A "universal" flu vaccine to protect against many strains of the virus for decades may be only three years away.

Flu viruses are notorious for their ability to mutate and become resistant to vaccines, but scientists believe they have cracked the problem of designing a "one fits all" jab using a new two-step approach to immunisation. Early safety trials of the vaccine have already started and its effectiveness could be tested in patients as early as 2013.

Working with mice, ferrets and monkeys, the US team first primed the immune system with influenza DNA. They then added a "booster" consisting of a regular flu vaccine. The prime-boost combination was effective against flu viruses of different subtypes and from different years, the researchers reported in the journal Science. Although the "priming" vaccine came from a 1999 virus, antibodies were generated that neutralised other strains. The vaccine was also effective against "bird flu".

In other experiments, the scientists measured how well the prime-boost vaccine protected mice and ferrets against deadly levels of flu virus. Three weeks after receiving the boost, 20 mice were exposed to high levels of 1934 flu virus and 80 per cent survived. When mice were given only the "prime" or "boost" elements alone, or a sham vaccine, all died. Similar results were seen in ferrets – good predictors of effectiveness in humans.

Study leader Dr Gary Nabel, from the US National Institute of Allergy and Infectious Diseases in Bethesda, Maryland, said: "It would be similar to vaccination against such diseases as hepatitis, where we vaccinate early in life and then boost immunity through occasional, additional inoculations in adulthood. We may be able to begin efficacy trials of a broadly protective flu vaccine in three to five years."