U.S. scientists say they have identified key human antibodies that will help them develop a new drug for influenza and bird flu. The researchers say this discovery could lead eventually to a universal vaccine that would give lifetime immunity against the flu.   

The influenza season is underway in the United States, and health officials are once again urging Americans to get their yearly flu shot.

Influenza epidemics occur in cycles around the world.  The World Health Organization says between three and five million people get sick every year, and 200,000 to 500,000 people die from the virus.  

The very young and the very old are often the most vulnerable.

Each year scientists produce a vaccine designed to kill the strain of virus that is in circulation.

But even with the best scientific calculations, some strains of the influenza virus confound doctors by mutating and spreading again.

Now a team of researchers from the Dana Farber Cancer Institute in Boston and the Burnham Institute in California have identified human antibodies they say will knock out the virus' ability to reinvent itself.

"This is the first step in a well defined pathway to develop these [antibodies] into drugs that can be used to fight and treat influenza in people," said Dr. Wayne Marasco of Dana Farber, a lead author of the research.

Dr. Marasco says more research is needed before clinical trials on humans will be conducted.  That could take at least three years.