The outbreak of a strain of N1H1 influenza commonly known as swine flu remains primarily confined to North America, the World Health Organization said Saturday, though it still is expected eventually to reach pandemic levels by spreading around the globe.
Isolated cases of the flu continue to pop up in countries from Costa Rica to Denmark to New Zealand, though only 17 people worldwide have been confirmed dead from the virus and all have been from Mexico. (The one death in the United States was a Mexican boy.)
Hundreds of schools around the United States have closed temporarily because of the swine flu outbreak as federal officials reported Saturday about two dozen new cases of swine flu had been confirmed, bringing the total to 160 in 21 states.
There are signs that the virus isn't as ominous as once feared, but health officials warned Saturday the U.S. may not be out of the woods yet.
"While reports in Mexico appear encouraging, there should be some cautious optimism," said Dr. Anne Schuchat, interim deputy director for the Centers for Disease Control's Science and Public Health Program. "We can't afford to let our diligence down."
There has not yet been a sustained spread of H1N1 flu outside North America but a pandemic is still "imminent," Michael Ryan, the World Health Organization's director of global alert and response, said Saturday.
Ryan said it remained probable that the alert level would be raised from its current level of 5 to the top of the 6-stage scale.
"We have no evidence of sustained community spread outside of North America," he told a news briefing at the U.N. agency's Geneva headquarters. "We are still at Phase 5."
President Obama said the U.S. is acting aggressively.
"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," the president said in his radio and Internet address Saturday. "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 government issued new guidance for schools with confirmed cases, saying they should close for at least 14 days because children can be contagious for seven to 10 days from when they get sick. That means parents can expect to have children at home for longer than previously thought.
Schuchat said the new guidance is reflective of new information showing the longer duration that a child might be infected.
"It takes longer to shed (the flu) in younger children than adults," Schuchat said.
The Education Department said that more than 430 schools had closed, affecting about 245,000 children in 18 states. That was about 100 more schools reported closed than reported on Thursday.
The latest developments in the flu scare — more intense in neighboring Mexico than in the United States — came as the CDC reported the virus had been confirmed in more U.S. states.
Confirmed cases had risen from 141 on Friday to 160 Saturday in 21 states, the CDC said. The U.S. death toll remained at one — the Mexican toddler who visited Texas with his family and died there.
Mexico has confirmed more than 300 swine flu cases and has 16 confirmed deaths, although reports have indicated that roughly 120 may have died from it.
Worldwide, the total confirmed cases numbered close to 600, although officials believe there are many more.
The CDC said that the virus was continuing to spread, though no faster than the rate of the regular winter flu.
"We think the cases do continue to occur," said CDC's Dr. Anne Schuchat. But CDC also said the new swine flu virus lacks genes that made the 1918 pandemic strain so deadly.
While emphasizing at a news conference that the closures to date represent a tiny fraction of the almost 100,000 schools in the country, Duncan instructed teachers, parents and students to be prepared if their school does close.
To teachers, Duncan said: "Think about reworking upcoming lesson plans so students can do their schoolwork at home if necessary."
To parents: "Learn about what they're learning at school. Keep them on task."
And to students: "Don't fall behind your peers at other schools that are still in session. Keep working hard."
Major U.S. airlines announced plans to curtail flights into flu-ravaged Mexico.
Many travelers have become increasingly concerned about going to Mexico, though authorities there said new cases were leveling off.
U.S. travelers have been advised to avoid nonessential travel to Mexico. Continental Airlines Inc., the biggest U.S. carrier to Mexico, said Friday it would halve the number of seats it sells to fly there. Delta Air Lines Inc. and UAL Corp.'s United Airlines also announced plans for reduced flights to Mexico, while smaller carriers were following suit.
Though most U.S. cases have been relatively mild and have not required a doctor's visit, U.S. precautions include shipping millions of doses of anti-flu drugs to states in case they're needed, replenishing the U.S. strategic stockpile with millions more treatment courses, and shipping 400,000 treatment courses to Mexico.
The Associated Press and Reuters contributed to this report.