The seasonal flu vaccine will offer no protection against the H1N1 swine flu, Imperato added. "This means that some groups in the population will need to receive more than just one flu shot in order to be protected against all of the influenza strains in circulation," he said.
There's also the lingering memory of the 1976 swine flu vaccination program, during which some 500 Americans came down with a rare neurodegenerative condition called Guillain-Barre syndrome, which many experts believe was linked to the shot. Twenty-five of those 500 people died.
"The potential for more severe illness and many more deaths caused by this new strain of influenza weighs heavily on our minds -- as does the unfortunate outcome of the 1976 swine flu vaccination program," said CDC spokeswoman Arleen Porcell-Pharr.
But the H1N1 flu vaccine will be very much like seasonal flu vaccines, which have an excellent safety profile, Porcell-Pharr said. "However, no vaccine is 100 percent safe. This vaccine will be no exception," she added.
If the vaccines are recommended for use, those who choose to be inoculated will receive information sheets describing the vaccines' risks and benefits, signs of side effects to look for after vaccination, and information on how to report adverse events, she added.
"We will be watching very closely for any signs that the vaccine is causing unexpected side effects, and we have systems in place to investigate those signals rapidly," Porcell-Pharr said.
For more information on swine flu, visit the U.S. Centers for Disease Control and Prevention.
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