New Delhi, May 12: The new influenza virus that has touched 30 countries is moving much faster than seasonal flu, displaying a severity and spread similar to the 1957 flu pandemic that had claimed an estimated two million lives worldwide.

Scientists investigating the Mexico outbreak and its international spread have said the virus appears associated with a fatality rate of 4 per 1000 infected persons, and its dispersal resembles the early stages of a global pandemic.

The researchers from the UK, Mexico and the World Health Organisation cautioned that the number of people infected with the virus is still unknown, but it appears that for every 10 persons infected, there would be 12 to 16 secondary cases.

“It has been following a very similar pattern to the flu pandemic of 1957, in terms of the proportion of people becoming infected and the percentage of potentially fatal cases,” said Neil Ferguson, the head of the research group whose findings will appear in the journal Science this week. The 1957 pandemic was mild compared to the 1918 pandemic that had killed 40 to 50 million people around the world.

The WHO had declared on April 29 that the new H1N1 flu virus was showing sustained human-to-human transmission. The virus has infected more than 5200 people in 30 countries and caused 56 deaths, a WHO update said today. India's health ministry has said that it has not detected any case of H1N1 yet, but most public health experts believe it is only a matter of time before the virus surfaces here.

The speed of spread of the new virus is higher than seasonal flu which infects 10 to 15 per cent of the population, but lower than in a pandemic influenza where 20 to 30 per cent of the population would be expected to become infected.

“As the epidemic spreads further, it is likely that the severity will vary from country to country depending on health care resources and the public health measures adopted to mitigate impact,” Ferguson and his coauthors said in their report.

They said it is still unclear whether past infections with flu viruses can provide protection from this new virus. In a town called La Gloria in Mexico, children were twice as likely to become infected as adults — 61 per cent of those under 15 years were infected compared with only 29 per cent of those above 15. This could be explained in two ways — adults may be protected by previous flu infections, or children are more susceptible because they mix around in school environments more than adults.

Their analysis suggests the new H1N1 epidemic started around mid-February this year in Mexico and the virus had infected between 6000 and 32,000 people in that country by mid-April.

The researchers said uncertainty over fatality rate persists because the number of people infected in Mexico remains unknown. The rate appears to be between 3 per 1000 to 15 per 1000. The researchers believe 4 is the most likely figure — for now.