CDC officials have said they expect the new swine flu to spread to all 50 states, to cause severe disease and some deaths, although most cases have been mild.

Mexico has confirmed 42 deaths and said it was impossible to get samples from about 70 more people who died of flu-like illness recently. Globally, more than 1,600 cases have been reported in 23 countries.

Second strain of flu may complicate picture-study

* Mutations seen in seasonal flu strain

* May have caused Canadian late-season outbreak

* May complicate picture in Mexico

Health and Science Editor WASHINGTON, May 6 (Reuters) - A second strain of influenza, one of the seasonal strains, may have mutated and may be complicating the picture in Mexico, Canadian researchers reported on Wednesday. They have found a strain of the H3N2 virus that appears to have made a shift and could have complicated the flu picture in Mexico, epicenter of an outbreak of a new strain of the H1N1 swine flu virus. One was seen in a traveler returning from Mexico, the team at the British

Columbia Centre for Disease Control reported to Pro-MED, an online forum for infectious disease experts. And it may have been involved in an unusually late outbreak of flu in long-term care facilities this year. The new H1N1 virus has killed at least 42 people in Mexico and two in the United States, has spread globally and brought the world to the brink of a pandemic.

It appears to act like seasonal flu but doctors have been confused because it has also killed some young and apparently healthy adults -- not the usual pattern for influenza, which picks off the elderly, chronically ill and very young. Danuta Skowronski and colleagues said they routinely sequence the hemagglutinin gene from a sample of influenza viruses submitted each season by community doctors, hospitals and care facilities across the province of British Columbia, Canada.

Hemagglutinin gives a flu virus the "H" in its name, as in H1N1 or H3N2, and is found on the surface of the virus. Vaccines target hemagglutinin and when it changes, the vaccine must be changed, too. This year the vaccine targets strains of H3N2 influenza, an H1N1 strain different from the new swine flu strain, and an influenza B strain.

"Until mid-February 2009, amino acid sequences of the hemagglutinin gene of H3 viruses in British Columbia were virtually identical to the vaccine strain," Skowronski wrote. "In early March 2009, however, we detected additional differences from the vaccine strain among British Columbia viruses collected from facility outbreak settings.

" They only found these changes in flu samples taken from patients in care facilities. When news broke of the new H1N1 strain, they ran more tests. "We have sequenced the hemagglutinin gene of one of the H3 viruses from an ill traveler returning from Mexico and find it shares the same ... changes," they wrote. "In British Columbia, these H3 mutations arose sometime in early March 2009 and we observe at least one returning traveler to have likely acquired illness due to this virus in Mexico," they wrote. "We thus also wonder to what extent the profile of influenza-like illness initially reported from mid-March in Mexico may in part be attributed to this H3N2 variant in addition to emergence of the novel A/H1N1 virus."