Mexican H1N1 flu spreads easily: study

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - The new strain of H1N1 flu that has killed 56 people in Mexico and has been carried around the world by travelers acts more like a pandemic strain than regular seasonal flu, researchers reported on Monday.

They said it was transmitted more easily and affects young adults more often than annual flu strains, justifying the World Health Organization's stepped-up level of pandemic alert, currently a 5, one step down from a full pandemic.

As many as 23,000 Mexicans were likely infected with the swine flu virus, Neil Ferguson of Imperial College London and colleagues reported in the journal Science.

They also found evidence to support the theory that the outbreak originated in the village of La Gloria in the state of Veracruz, which had been the subject of intense speculation.

"The dating (of the outbreak) was consistent with that," Christophe Fraser of Imperial College London said in a telephone interview.

The World Health Organization Rapid Pandemic Assessment Collaboration looked at both the pattern of disease spread and the early genetic analysis of the virus.

"I think the most important thing we found was the rate of spread and the extent of spread is similar to what has been seen in previous pandemics," Fraser added.

"Our estimates suggest that 23,000 individuals had been infected in Mexico by late April, giving an estimated case fatality ratio of 0.4 percent based on confirmed and suspect deaths reported to that time," they wrote.