"While reports from Mexico appear to be encouraging and some are cautiously optimistic, we can't afford to let down our vigilance," Schuchat said during the Saturday teleconference.
The U.S. Education Department said that more than 430 schools had closed, affecting about 245,000 children, according to the Wall Street Journal.
However, Nancy Cox had some welcome news on the nature of the virus itself. She said during the teleconference that a preliminary analysis of the H1N1 strain finds it lacks certain "virulent characteristics" that made the 1918 flu pandemic strain so deadly.
And the new Secretary of Health and Human Services, Kathleen Sebelius, has made the decision to buy 13 million more courses of antivirals to replenish the antiviral stockpile, Schuchat said. "We don't know if we are going to need them, we just wanted to be ready," she said.
In addition, in the last 24 hours, the United States has shipped 400,000 regimens of antivirals to Mexico, believed to be the source of the global outbreak, at the request of the Mexican government, Schuchat added.
Meanwhile, President Barack Obama has urged Americans to stay calm, noting that it was not clear whether the global outbreak of the never-before-seen flu strain was any worse than "ordinary flus." But, he added, agencies across the U.S. government are preparing for the worst, according to the Associated Press.
If swine flu "is relatively mild on the front end, it could come back in a more virulent form during the actual flu season," he said at the end of a Cabinet meeting on Friday.
Elsewhere, the World Health Organization reported late Friday that the number
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