Swine flu H1N1: evolution of a virus

The influenza strain that has struck Mexico and the United States involves, in many cases, a never-before-seen strain of the H1N1 virus. The flu virus is notorious for its ability to mutate and evolve new features that allows it to escape the immune defence systems of its victims. The appearance of new flu strains is therefore a perennial problem for doctors and scientists.

And experts warned last night that it may be too late to contain the new outbreak, given how widespread the known cases are. If the confirmed deaths are the first signs of a pandemic, then cases are probably incubating around the world by now, said Dr Michael Osterholm, a flu expert at the University of Minnesota.

In New York City, health officials say about 75 students at a Queens high school have fallen ill with flu-like symptoms and testing is under way to rule out that is the same strain of swine flu found in Mexico.

No vaccine specifically protects against swine flu and it is unclear how much protection current generic vaccines might offer. A version genetically matched to the new swine flu virus has been created by the US Centres for Disease Control, said Dr Richard Besser, the agency's acting director. However, it could take months to create enough supplies for mass vaccination programmes - if governments decide vaccine production is necessary.

Two drugs, Tamiflu and Relenza, seem effective against the new strain. Roche, the maker of Tamiflu, said it was prepared to immediately deploy a stockpile of the drug if requested. Both drugs must be taken early, within a few days of the onset of symptoms, to be most effective.