There have been 20 infections in the US and six in Canada, and possible cases as far as Europe, Israel and New Zealand, according to Reuters.

Everything slowed down in the Mexican capital as millions of people stayed at home, fearing infection. Those going out wore masks.

Mexican President Felipe Calderon said a clean bill of health has been given to most of the some 1,300 people suspected of being infected with the flu.

Meanwhile, the US declared a public health emergency and a top health official said she feared there would be fatalities in the US as the new strain of flu transmits.

The World Health Organization has declared the flu a "public health emergency of international concern". Such a pandemic could deal a major blow to the world economy, costing trillions of dollars.