WASHINGTON, DC (AP):

The number of cases of swine flu may have been several times higher than reported and the potential for rapid spread of the illness justified the World Health Organisation's decision to raise the global pandemic alert, a new study concludes.

While about 4,800 confirmed cases have been reported in 30 countries, the new analysis estimates there have been between 6,000 and 32,000 cases in Mexico alone. While there have been 1,626 cases of the flu confirmed in Mexico, the researchers note that there have been more than 11,000 suspected infections.

No estimate

The US Centers for Disease Control and Prevention (CDC) has not offered an estimate of how many total swine infections have hit the United States so far. However, the number of confirmed cases is no doubt just "the tip of the iceberg," said Dr Anne Schuchat, the CDC's interim deputy director for science and public health.

About 40 per cent of confirmed US flu cases are probably swine flu, according to CDC data from about nine days ago.

"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.