Mexican and US health officials searched today for signs an outbreak of a new flu strain is spreading further, after it killed up to 68 people in Mexico and infected eight in the United States.
As Mexico shut schools and museums and axed public events, global health officials stopped short of declaring a pandemic.
But they warned more cases could come to light, making up a major outbreak, as the flu spreads between people and infected some individuals who had no contact with one another.
The World Health Organisation said the virus from 12 of the Mexican patients was the same genetically as a new strain of swine flu, designated H1N1, seen in eight people in California and Texas who later recovered.
The Mexican government said the flu had killed 20 people and it may also be responsible for 48 other deaths. In all, 1,004 suspected cases have been reported nationwide.
Health Minister Jose Angel Cordova, speaking on the evening television news, encouraged people to avoid crowds and wear face masks, noting there was no guarantee that going to get a flu vaccine would help against the new strain.
He said the death rate appeared to have steadied and hospitals in the past few days had not seen the exponential rise in the number of people infected that many had feared.
Genetic analysis shows the flu strain is a never-before-seen mixture of swine, human and avian viruses.
The fact most of the dead were aged between 25 and 45 was seen as a worrying sign linked to pandemics, as seasonal flu tends to be more deadly among the elderly and the very young.
"We realise the seriousness of this problem," Mexican President Felipe Calderon told health officials yesterday.
In California, Dr Gil Chavez, director of the Centre for Infectious Diseases at the California Department of Public Health and the state's chief epidemiologist, said many more cases could come to light as patients are tested. "The more we look the more we are likely to find," he said.
In New York City, health officials were investigating what had sickened scores of students who fell ill with flu-like symptoms in a Queens high school on Thursday and yesterday. The symptoms were reported as mild and a city health official said he could not speculate about which flu strain was responsible.
The US government said it was taking the situation seriously and monitoring for any new developments.
Reuters