The first adjuvanted bird flu vaccine has been approved by the U.S. Food and Drug Administration (FDA), Medical Daily reported.

A vaccine adjuvant is a substance added to a vaccine in order to improve the body’s immune response to the drug. The two-dose vaccine, used to protect against the H5N1 strain of bird flu, won’t be available for commercial use. Instead, it will be distributed by public health officials only in the event of an outbreak, according to the FDA.

“This vaccine could be used in the event that the H5N1 avian influenza virus develops the capability to spread efficiently from human to human," Dr. Karen Midthun, director of the Center for Biologics Evaluation and Research (CBER) at the FDA, said in a news release. “Vaccines are critical to protecting public health by helping to counter the transmission of influenza disease during a pandemic.”

There have been limited cases of the H5N1 bird flu spreading from person to person, and most humans can only contract the virus through contact with a dead animal or infected environments. Since 2003, there have been a total of 641 human cases of bird flu including 381 deaths.

"H5N1 bird flu is a highly pathogenic virus that has killed millions of birds but rarely infects humans. Studies in the laboratory have shown that it is very difficult to manipulate it genetically to pass easily from human to human," Dr. Marc Siegel, Fox News Medical A Team member, told FoxNews.com. "Yet some scientists remain concerned that such a mutation will occur and lead to a sudden disastrous pandemic, despite the fact that an H5 strain has never done so before. This is the rationale behind a pandemic vaccine."

The H5N1 vaccine is the first vaccine adjuvant to receive a license in the United States, according to Medical Daily. Previous concerns over safety had led researchers in the U.S. to rely on non-adjuvanted vaccines. However, some believe adjuvanted vaccines may be able to fight the flu more efficiently.

“One of the advantages with adjuvanted vaccines is their ability to protect against drifted strains. It opens the door for a whole new strategy in dealing with flu,” Professor David Salisbury, head of immunization at the U.K. Department of Health, told the Telegraph.

However, some believe that scientists would be better off focusing their efforts elsewhere.

"An adjuvant boosts the immune response of a vaccine, which would make it more effective in the event of a pandemic, and adjuvants have been used safely in Europe...," Siegel said. "But it is highly unlikely that H5N1 will ever infect humans on a grand scale and scientists would be better off focusing efforts on perfecting the universal flu vaccine."

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