(AP) -- Mexico City closed schools across the metropolis of 20 million Friday after at least 16 people died and more than 900 others fall ill from what health officials suspect is a new strain of swine flu. World health officials worried that it could mark the start of a flu pandemic.
The World Health Organization in Geneva, Switzerland said at least 57 have died in the outbreak, although it wasn't yet clear if this larger number of deaths was due to swine flu.
"We are very, very concerned," said Thomas Abraham, a spokesman for the agency. "We have what appears to be a novel virus and it has spread from human to human." If international spread is confirmed, that meets WHO's criteria for raising the pandemic alert level, he added.
Abraham said WHO on Friday raised their internal alert system, allowing them to divert more money and personnel to dealing with the outbreak. "It's all hands on deck at the moment." Abraham said.
Mexico's Health Secretary, Jose Cordova, said only 16 of the deaths have been confirmed to have been caused by the new strain, through testing at the government's laboratories. Samples from 44 other people who died were still being tested. The health department put the total number of people sickened at around 943 nationwide.
Cordova said samples were sent to the U.S. Centers for Disease Control and Prevention in Atlanta, Georgia, to determine whether it's the same virus infecting seven people in Texas and California. As of now, tests show the flu is a "new, different strain ... that originally came from pigs."
"We certainly have 60 deaths that we can't be sure are from the same virus, but it is probable," Cordova told MVS radio in Mexico City.
Cordova described a chilling new strain that had killed only people among the normally less-vulnerable young and mid-adult age range. One possibility is that the most vulnerable segments of the population - infants and the aged - had been vaccinated against other strains, and that those vaccines may be providing some protection.
But Dr. Anne Schuchat of the CDC said "at this point, we do not have any confirmations of swine influenza in Mexico" of the kind that sickened seven California and Texas residents.
All seven U.S. victims recovered from a strain of the flu that combines pig, bird and human viruses in a way that researchers have not seen before.
Cordova also told MVS radio in Mexico City that Mexican health officials can't be sure that the deaths "are from the same virus, but it is probable."
Closing the schools kept 6.1 million students home from day care centers through high schools, and thousands more were affected as colleges and universities closed down. Parents scrambled to juggle work and family concerns due to what local media said was the first citywide schools closure since Mexico City's devastating 1985 earthquake.
Lillian Molina and other teachers at the Montessori's World preschool scrubbed down their empty classrooms with Clorox, soap and Lysol on Friday between fielding calls from worried parents. While the school has had no known cases among its students, Molina supported the government's decision to shutter classes, especially in preschools.
"It's great they are taking precautions," she said. "I think it's a really good idea."
Authorities advised capital residents not to go to work if they felt ill, and to wear surgical masks if they had to move through crowds. A wider shutdown - perhaps including shutting down government offices - was being considered.
"It is very likely that classes will be suspended for several days," Cordova said. "We will have to evaluate, and let's hope this doesn't happen, the need to restrict activity at workplaces."
Still, U.S. health officials said it's not yet a reason for alarm in the United States. The five in California and two in Texas have all recovered, and testing indicates some common antiviral medications seem to work against the virus.
Schuchat of the CDC said officials believe the new strain can spread human-to-human, which is unusual for a swine flu virus. The CDC is checking people who have been in contact with the seven confirmed U.S. cases, who all became ill between late March and mid-April.
The U.S. cases are a growing medical mystery because it's unclear how they caught the virus. The CDC said none of the seven people were in contact with pigs, which is how people usually catch swine flu. And only a few were in contact with each other.
CDC officials described the virus as having a unique combination of gene segments not seen in people or pigs before. The bug contains human virus, avian virus from North America and pig viruses from North America, Europe and Asia.
Health officials have seen mixes of bird, pig and human virus before, but never such an intercontinental combination with more than one pig virus in the mix.
Scientists keep a close eye on flu viruses that emerge from pigs. The animals are considered particularly susceptible to both avian and human viruses and a likely place where the kind of genetic reassortment can take place that might lead to a new form of pandemic flu, said Dr. John Treanor, an infectious disease specialist at the University of Rochester Medical Center.
The virus may be something completely new, or it may have been around for a while but was only detected now because of improved lab testing and disease surveillance, CDC officials said.
The virus was first detected in two children in southern California - a 10-year-old boy in San Diego County and a 9-year-old girl in neighboring Imperial County.
It's not known if anyone is getting sick from the virus right now, CDC officials said.
It's also not known if the seasonal flu vaccine that Americans got last fall and early this year protects against this type of virus. People should wash their hands and take other customary precautions, CDC officials said.
Associated Press Writers Maria Cheng in London, Traci Carl in Mexico City and Mike Stobbe in Atlanta, Georgia, contributed to this report.
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