About two weeks ago, an unusually large number of about 100 students and staff members sought medical attention from the school nurse in one day, including six students and a teacher who had travelled to Mexico.

This prompted St Francis Preparatory School in Queens, New York to survey all its 2,600 students and 220 staff members.

It found out that more than 600 people were suffering from a flu-like illness, many more than those who had actually sought help.

And most of these were likely to have caught the new Influenza A (H1N1) strain, based on test results from some of them.

Tests on viral samples from 50 of those who were ill found that 88 per cent - 43 students and one teacher - were positive for the new flu strain.

'This gives you an indication that if you had the resources to sample all the people, you might actually get a lot more patients,' said Dr Lyn James, director of the Health Ministry's communicable diseases division, who related the events to reporters yesterday.

New York's Department of Health, for example, estimated that the city has hosted more than 1,000 cases, though its official report to the World Health Organisation was less than 100 cases.

Such under-reporting of the total number of people stricken by the new flu strain could have created the impression of it being more deadly than it actually was, Dr James said.

An initial estimate of the death rate in Mexico - where all but two of the 44 deaths from the new flu strain have now occurred - put it at about 6 per cent of all those who became ill.

But experts now believe it is lower than that because many mild cases would have gone unreported, she said.

Read the full story in Thursday's edition of The Straits Times.