The swine flu strain that set off the latest pandemic could do some good too. This 2009 H1N1 virus might, scientists say, hold the key to a universal flu vaccine. Unlike present flu vaccines that need to be matched to a particular strain, a universal vaccine would be able to provide immunity against several different strains.

But creating a universal flu vaccine is no easy task. The flu virus is good at mutating. By endlessly changing the shape of its protein coat in this manner, the virus is able to dodge the human immune system.

Scientists therefore need to find a protein segment that remains almost unchanged across flu strains. In addition, when given as a vaccine, that bit of protein must also evoke a strong immune response.

The 2009 H1N1 virus may help in this search. Scientists at the Emory University in the United States tested mice that had first been injected with non-lethal doses of a mouse-adapted form of the swine flu virus.

They found that the mice were then protected against disease and death when given lethal doses of several older H1N1 strains.

More surprisingly, it turned out that the mice survived even when they were innoculated with an entirely different sub-type of flu, a H3N2 strain.

“We may have stumbled upon an influenza virus that can induce cross-reactivity against a larger range of influenza viruses,” observed Joshy Jacob, an associate professor at the university who led the research team, in an email. Their work has been published online by The Journal of Immunology.

Other scientists had shown that the immune response to the 2009 H1N1 virus protects against the H5N1 strain of bird flu as well, he pointed out.

The Emory scientists found that both antibodies and the immune system's killer cells have a part in generating the protection provided by the 2009 H1N1 strain.

Most of the protection comes from antibodies, much of it targeting the hemagglutinin protein on the virus' outer coat.

By latching on to this protein, the antibodies prevent the virus from infecting cells and reproducing itself inside them.

But, in addition, the swine flu virus primes CD8 T cells of the immune system. If antibodies are like missiles, “the CD8 T cells are akin to the army that does the hand-to-hand combat and decimates virus-infected cells,” explained Dr. Jacob.

Mice that had their CD8 T cells depleted after being immunised with the 2009 H1N1 virus were more likely to die and show more severe signs of sickness when faced with lethal doses of another flu strain.

Identifying the mechanism behind the protective response evoked by the 2009 H1N1 virus “may help us to design broadly cross-reactive universal influenza vaccine,” the scientists noted in their paper.