"Our early analysis would suggest this is going to be an outbreak comparable to that of 20th century pandemics regarding the extent of its spread - it's very difficult to quantify the human health impact at this stage, however," said lead author Neil Ferguson of Imperial College, London.
Ferguson's analysis was released by the journal Science. Normally Science releases its reports on Thursdays but the journal said it was issuing this study early because it contains important public health information.
The researchers said the 2009 H1N1 flu appears to be about equal in severity to the flu of 1957 and somewhat less severe than the deadly 1918 version.
The new analysis estimated that between 0.4 percent and 1.4 percent of cases were fatal.
The outbreak appears to have originated in mid-February in the village of La Gloria, Veracruz, where over half the population suffered acute respiratory illness, affecting more than 61 percent of under 15-year olds in the community, the report added.
Using a variety of methods to estimate how easily the virus is transmitted, the researchers said that each case of the flu resulted in between 1.4 and 1.6 infections to others.
Data on the spread and strength of the illness is still incomplete, the researchers stressed. But they said their findings can help policymakers make such decisions as whether to close schools, balancing the cost of such actions against the potential to prevent spread of disease.
The potential spread of the illness in the Southern Hemisphere, which is just beginning its flu season, needs to be closely monitored, Ferguson's team wrote.
WHO's announcement of a Level 5 alert meant that a virus has caused sustained community level outbreaks in at least two countries in one region, and a worldwide pandemic is considered imminent. It alerts countries that do not have the illness yet to prepare for its arrival and institute their pandemic preparedness plans.
The Centers for Disease Control and Prevention reports that there have been 2,618 flu cases in 44 states with three deaths.
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