MEXICO CITY (AP) - A unique strain of swine flu is the suspected killer of dozens of people in Mexico, where authorities closed schools, museums, libraries and theaters in the capital on Friday to try to contain an outbreak that has spurred concerns of a global flu epidemic.
The worrisome new virus _ which combines genetic material from pigs, birds and humans in a way researchers have not seen before _ also sickened at least eight people in Texas and California, though there have been no deaths in the U.S.
"We are very, very concerned," World Health Organization spokesman Thomas Abraham said. "We have what appears to be a novel virus and it has spread from human to human ... It's all hands on deck at the moment."
The outbreak caused alarm in Mexico, where more than 1,000 people have been sickened. Residents of the capital donned surgical masks and authorities ordered the most sweeping shutdown of public gathering places in a quarter century. President Felipe Calderon met with his Cabinet Friday to coordinate Mexico's response.
The WHO was convening an expert panel to consider whether to raise the pandemic alert level or issue travel advisories.
It might already be too late to contain the outbreak, a prominent U.S. pandemic flu expert said late Friday.
Given how quickly flu can spread around the globe, if these are the first signs of a pandemic, then there are probably cases incubating around the world already, said Dr. Michael Osterholm at the University of Minnesota.
In Mexico City, "literally hundreds and thousands of travelers come in and out every day," Osterholm said. "You'd have to believe there's been more unrecognized transmission that's occurred."
There is no vaccine that specifically protects against swine flu, and it was unclear how much protection current human flu vaccines might offer. A "seed stock" genetically matched to the new swine flu virus has been created by the U.S. Centers for Disease Control, said Dr. Richard Besser, the agency's acting director. If the government decides vaccine production is necessary, manufacturers would need that stock to get started.
Authorities in Mexico urged people to avoid hospitals unless they had a medical emergency, since hospitals are centers of infection. They also said Mexicans should refrain from customary greetings such as shaking hands or kissing cheeks. At Mexico City's international airport, passengers were questioned to try to prevent anyone with flu symptoms from boarding airplanes and spreading the disease.
Epidemiologists are particularly concerned because the only fatalities so far were in young people and adults.
The eight U.S. victims recovered from symptoms that were like those of the regular flu, mostly fever, cough and sore throat, though some also experienced vomiting and diarrhea.
U.S. health officials announced an outbreak notice to travelers, urging caution and frequent handwashing, but stopping short of telling Americans to avoid Mexico.
Mexico's Health Secretary Jose Angel Cordoba said 68 people have died of flu and the new swine flu strain had been confirmed in 20 of those deaths. At least 1,004 people nationwide were sick from the suspected flu, he said.
Scientists have long been concerned that a new flu virus could launch a worldwide pandemic of a killer disease. A new virus could evolve when different flu viruses infect a pig, a person or a bird, mingling their genetic material. The resulting hybrid could spread quickly because people would have no natural defenses against it.
Still, flu experts were concerned but not alarmed about the latest outbreak.
"We've seen swine influenza in humans over the past several years, and in most cases, it's come from direct pig contact. This seems to be different," said Dr. Arnold Monto, a flu expert with the University of Michigan.
"I think we need to be careful and not apprehensive, but certainly paying attention to new developments as they proceed."
The CDC says two flu drugs, Tamiflu and Relenza, seem effective against the new strain. Roche, the maker of Tamiflu, said the company is prepared to immediately deploy a stockpile of the drug if requested.
Both drugs must be taken early, within a few days of the onset of symptoms, to be most effective.
Cordoba said Mexico has enough Tamiflu to treat 1 million people, but the medicine will be strictly controlled and handed out only by doctors.
Mexico's government had maintained until late Thursday that there was nothing unusual about the flu cases, although this year's flu season had been worse and longer than past years.