MEXICO CITY (AP) -- Mexico reported no new deaths from swine flu overnight - more reason to be optimistic that the worst is over at the epicenter of the outbreak. But the virus keeps spreading around the world, with new cases confirmed in Latin America, Europe and Asia, and governments banning flights and preparing quarantines.
The World Health Organization said it has sent 2.4 million treatments of anti-flu drug Tamiflu to 72 developing countries, taking the drugs from a stockpile donated by Roche Holding AG.
"At this point it's important that all countries have access to antivirals," said Dr. Mike Ryan, WHO's global alert and response director.
The WHO has decided not to raise its alert to a full pandemic, since the virus has yet to cause sustained transmission outside North America. But Ryan warned against complacency.
"These viruses mutate, these viruses changes, these viruses can further reassort with other genetic material, with other viruses. So it would be imprudent at this point to take too much reassurance" from signs the virus is weaker than feared.
The U.S. Centers for Disease Control and Prevention also said it's too early to declare victory.
"We have seen times where things appear to be getting better and then get worse again," said Dr. Anne Schuchat, the U.S. agency's interim science and public health deputy director. "I think in Mexico we may be holding our breath for sometime."
Costa Rica reported its first confirmed swine flu case - and the first flu case in Latin America outside Mexico.
And China worked aggressively to track down people who may have been near a sick Mexican tourist, sealing 305 people inside a Hong Kong hotel where he stayed and hospitalizing 15 fellow passengers. The man developed a fever after arriving in the Chinese territory and was isolated in stable condition Saturday.
South Korea reported Asia's second confirmed case - a woman just back from Mexico - and other governments also prepared to quarantine passengers, eager to show how they have learned from the deadly SARS epidemic in 2003, when Hong Kong was criticized for imposing quarantines too slowly.
The U.S. is taking "all necessary precautions" now to be prepared if the swine flu develops into "something worse" President Barack Obama said Saturday.
"This is a new strain of the flu virus, and because we haven't developed an immunity to it, it has more potential to cause us harm," Obama said. "Unlike the various strains of animal flu that have emerged in the past, it's a flu that is spreading from human to human. This creates the potential for a pandemic, which is why we are acting quickly and aggressively."
The global caseload was 718 and growing - the vast majority in Mexico, the U.S. and Canada. Swine flu cases also were confirmed in 14 other countries - in Europe, the Middle East, Asia and the Pacific - and experts believe the actual spread is much wider than the numbers suggest.
Mexico has taken extraordinary measures against the epidemic, ordering all nonessential government and private businesses to shut down for five days, at a tremendous cost to its economy. In the wide valley where traffic and crowds can be stifling even on a Saturday, Mexico City streets were strangely quiet, its usually crowded markets shuttered and even parks locked down.
"I'm going crazy in my house with this confinement," retiree Rocio Lara said in Mexico City. "There is nowhere to go, nowhere to spend your time."
Mexico City's mayor Marcelo Ebrard said they had expected exponential growth in the number of persons complaining of swine flu symptoms, and that the outbreak seems to be slowing instead.
It should soon become clear whether the epidemic is really stabilizing in Mexico, but many questions remain about how the disease kills, said the leader of an international team of flu-fighters now operating in the capital.
"That is the big question: Is it stabilizing or not? And it is too early to say, but I think we are getting systems in place where we are going to be able to get a handle on this soon," said Dr. Steve Waterman of the CDC.
Waterman also warned against taking false comfort from the fact that only one person has died outside Mexico, saying more deaths are likely as the epidemic evolves.
The U.S. government said schools with confirmed cases should close for at least 14 days because children can be contagious for seven to 10 days from when they get sick. More than 430 U.S. schools had closed, affecting about 245,000 children in 18 states.
With the disease on its doorstep, mainland China suspended all direct flights from Mexico, and sealed off the Metropark Hotel, where the 25-year-old Mexican stayed before he became Asia's first confirmed case late Friday. Health workers in white bodysuits patrolled the lobby, and mask-wearing police enforced a seven-day quarantine. One guest, Olivier Dolige of Paris, said they were taking Tamiflu as a precaution - and trying to make the best of it.
Speaking with The Associated Press using his computer's video-conferencing program, Dolige said he will turn 43 on Tuesday in quarantine. "I think about having my birthday with water and bad cake," he wrote. "No champagne."
Scientists trying to determine the mortality rate said this virus does not appear to match the ferocity of past killers.
"Most people think it is unlikely this is going to be as virulent as the 1918 epidemic. From what we know so far, it doesn't seem like it is as virulent," Waterman said.
"The virus has been circulating for over a month in a city of 20 million of high population density. It could have been much worse," agreed CDC epidemiologist Marc-Alain Widdowson.
The two CDC doctors spoke during a tour of Mexico's Intelligence Unit for Health Emergencies, where teams of doctors and scientists monitor the outbreak in real time and plasma screens enable frequent video conference calls with leaders from the Atlanta-based CDC, the World Health Organization and other institutions.
Health Secretary Jose Angel Cordova cited other indications that the disease is not very contagious: Mexican investigators who visited 280 relatives of victims found only 4 had contracted the disease, and that the number of people hospitalized with suspected cases is declining. But he stressed that it's too early for the government to declare the epidemic is subsiding.
Getting fast and effective care is important, said Hugo Lopez-Gatell Ramirez, deputy director general of epidemiology at the center. Among the 16 confirmed swine flu deaths in Mexico, the average time victims waited before going to a doctor was seven days. For those who were sickened but recovered, the average wait was three days.
Lopez-Gatell said that even before the swine flu outbreak, Mexican authorities had been monitoring a higher-than-usual number of flu cases and an unusual phenomenon in which otherwise healthy young adults were falling ill with pneumonia in greater numbers. There had been 15 flu outbreaks in this year's flu season, as opposed to the 5 or 6 that Mexico normally sees.
He said that put Mexico on guard and led to a fast reaction when unexplained illnesses began in March. Despite some international criticism of the Mexican response, Lopez-Gatell said no mistakes were made.
"We would have done everything the same if we had it to do over again," he said.
Associated Press writers David B. Caruso and Malcolm Ritter in New York, Lauran Neergaard in Washington and Paul Haven and Juan Carlos Llorca in Mexico City contributed to this report.