Thursday.

A divided committee advising the U.S. Food and Drug Administration said the new Protein Sciences Corp FluBlok vaccine appears effective among adults under age 50 but said the company has not shown that it is safe enough to be approved.

The vaccine is made using genetically engineered pieces of flu virus inserted into caterpillar cells, instead of the current method using whole virus grown in chicken eggs.

The U.S. government is struggling to vaccinate the population against H1N1, which the U.S. Centers for Disease Control and Prevention estimates has infected 22 million Americans and killed 3,900 since April.

Fewer than 50 million doses of H1N1 vaccine are now available.

Some panel members were concerned about some side-effects seen in people who volunteered to test it, such as a woman who had Bell's palsy, a temporary paralysis of the face, that flared up an hour after she was vaccinated.

"I don't feel the safety database is large enough," said Pamela McInnes of the National Institutes of Health. She said the vaccine is a new formulation and so the burden of proof is higher than simply changing the influenza strain from one season to another.

"I feel the safety data do not raise significant red flags," disagreed Dr. Jack Stapleton of the University of Iowa.

"What we are dealing with, rock bottom, is a pretty mediocre vaccine," said Dr. Theodore Eickhoff, a professor emeritus at the University of Colorado School of Medicine and a panel member.

The vaccine is made using cells from a caterpillar and would be the first influenza vaccine made using cell cultures approved in the United States.

The FDA usually follows the recommendations of its advisory committees.

The company says it could produce a vaccine much more quickly than traditional egg-based techniques - in two months, versus five to six months for vaccines made using eggs.

The Vaccines and Related Biological Products Advisory Committee said the data show the vaccine works well enough in adults aged up to 50, but six of 11 members said studies do not show the vaccine works safely.

Connecticut-based Protein Sciences, a closely held biotech, won a $35 million($48 million) federal contract in June to make vaccines for pandemic influenza. The contract could be expanded over five years for a total of nearly $150 million.

Protein Sciences tested a formulation for seasonal influenza vaccine, but it could easily be changed to cover pandemic H1N1 swine flu.