Researchers have discovered a way to trigger a preventative response to a flu infection without any help from the virus.

It involves using a protein called IFITM3 which is usually only produced in large quantities after a person has caught flu. IFITM3 works by trapping the virus and preventing it from making new copies before switching off when the invader has been vanquished.

But US scientists at Ohio State University have discovered that it can be switched on permanently using drugs. Those at high risk of getting flu such as the elderly and pregnant women could be given the drug which would ramp up their own defences so that they would never catch the virus.

Study senior author Doctor Jacob Yount, assistant professor of microbial infection and immunity at Ohio State, said: "The flu vaccine needs to change every year because the virus is constantly mutating.

"What we're doing is targeting a more fundamental process that is not specific to any particular strain of the virus."

The findings, in both mouse and human cells, suggests that manipulating a natural process could someday be an alternative way to not just reduce the severity of the flu, but prevent infection altogether.

After showing in cells that altering the role of one protein can stop the virus in its tracks, Dr Yount's team has begun using experimental drugs to test their flu prevention strategy in mice.

Dr Yount said any possibility for human use is still many years away, but their long-term goal is to develop a vaccine-independent method to prevent flu infections.

He added: "If we were to have an outbreak of some pandemic influenza virus similar to what we experienced in 2009, I could envision using this technique to help people who are particularly vulnerable to infection.

"It would work best if used before an infection, because the strategy prevents cells from becoming infected in the first place."