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.
"Thus while substantial uncertainty remains, clinical severity appears less than that seen in 1918 but comparable with that seen in 1957."
The 1918 pandemic was the worst of the 20th century, killing anywhere between 25 million and 100 million people, depending on estimates. It was the first appearance of the H1N1 virus. The 1957 pandemic of H2N2 killed an estimated 2 million people globally. Seasonal flu kills about 250,000 to 500,000 people annually with a fatality rate of less than 0.1 percent.
Looking at the pattern in La Gloria, the researchers said it looked as if the virus was transmitted human to human over 14 to 73 generations -- meaning one person infected another, who infected another, up to 73 times.
"Transmissibility is therefore substantially higher than seasonal flu," they wrote.
The first case was seen in La Gloria around February 15, they wrote, citing reports and not hard evidence. But quick genetic analysis suggests the virus may have first infected someone around January 12.
"As the epidemic spreads further, it is likely that severity will vary from country to country depending on health care resources and the public health measures adopted to mitigate impact," they added.
Globally, WHO has confirmed 4,694 infections in 30 countries. Sixty deaths have been confirmed, most in Mexico with three in the United States and one in Canada.
While it is widespread across the United States, WHO said the new H1N1 virus shows no signs of sustained person-to-person spread outside of North America.
Editing by Julie Steenhuysen and Will Dunham