People aged 60 and older may have some protection against the new swine flu virus, which doesn't seem to be as deadly as first feared, based on preliminary research, the U.S .Centers for Disease Control said Friday.

Older people have antibodies from other strains of flu that appear to beat back the new virus, flu influenza chief Nancy Cox said.

It is too early to draw definitive conclusions about how the new strain of H1N1 swine flu in terms of its pandemic potential, infectious disease experts caution.

"We do not see the markers for virulence that were seen in the 1918 virus," Cox told reporters.

Experts estimate about 40 to 50 million people worldwide were killed in the 1918 pandemic.

About 25 per cent of family members of cases in U.S. households appear to be falling ill from swine flu, said Dr. Anne Schuchat, CDC's interim deputy director for science and public health.

Seasonal flu tends to sicken between five per cent to 20 per cent of those exposed.

The CDC also published an updated report on the origins of the outbreak in Mexico on Thursday.

The Weekly Morbidity and Mortality Report includes:

  • A timeline from the first report of an outbreak of influenza-like illness in a small community in the state of Veracruz to the Pan American Health Organization.
  • Graph of the 1,918 suspected cases were reported between March 1 and April 30, including 286 probable and 97 cases.
  • Age group breakdown of laboratory-confirmed infections.
  • Review of medical records, including the number of cases reporting fever, cough and other symptoms.

"Understanding the epidemiology and clinical profiles of recent cases of [Swine-Origin Influenza A (H1N1)] infection in Mexico can help inform regional, national, and global control measures in response to the emergence of S-OIV infection," an editor's note with the report said.

"Important areas for investigation worldwide include evidence of person-to-person transmission, the geographic distribution of disease, the clinical spectrum of disease, and the effectiveness of mitigation strategies."

Genetic information for the new virus has been added to publicly available databases to allow researchers to investigate these questions, Cox said.

With files from The Associated Press