A new strain of swine flu has sickened two American children, although both have recovered and so far, there is no sign that the virus is spreading.

The viruses in the two children were similar but not identical. Both were resistant to older flu medicines but not to Tamiflu or Relenza. The Center for Disease Control and Prevention says that the virus includes a gene from the 2009 H1N1 strain. It is the first combination virus to turn up in people since the pandemic and is classified as an H3N2 virus. This year's vaccine, which is the same as last year's, is unlikely to protect against the new strain.

Flu viruses can easily spread from one species to another and exchange genes with other viruses to form new hybrids. The 2009 H1N1 strain is the hybrid of four bird flu viruses and the swine flu virus, which makes it hard to prevent.

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