MEXICO CITY: Fears of a global swine flu pandemic grew with new infections in the United States and Canada yesterday and millions of Mexicans hid indoors to avoid a virus that has already killed up to 81 people.

While the only deaths so far have been in Mexico, the flu is spreading with 20 cases in the United States and four in Canada, and possible infections also popped up as far afield as Europe and New Zealand.

New York Mayor Michael Bloomberg confirmed that eight schoolchildren there had caught the swine flu virus, although the cases were mild and the illness did not appear to be spreading rapidly to the general population.

Another 12 cases have been confirmed in California, Kansas, Texas and Ohio, and Homeland Security Secretary Janet Napolitano said the government would declare a public health emergency.

Canadian health authorities said four cases were confirmed there.

In New Zealand, 10 students from a school party that had been in Mexico were being tested after showing flu-like symptoms.

The World Health Organization has declared the flu, of a type never seen before, a “public health emergency of international concern” and says it could become a pandemic, or a global outbreak of serious disease. A pandemic would deal a major blow to a world economy already suffering its worst recession in decades, and experts say it could cost trillions of dollars.

A 1968 “Hong Kong” flu pandemic killed about one million people globally.

Mexico City, one of the world’s biggest cities, practically ground to a halt yesterday with restaurants, cinemas and churches closing their doors and millions staying at home as officials tested 1,300 suspect cases.

People were told to follow Sunday church services on television and some residents abandoned the capital, a rambling, chaotic city of some 20 million people.

US officials said they would release a quarter of the country’s stockpile of the antiviral drugs Tamiflu, made by Roche AG, and Relenza, made by GlaxoSmithKline. Both have been shown to be effective against the new swine flu.

Flu is characterized by a sudden fever, muscle aches, sore throat and dry cough. Victims of the new strain have also suffered more vomiting and diarrhea than is usual with flu.