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The World Health Organization is calling the swine flu outbreak a "public health emergency of international concern" and its asking countries to ramp up surveillance for the virus.

But the organization decided Saturday to hold off on raising the global pandemic alert level, even as more cases are being reported in Mexico, and the United States.

Gregory Hartl, a spokesperson for the WHO, said Saturday the threat level will stay at its current Phase 3 until the organization gets a clearer picture at what is going on. Phase 6 would mean officials are calling the outbreak a pandemic.

Countries around are being asked by the WHO to step up their reporting and surveillance of the disease. WHO fears the outbreak could spread from Mexico and the United States.

Mexico officials are closing schools in the capital and two other states for the next 10 days as the country tries to slow the spread of a deadly swine flu outbreak.

Health Secretary Jose Angel Cordova told a news conference Saturday evening that 20 deaths have been confirmed to be from the swine flu outbreak and another 1,324 have likely been sick from the virus since April 13.

He added that the virus may have killed at least 81 people in Mexico, but not all the deaths have been confirmed to be from the virus, known as A H1N1.

The strain has not been previously diagnosed in either pigs or humans and appears to spread easily from human to human.

On Saturday evening, New York City health officials said that eight high school students in the city have likely contacted the virus after visiting Mexico. The results of tests won't be known until Sunday.

Kansas state officials confirmed two cases there Saturday, in addition to seven cases in California and two in Texas.

The head of the WHO, Margaret Chan, said at a news conference earlier on Saturday that the WHO has not received reports of swine flu beyond Mexico and the southwestern United States, but urged all countries to be vigilant for an increase in pneumonia cases or other flu-like illness.

"The viruses causing cases in some parts of Mexico and some parts of the USA are genetically the same," she said, adding the virus "has pandemic potential" because it is spreading between people.

Quarantine in Montreal

Meanwhile, doctors in Montreal placed a woman in quarantine, but released her later when it became clear she did have the virus.

The woman, Patricia Whelan, vacationed in Mexico two weeks ago. Another Canadian was also placed in isolation, but has since been released.

Doctors "have no reason to believe it's swine flu, but they're taking absolutely no chances. It's precautionary," said CTV Montreal's Rob Lurie.

Monica Whelan, the daughter of the woman who was in quarantine, said her family was surprised to learn her mother was so sick.

"My mother never catches a cold, and she's always very strong," she said.

"She still felt very ill after having been on antibiotics. It's been a month since she's been home and her symptoms have not gone away," said Whelan.

Mexican cases

According to Chan, Mexican authorities are reporting that cases range from the very mild to severe, which means it is difficult to tell how widespread the virus really is.

"We need to really comb through those data and get the granularities and understand exactly who is suffering and which (age) group has disease more severe than others," Chan said.

The virus is unusual in that it is primarily affecting healthy young adults. Influenza usually affects the very young and the very old.

Unlike recent bird flu cases, where people who became ill had contact with infected birds, it appears that this strain of swine flu is infecting people with no history of contact with animals, according to Canadian infectious disease specialist Dr. Neil Rau.

"What is concerning is that even though it has the word swine flu, meaning it originates from pigs, people who get this infection have no history of contact with pigs," Rau said Saturday during an interview on Â鶹ӰÊÓnet.

Mexican authorities have closed schools, museums, libraries and theatres in an attempt to prevent the virus's spread, while officials at the Mexico City airport are questioning travellers in an effort to prevent a sick passenger from boarding a plane.

On Saturday, Mexico City's mayor cancelled all public events for the next 10 days in order to control the outbreak.

Mayor Marcelo Ebrard also announced that the city has enough medicine to treat those who are infected.

The Mexican government plans to administer 500,000 doses of influenza vaccine to health care workers, who are at high risk of infection.

However, it is unclear if this year's influenza vaccine offers protection against the swine flu.

The WHO said that 12 of the Mexican cases have been linked genetically to a swine flu virus that has appeared in California.

In Canada, Health Minister Leona Aglukkaq said Friday that so far, the virus has not been detected in Canada.

According to Rau, the current outbreak has not been linked to resort areas frequented by Canadian tourists.

However, public health officials are probing a small number of flu-like cases in Canada to determine if they match the Mexican illness.

Experts say the best protection against the virus is to avoid contact with anyone who is coughing or sneezing, wash hands regularly and see a doctor if any of these symptoms become apparent:

• fever
• cough
• sore throat
• eye pain
• shortness of breath 
• muscle and joint pain
• extreme fatigue

With files from The Associated Press and The Canadian Press