Hartl added Sunday that a time frame for an upgrade in alert level "is not very easy to predict, because transmission of influenza virus, or of any virus for that matter, does not necessarily always move at the same speed. There could be hiatus in its spread, especially [because] we have not seen any confirmed instances of sustained human-to-human transmission in communities outside of the Americas."
"We don't know when that will happen; certainly we remain on alert in case that happens," he said. "But ... it would not be prudent to try to make a prediction" on the timing of a potential decision to step up the alert level.
The next level, six, would indicate that "a global pandemic is under way," the guidelines say.
There have been concerns about the safety of eating pork since the outbreak of the virus, though officials have maintained that the meat is safe. Global health officials have ceased calling it swine flu in favor of its scientific designation, H1N1, as part of the effort to reinforce that notion.
But on Saturday, concerns grew after Canadian officials confirmed that pigs on a farm in that country tested positive for the virus. The farm has been placed under quarantine.
WHO food safety scientist Dr. Peter Ben Embarek said today that a farm worker who had recently visited Mexico likely brought the virus to the farm, but that the humans and animals infected in that case are recovering.
Embarek said the virus infected only about 10 percent of the herd of 2,200 pigs.
"This tells us that, also for the animal population, it doesn't seem to be a very serious disease," Embarek said. "And it's apparently, from what we know, something that is not surprising and could be expected from a virus like this."
He added that, though farm workers and those who slaughter and process infected animals could be at a greater risk for picking up the disease, "from the consumer point of view, there is absolutely no risk of consuming cooked products," and that the meat trade should not be restricted.
President Obama said in his weekly address to the nation Saturday that the United States is acting "quickly and aggressively" to prepare for a potential pandemic, and is "investing in every resource necessary to treat this virus and prevent a wider outbreak."
His newly-confirmed secretary of health and human services, Kathleen Sebelius, said on "This Week" that the government is taking action to develop a vaccine for the H1N1 virus in addition to preparing for the fall flu season.
"The good news is we're in the right seasonal time. We can accelerate the seasonal flu vaccine which we're doing right now, to be prepared and ready for what we know will hit this fall and winter," Sebelius said, "At the same time, we are in the stages of growing the virus, testing it, and we can be ready to do both simultaneously."
Obama added the nation's stockpile of antiviral medications has been opened up to the states, with a quarter of the supply of 50 million courses of treatment already released. The U.S. government also purchased 13 million treatments to replenish the stockpile, he said.
The WHO said Saturday that it released 2.4 million antiviral courses to 72 developing countries, including Mexico.