(Removes incorrect name Pandemrix, paragraph 2)

By Toni Clarke

Nov 22 (Reuters) - The U.S. Food and Drug Administration said on Friday it has approved a vaccine made by GlaxoSmithKline Plc for use in the event of an H5N1 bird flu epidemic.

The vaccine will be added to the national stockpile and will not be available for commercial use, the FDA said. The vaccine does not have a trade name in the United States.

It is the first H5N1 vaccine approved in the United States to contain an adjuvant, or booster, that turbo-charges the body's immune response to the vaccine.

"This vaccine could be used in the event that the H5N1 avian influenza virus develops the capability to spread efficiently from human to human, resulting in the rapid spread of disease across the globe," Dr. Karen Midthun, director of the FDA's biologics division, said in a statement.

The FDA approved the vaccine for use in people over the age of 18 who are at increased risk of exposure to the virus. It would be distributed by public health officials if needed.

The approval comes amid some uncertainty over the safety of modern adjuvants such as the one used in GSK's bird flu vaccine.

However, there are no truly effective traditional vaccines against bird flu. Glaxo's super-charged product is the first to show it can confer protection in the event of a pandemic. European regulators have approved it under the brand name Pumarix.

Data shows that children in England who received a similar adjuvanted vaccine, Pandemrix, during the 2009-10 H1N1 swine flu epidemic had a 14-fold heightened risk of developing narcolepsy, a chronic and potentially debilitating sleep disorder that can cause hallucinations, daytime sleepiness and cataplexy, a form of muscle weakness precipitated by strong emotion.

According to GSK, some 30 million doses of Pandemrix were administered across Europe and 800 people, mostly children, developed narcolepsy. While GSK has acknowledged an association between the vaccine and the narcolepsy cases, it has said there is insufficient evidence to prove the adjuvant was to blame.

Adjuvants have been used in vaccines for diseases such as diphtheria and tetanus for decades. Now, a new, more powerful generation of adjuvants, including one known as AS03 that is contained in the bird flu vaccine, is being developed whose safety is relatively untested.

Previously, the United States has taken a more cautious approach toward adjuvants than some other countries. The FDA did not, for example, approve Pandemrix. U.S. experts felt that traditional, non-adjuvanted H1N1 vaccines made by Sanofi SA and others offered enough protection against the virus.