As the United States suffers through an unusually nasty flu season, scientists say they may be getting closer to a "universal vaccine" that can effectively fight multiple strains of influenza with a lifetime dose.

It's the fact of multiple strains that has made this flu season so severe, they point out; one strain, not thought to be serious when planning began in February of last year to recommend which strains the current vaccine should target, has turned out to be the one hitting the U.S. hard.

Flu remains a global problem, killing between 250,000 and 500,000 people around the world every year.

It takes the better part of a year to test a flu vaccine and manufacture 150 million doses, so in the planning stages it's difficult to assess what flu strains will be most prevalent when the following winter arrives.

Complicating that is the fact that flu strains are in a constant state of mutation that can make a vaccine less effective when it does become available.

That's why the current flu shot is only around 23 percent effective, as opposed to the 50 to 60 percent effectiveness when the vaccine is a good match to the most common strains winter has brought.

Still, there may be hope on the horizon; researchers have announced the start of clinical trials of a "one and done" vaccination similar to the ones offering lifetime protection against measles, mumps and rubella.

"The vaccine could become a reality in as few as five to seven years, if clinical trials go smoothly," says Matthew Miller at McMaster University in Ontario.

The "universal" vaccine takes advantage of a class of antibodies that can combat wide variety of influenza A viruses by targeting a part of a virus protein called the hemagglutinin stalk domain.

That's like the stick in a viral protein "lollipop," the researchers explain; while the flavors in the candy in the lollipop can change when a virus mutates, the "stick" remains unchanged and continues to be vulnerable to the universal antibody.

The researchers tested the universal vaccine in ferrets and mice, and found it performed as well as conventional vaccines that were well matched to the particular flu virus the animals were exposed to.

"However," Miller says, "when animals were infected with a 'mismatched' virus, those given the conventional vaccine died, while those given the universal vaccine survived.

"This is the huge breakthrough."