The U.S. government has outlined a multiagency response, but seemed determined to project an air of calm, reports CBS News correspondent Kimberly Dozier. They likened their efforts to planning for a hurricane that may never come.
"Really, that's what we're doing right now. We're preparing in an environment where we really don't know ultimately what the size or seriousness of this outbreak is going to be," Homeland Security Secretary Janet Napolitano told reporters.
Mexico, the outbreak's epicenter with up to 103 suspected deaths, canceled some church services and closed markets and restaurants. Few people ventured onto the streets, and some wore face masks. Canada became the third country to confirm cases, in six people, including some students who - like some New York City spring-breakers - got mildly ill in Mexico. Countries across Asia promised to quarantine feverish travelers returning from flu-affected areas.
The U.S. declared the health emergency so it could ship roughly 12 million doses of flu-fighting medications from a federal stockpile to states in case they eventually need them - although, with 20 confirmed cases of people recovering easily, they don't appear to for now.
There is not a global pandemic - at least not yet. It's not clear how many people truly have this particular strain, or why all countries but Mexico are seeing only a mild form of the disease. Nor is it clear if the new virus spreads easily - one milestone that distinguishes a bad flu from a global crisis. But waiting to take protective steps until after a pandemic is declared would be too late.
"We do think this will continue to spread but we are taking aggressive actions to minimize the impact on people's health," said Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention.
Earlier, White House press secretary Robert Gibbs said the outbreak was serious, but that the public should know "it's not a time to panic." He told NBC's "Meet the Press" that President Obama was getting updates "every few hours" on the situation.
New York City was dealing with a growing public health threat Sunday after tests confirmed that eight students at a private Catholic high school had contracted the same strain of the swine flu that has affected Mexico. Some of the school's students had visited Cancun on a spring break trip two weeks ago.
New York officials previously had characterized the cases as probable, but Centers for Disease Control and Prevention confirmed that it was swine flu, Mayor Michael Bloomberg said.
About 100 students at St. Francis Preparatory School complained of flu-like symptoms; further tests will determine how many of those cases are swine flu.
Officials in New York City tried to dampen fears of an epidemic, reports CBS News correspondent Bianca Solorzano. Bloomberg stressed that the New York cases were mild and many are recovering, but said that some family members of students also had flu symptoms, "suggesting it is spreading person to person."
He said that the virus likely came from Mexico, although that has not officially been determined.
In Mexico, soldiers handed out 6 million surgical-style masks to deal with a deadly flu strain that officials say may have sickened 1,400 people since April 13. Special laboratory tests to confirm how many died from it - 22 have been confirmed so far out of 103 suspected deaths - are taking time.
The World Bank said it would send Mexico $25 million in loans for immediate aid and $180 million in long-term assistance to address the outbreak, along with advice on how other nations have dealt with similar crises.
The World Health Organization and the U.S. were following a playbook of precautions developed over the past five years to prepare for the next super-flu. The WHO on Saturday asked all countries to step up detection of this strain of A/H1N1 swine flu and will reconsider on Tuesday whether to raise the pandemic threat level, in turn triggering additional actions.
A potential pandemic virus is defined, among other things, as a novel strain that's not easily treated. This new strain can be treated with Tamiflu and Relenza, but not two older flu drugs. Also, the WHO wants to know if it's easily spread from one person to a second who then spreads it again - something U.S. officials suspect and are investigating.
"Right now we have cases occurring in a couple of different countries and in multiple locations, but we also know that in the modern world that cases can simply move around from single locations and not really become established," cautioned WHO flu chief Dr. Keiji Fukuda.
There is no vaccine against swine flu, but the CDC has taken the initial step necessary for producing one - creating a seed stock of the virus - should authorities decide that's necessary. Last winter's flu shot offers no cross-protection to the new virus, although it's possible that older people exposed to various Type A flu strains in the past may have some immunity, CDC officials said Sunday.
Worldwide, attention focused sharply on travelers.
"It was acquired in Mexico, brought home and spread," Nova Scotia's chief public health officer, Dr. Robert Strang, said of Canada's first four confirmed cases, in student travelers.
New Zealand said 10 students who took a school trip to Mexico "likely" had swine flu. Spanish authorities had seven suspected cases under observation. A New York City school where eight cases are confirmed will be closed Monday and Tuesday.
China, Russia and Taiwan began planning to quarantine travelers arriving from flu-affected areas if they have symptoms. Italy, Poland and Venezuela advised citizens to postpone travel to affected parts of Mexico and the U.S.
The U.S. hasn't advised against travel to Mexico but does urge precautions such as frequent hand-washing while there, and began questioning arriving travelers about flu symptoms.
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