Scientists are close to developing a vaccine that would treat all different strains of the flu, making annual flu shots unnecessary.

Flu viruses are able to mutate rapidly in response to new vaccinations, something that has forced doctors to develop new treatments for every flu season. But Dr. Francis Collins, head of the National Institutes of Health, told USA Today that he was "guardedly optimistic" about the prospects of developing an all-encompassing vaccine within the next five years.

"There are parts [of the virus] that don't change," Collins said. "If you designed a vaccine to go after the constant part of the virus, you'd be protected against all strains."

If true, the predictions would represent a significant breakthrough for a vaccine that has long eluded scientists and that "seemed completely out of reach only a few years ago," Collins said.

The flu represents one of the leading preventable causes of death, killing between 250,000 to 500,000 lives yearly, of which 36,000 are in the U.S., and hospitalizing about 200,000. A universal vaccine would likely incorporate antibodies that are able to recognize the recurring parts of the flu virus.

The National Institute of Allergy and Infectious Diseases has been hard at work testing various vaccines on mice, ferrets and monkeys, and researchers have been able to produce antibodies that attack a variety of different influenza strains, according to the organization'se website.

There are various different types of the flu. The seasonal flu occurs annually and affects between 5 and 20 percent of Americans, and would be the target of the universal vaccine. Particularly virulent strains of the flu -- that is, those with the capacity to rapidly infect a large number of people -- are subsumed under the category of pandemic flu, which includes the H1N1 virus.