| MEXICO CITY
MEXICO CITY Fears of a global swine flu pandemic grew with new infections in the United States and Canada on Sunday, and millions of Mexicans hid indoors to avoid a virus that has already killed up to 81 people.
While the only deaths have been in Mexico, the flu is spreading with 20 cases in the United States and six in Canada, and possible cases as far afield as Europe, Israel and New Zealand.
Offering a glimmer of hope in Mexico, President Felipe Calderon said most of the roughly 1,300 people in the country suspected of having the fatal flu have been given a clean bill of health.
The United States declared a public health emergency and a top official at the Centers for Disease Control and Prevention, or CDC, said she feared there would be deaths in the United States as the new strain of flu spreads.
The Mexican capital, one of the world's biggest cities, slowed to a snail's pace as millions of residents stayed at home, fearing infection.
"This is the first time I've left the house in two days. I had to get some air," said Juan Casiano, a 39-year-old office worker, walking briskly through a city park.
The Roman Catholic faithful listened to mass on the radio rather than go to church, professional soccer games were played in empty stadiums and weekend cyclists stayed off the road in a lock-down of a normally chaotic city of 20 million people.
The World Health Organization has declared the flu a "public health emergency of international concern" that could become a pandemic, or global outbreak of serious disease.
A pandemic would deal a major blow to a world economy already suffering its worst crisis in decades, and experts say it could cost trillions of dollars.
A 1968 "Hong Kong" flu pandemic killed about 1 million people globally.
U.S. DEATHS EXPECTED
Deaths are now feared in the United States, a CDC official said.
"I do fear that we will have deaths," the agency's Dr. Anne Schuchat said. She told reporters officials were preparing for the illness's possible spread beyond the 20 cases confirmed in the United States.
Eight schoolchildren in New York were confirmed as having had caught the swine flu virus, although like other cases reported outside Mexico they were relatively mild.
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.
Although it is called "swine flu" there is no evidence that any of the cases stemmed from contact with pigs.
Health officials say they do not understand why deaths have been reported in Mexico but nowhere else. "I think right now it is really premature to say the disease is different in Mexico from here," Schuchat said.
The outbreak has snowballed into a major headache for Mexico, already grappling with a violent drug war and economic slowdown, and has become one of the biggest global health scares in years.
Mexico's tourism and retail sectors could be badly hit by the crisis.
"This issue can have an important impact on the economy, although the most important impact is the one on human life and human well-being," Finance Minister Agustin Carstens said.
Officials said they would release a quarter of the U.S. stockpile of the antiviral drugs Tamiflu, made by Roche AG, and Relenza, from GlaxoSmithKline. Both have been shown to be effective against the new swine flu.
(Additional reporting by Stephanie Nebehay in Geneva, Maggie Fox, Emily Kasier and Lesley Wroughton in Washington, Mica Rosenberg and Catherine Bremer in Mexico City)