MEXICO CITY - A new flu strain that has killed up to 68 people in Mexico could become a pandemic, the World Health Organization warned on Saturday, as the nation's crowded capital hunkered down in fear of the disease.
Hospitals tested patients with flu symptoms for the never-before-seen virus, which has also infected eight people in the United States. There have been no further deaths in Mexico City since Friday, but 24 new suspected cases were being tested and officials warned the strain was spreading fast between people, meaning there was a risk of a major outbreak.
"It has pandemic potential because it is infecting people," WHO Director-General Margaret Chan said in Geneva.
"However, we cannot say on the basis of currently available laboratory, epidemiological and clinical evidence whether or not it will indeed cause a pandemic.
The new flu strain -- a mixture of swine, human and avian flu viruses -- is still poorly understood and the situation is evolving quickly, Chan said.
As far away as Hong Kong and Japan, health officials said they were stepping up surveillance of travelers for flu-like symptoms.
Mexico has said the new flu had killed at least 20 people and could be to blame for 48 more deaths. In all, more than 1,000 suspected cases have been reported nationwide. Most of the dead were aged 25 to 45, a worrying sign because a hallmark of past pandemics has been high fatalities among healthy young adults.
Mexico has shut schools and museums and canceled hundreds of public events in its sprawling, overcrowded capital of 20 million people to try to prevent further infections.
A foot race along Mexico City's most famous avenue was canceled on Saturday and two major soccer matches on Sunday were to be played in empty stadiums. Shopping centers were hushed, restaurant reservations were canceled and people out on the street wore face masks.
"Our goal is to break the spread of the virus in the city," Mexico City Mayor Marcelo Ebrard said after suspending 553 public events planned for coming days.
No countries or global bodies have issued travel alerts about Mexico, but a significant worsening of the flu outbreak could damage the tourism and retail sectors in an economy already hit hard by the global crisis.
MORE CASES COULD EMERGE
The WHO says the virus from 12 of the Mexican patients is genetically the same as a new strain of swine flu, designated H1N1, seen in eight people in California and Texas. All of the eight later recovered.
An emergency committee of WHO experts, convening on Saturday, will advise Chan on issues including possibly changing the WHO's pandemic alert level, currently at 3 on a scale of 1 to 6.
"We do not yet have a complete picture of the epidemiology or the risk, including possible spread beyond the currently affected areas," Chan said, adding the situation was seen as "serious."
Mexican soldiers handed out surgical masks and Health Minister Jose Angel Cordova told people to wear face masks and avoid crowds and close physical contact. He said there was no guarantee current vaccines will help against the new strain.
The seasonal flu vaccine protects against one strain of the H1N1 virus, which is also circulating, but this new version is genetically different. Experts at the U.S. Centers for Disease Control and Prevention are working on a vaccine against the new strain but it could take months to make.
"We realize the seriousness of this problem," Mexican President Felipe Calderon told health officials on Friday.
Cordova said he was encouraged by a steadying in the death rate and the fact hospitals had not seen the exponential rise in the number of people infected that many had feared.
More cases could come to light as patients are tested in California, said Dr. Gil Chavez, director of the Center for Infectious Diseases at the California Department of Public Health and the state's chief epidemiologist. "The more we look the more we are likely to find," he said.
In New York City, health officials were looking into what had sickened scores of students who fell ill with flu-like symptoms in a Queens high school on Thursday and Friday, although the symptoms were reported as mild.
"At this point we have no evidence to say what's going on in New York is related to this," Dr. Richard Besser, acting director of the CDC, told CNN.
Little can be done to prevent an outbreak of flu from spreading, health experts warn, but common sense measures can help people protect themselves. No. 1 is hand-washing.
In Hong Kong, the epicenter of the 2003 SARS epidemic and a city especially vigilant about any threat of infectious disease, the government said it was ramping up safety checks at airports and the border. Authorities have also said they would analyze flu samples in the territory.
Cordova said Mexico had 1 million doses of antiviral medicine, easily enough to treat the cases reported so far. Roche AG's Tamiflu and GlaxoSmithKline's Relenza are both recommended to treat flu and have been shown to work against viral samples taken from the eight people infected in the United States.
The last flu pandemic was in 1968 when "Hong Kong" flu killed about a million people globally.