TOKYO — A group of researchers has developed a new vaccine effective against many types of influenza, a breakthrough that could be a possible silver bullet against new strains of the virus.

Although practical application of the vaccine is still several years away, trials conducted on mice have shown promising results even on flu strains that can quickly mutate.

Researchers from the National Institute of Infectious Diseases, Hokkaido University, Saitama Medical University and chemical maker NOF Corp. developed the groundbreaking vaccine. The team was working under Japan’s Health, Labor and Welfare Ministry.

Previous vaccines were developed based on proteins that look like barbs covering the outer surface of the virus. After a virus enters the body of a person who had been inoculated, antibodies detect the barbs and try to suppress and fight off the virus.

However, the shape of these proteins differs between influenza strains.

New vaccines must be formulated almost every year because these proteins can quickly change their shape, a problem that becomes all the more troublesome when several strains of influenza with differently shaped proteins are making the rounds.

When predictions about which strain will become an epidemic miss the mark, inoculations lose much of their effectiveness.

The researchers this time targeted the proteins inside the virus, which change little over time compared with those on the outer surface. The vaccine consists of an artificial version of the protein developed by the team that is attached to a special lipid membrane. When the vaccine is injected, immune system cells attack the cells infected by the virus.

The researchers examined the proteins of three common influenza strains — the Hong Kong A strain, the Soviet A type, and the highly pathogenic H5N1 bird flu.

The team inoculated mice implanted with human genes that boost immunity and then infected them with the three viruses. The mice showed no symptoms of the illness, and the vaccine suppressed the viruses’ ability to multiply.

Research will continue into whether the revolutionary vaccine causes any serious side effects in humans.

A group of researchers at England’s Oxford University reportedly is conducting trials of a similar universal flu vaccine.