Washington, May 2 (Reuters): An experimental bird flu vaccine may show the potential to protect not only against the feared H5N1 virus but perhaps other strains of influenza as well, researchers said today.

Tests in mice and ferrets show the vaccine, being developed by San Diego-based Vical Incorporated, protects mice and ferrets against the H5N1 avian influenza virus. And it protected mice against seasonal human flu viruses, too ? meaning it may offer potential as a “universal” flu vaccine.

But Dr Richard Webby of St Jude Children’s Research Hospital in Memphis, Tennessee, who tested the vaccine, played down its immediate significance. “It is something that is promising at least in mice, but mice are probably the easiest of all animals to achieve this cross-protection in, I think,” Webby said in a telephone interview.

Webby said the next step is to test the vaccine in ferrets, considered the best animal “model” for human flu infection, to see if the shot offers cross-protection against other flu strains. “A universal vaccine is the big goal. There are various approaches trying to get there. This is one of them,” Webby said.

Achieving cross-protection would mean that new vaccines would not have to be formulated every flu season.

Influenza is a virus that makes “mistakes” easily when replicating itself and thus mutates, or evolves, constantly. For this reason, the seasonal flu vaccine is re-formulated every year and people have to get new shots regularly.

The H5N1 avian influenza virus does not yet easily infect people but it has spread in birds across Asia, Europe and parts of Africa. It has infected 205 people and killed 113, but if it evolves the ability to spread easily from human to human it would spark a deadly pandemic.

Current vaccines activate an immune response against the most mutation-prone regions of the virus, which is why they must be changed every year. For this reason, experimental H5N1 vaccines being worked on now are unlikely to provide very good protection against a future pandemic strain.

But there are parts of the flu virus that are conserved ? that do not change as strains mutate.

Experts have been trying to formulate a vaccine that helps the immune system recognise these proteins. Vical’s vaccine uses three bits of DNA ? the “H5” part of the H5N1 avian influenza virus, and genes not so subject to mutation ? the nucleoprotein, or NP, and matrix protein, or M2.