Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus amid global concern about a possible pandemic, which means a prevalent and rapidly spreading disease over a large region.
But Gregory Hartl, a World Health Organization spokesman, told reporters that it did not recommend border closure or travel restrictions as a countermeasure.
He noted that infected people may not show symptoms at the airport or when they reach a border crossing. "Certainly, if you feel that you are ill, you should not travel, in any case, to anywhere," Hartl said.
Lessons learned from the SARS (a viral respiratory disease) outbreak in Asia in 2003 showed that border closure was economically disruptive and not particularly effective. "In public health terms, it didn't work, so we don't want to repeat something that didn't work" Hartl said.
But governments in Asia were not taking any chances. Singapore, Thailand, Japan, Indonesia and the Philippines dusted off thermal scanners used in the SARS crisis as they checked for signs of fever among passengers from North America.
South Korea and Indonesia introduced similar screening. In Malaysia, health workers in face masks took the temperatures of passengers as they arrived on a flight from Los Angeles.
China said anyone experiencing flulike symptoms within two weeks of arrival had to report to authorities.
India will start screening people arriving from Mexico, the United States, Canada, New Zealand, Spain, Britain and France for flulike symptoms, said Vineet Chawdhry, a top health ministry official. It also will contact people who have arrived from Mexico and other affected countries in the past 10 days to check for the symptoms, he said.
These measures came after the World Health Organization announced that it had raised its pandemic alert level to Phase 4 from Phase 3, the first time the alert level has been raised above 3 since the system was adopted in 2005.
The Phase 4 designation signifies that the new swine flu virus can cause sustained outbreaks and is adapting itself to spread among humans, significant steps toward a pandemic. But the ranking does not necessarily mean that a pandemic is a foregone conclusion.
"If the virus is an efficient virus, if it spreads easily from human to human, it will probably continue to spread," Hartl said.
All transmission of the disease so far appears to have been human to human and not from animal or other contact, according to the WHO. "There is no danger from eating pork," Hartl said. "If you cook pork well, if you cook all meat well, it kills all virus."
Outside Mexico, where 159 people have died, the United States has been the country worse hit by the swine flu outbreak. Many of the cases appear to be students who recently visited Mexico.
Addressing the National Academy of Sciences Monday, President Obama acknowledged the efforts to keep pace with the spread of the disease within the United States.
"We are closely monitoring the emerging cases of swine flu in the United States," he said. "And this is obviously a cause for concern and requires a heightened state of alert. But it is not a cause for alarm."