Responding to what appeared to be the leading edge of a global pandemic emerging from Mexico, U.S. health officials declared a public health emergency Sunday as 20 cases of swine flu were confirmed in this country, including seven in California.

Other nations imposed travel bans or made plans to quarantine air travelers as confirmed cases appeared in Mexico, Canada and the United States and suspect cases emerged in other countries.

Top global flu experts struggled to predict how dangerous the new type A (H1N1) swine flu strain would be as it became clear that they had too little information about Mexico's outbreak - in particular whether the virus was mutating to be more lethal or less so.

"We're in a period in which the picture is evolving," said Dr. Keiji Fukuda, deputy director general of the World Health Organization. "We need to know the extent to which it causes mild and serious infections."

Without that knowledge - which is unlikely to emerge soon because only two laboratories, in Atlanta and Winnipeg, Manitoba, can confirm a case - his agency's panel of experts was unwilling to raise the global pandemic alert level, even though it officially considered the outbreak a public health emergency and opened its emergency response center.

U.S. investigators said they expected more cases, but noted that virtually all so far have been mild and urged Americans not to panic.

The speed and scope of the world's response, public health experts said, showed the value of preparations made because of the scares over avian flu and SARS, or severe acute respiratory syndrome.

More money for drugs

The emergency declaration lets the government free more money for antiviral drugs and give some previously unapproved tests and drugs to children. One-fourth of the national stockpile of 50 million courses of antiflu drugs will be released.

Border patrols and airport security officers will begin asking travelers if they have had flu or fever; those who appear ill will be stopped, taken aside and given masks until they can arrange for medical care.

"This is moving fast, and we expect to see more cases," Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said at a news conference in Washington. "But we view this as a marathon."

He advised Americans to wash their hands frequently and before touching their eyes or mouth, to cover coughs and sneezes and stay home if they feel ill. He stopped short of advice being given in Mexico to wear masks and not kiss or touch anyone.

Homeland Security Secretary Janet Napolitano, at the same news conference, called the emergency declaration "standard operating procedure" and said she would rather call it a "declaration of emergency preparedness."

"It's like declaring one for a hurricane," she said. "It means we can release funds and take other measures. The hurricane may not actually hit."

Besides the seven cases in California, officials said they had confirmed eight in New York, two in Kansas, two in Texas and one in Ohio. Health authorities are investigating another suspected case at a school in Fair Oaks, near Sacramento.

The virus looked identical to the one in Mexico believed to have killed 103 people and sickened 1,614. As of Sunday night, there had been no deaths in the United States.

Other governments tried to contain the infection amid reports of potential new cases from New Zealand to Hong Kong to Spain, raising fears that a global pandemic was under way.

Fukuda of the WHO said his agency would put off until Tuesday a decision on whether to raise the pandemic alert level to 4. Such a move would trigger more travel bans, and the agency is historically reluctant to take actions that hurt member nations.

Canada also confirmed six cases, at opposite ends of the country - four in Nova Scotia and two in British Columbia. Canadian health officials said the victims had only mild symptoms and had either recently traveled to Mexico or been in contact with someone who had.

Nations' travel advisories

Other governments issued travel advisories urging their citizens not to visit Mexico. China, Japan, Hong Kong and others set up quarantines for anyone possibly infected. Russia and other countries banned pork imports from Mexico, although one cannot get the flu from eating pork.

In the United States, the Centers for Disease Control confirmed that eight students at St. Francis Preparatory School in Fresh Meadows, Queens, had been infected with swine flu. At a news conference on Sunday, Mayor Michael Bloomberg said all those cases had been mild and city hospitals had not seen a surge in severe lung infections.

The central question, said Dr. Martin Cetron, director of global migration and quarantine for the Centers for Disease Control, is how many mild cases Mexico has had.

"We may just be looking at the tip of the iceberg, which would give you a skewed initial estimate of the case fatality rate," he said, meaning that there may have been tens of thousands of mild infections around the 1,600 cases of serious disease and 100 or more deaths. If that is true, as the flu spreads, it would not be surprising if most cases are mild.

Even in 1918, according to the Centers for Disease Control, the virus infected at least 500 million of the world's 1.5 billion people to kill 50 million. Many would have been saved if antiflu drugs, antibiotics and mechanical ventilators had existed.

Another hypothesis, Cetron said, is that some other factor in Mexico increased lethality, such as co-infection with another microbe or an unwittingly dangerous treatment.

Preventing spread of swine flu

-- If you have flu symptoms - fever, sore throat, cough, and possibly vomiting and diarrhea - stay home from work or school. Do not return until two days after your symptoms are gone.

-- Cover your nose and mouth when you cough or sneeze, and wash your hands frequently.

-- Go to the hospital if you have severe symptoms such as difficulty breathing. If your symptoms are mild, stay home to avoid spreading the virus to others at the hospital.

Source: Associated Press