U.S. must decide on flu vaccine formula

WASHINGTON, May 2 (UPI) -- U.S. public health officials must decide whether to change the formula of the vaccine for the next flu season to guard against swine flu, officials said.

The formula was approved about a month ago, The Washington Post reported Saturday. Researchers, still unsure how dangerous the H1N1 virus is, will have to make a final decision soon.

The government is preparing a vaccine aimed specifically at swine flu, which would supplement the other vaccine. Anne Schuchat, an acting public health executive at the Centers for Disease Control, said it will not be available until September or later.

The influenza virus is a notorious quick-change artist, mutating frequently.

"Last year's virus can evade today's immune response," said Peter Palese, a researcher at Mount Sinai Medical Center in New York. "We have all been infected with influenza and have a certain immunity, but the virus has changed enough so many of us can get sick again."

The most recent strain is an example of that, since it combines bird, pig and human strains that got together in a single animal.