(Rodrigo Abd ~ Associated Press)
With no suspected swine flu deaths since Wednesday and fewer people turning up at hospitals with virus symptoms, Mexican officials were guardedly optimistic that the worse was over in the outbreak's epicenter. Cases outside Mexico suggested the new swine flu strain is weaker than originally feared. But governments moved quickly anyway to ban flights and prepare quarantine plans.
The World Health Organization decided against a full pandemic alert, but that doesn't mean people can relax, said Dr. Mike Ryan, the WHO's global alert and response director.
"These viruses mutate, these viruses changes these viruses can further reassort with other genetic material, with other viruses," he said. "So it would be imprudent at this point to take too much reassurance" from the small number of deaths.
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 some time."
The global caseload was 763 and growing -- the majority in Mexico, the U.S. and Canada. Costa Rica reported its first confirmed swine flu case -- the first in Latin America outside Mexico.
Swine flu cases have been confirmed in 18 countries so far -- including Europe, the Middle East and the Asia-Pacific region -- and experts believe the actual spread is much wider than the numbers suggest.
President Obama urged caution 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. Later, he spoke with Mexican President Felipe Calderon for about 20 minutes to share information.
What started as a swine flu outbreak more than a week ago in Mexico quickly ballooned to a global health threat, with the WHO declaring a pandemic was imminent. Now public health officials are having to carefully calibrate their statements. Push the message too far, and they could lose credibility if the virus fizzles out. But if they back off and it suddenly surges, the consequences could be much more dire.
Health Secretary Jose Angel Cordova -- who has fended off criticism that his government reacted too slowly at first -- dismissed criticism that Mexico later overreacted in ordering a five-day, nationwide shutdown of all nonessential government and private businesses.
"It's absurd to think that Mexico was putting on a show," Cordova said. "I think it's preferable, at a certain moment, to take advanced measures and succeed in containing the problem than to not take them and ask, 'Why didn't we take them?"'
Mexico's last confirmed swine flu death occurred Tuesday, and the last suspected death came Wednesday, said Pablo Kuri, an epidemiologist and adviser to Cordova.
Cordova said hospitals are now handling fewer patients with swine flu symptoms, a sign that the disease at present is not very contagious. Mexican investigators who visited 280 relatives of victims found only 4 had the virus.
But experts said there is much they don't know about the outbreak in Mexico, where tests confirmed 16 deaths and nearly 450 people sickened. A multinational team of virus sleuths are trying to piece together the epidemiological puzzle.
Kuri said three of the dead were children: a 9-year-old girl, a 12-year-old girl and a 13-year-old boy. Four were older than 60.
The other nine were between 21 and 39 -- unusual ages for people to die of the flu because they tend to have stronger immune systems.
Although most of the dead were from Mexico City, they came from different neighborhoods in the metropolis of 20 million people, Kuri said. And he said health investigators haven't found any similarities in their medical backgrounds.
One explanation may be that they sought treatment too late -- an average of seven days before seeing a doctor. For those who recovered, the average wait was three days, said Hugo Lopez-Gatell Ramirez, deputy director of Mexico's Intelligence Unit for Health Emergencies.
Many of the sick around the world were people who had visited Mexico, including 13 of Britain's 15 cases.
China sealed 305 people inside a Hong Kong hotel where an infected Mexican tourist stayed.
South Korea reported Asia's second confirmed case -- a woman just back from Mexico -- and other governments prepared to quarantine airline 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.
With the disease on its doorstep, China suspended all direct flights from Mexico. Health workers in white bodysuits patrolled the lobby of the sealed Metropark Hotel where the 25-year-old Mexican stayed before he became Asia's first confirmed case late Friday.
Mexican Foreign Minister Patricia Espinosa complained that China had isolated 71 Mexican nationals in six cities, including Beijing, without reason -- and urged Mexicans not to travel to China until the situation was resolved.
"These are discriminatory measures," she said.
Governments must act cautiously, said Michael Osterholm, an infectious disease specialist at the University of Minnesota who has advised the U.S. government on flu preparations.
"This is a flu virus acting like a flu virus and causing, at worst, mild to moderate influenza," he said. "We have no room for complacency here, but we have to have a proportional response. What are the risks at the immediate time?"
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.