May 3 (Bloomberg) -- The World Health Organization may declare the outbreak of H1N1 influenza a pandemic even as many cases of swine flu show symptoms no more severe than seasonal flu, health officials said.

The WHO raised its six-tier alert to 5 on April 29 and a further elevation would signal a pandemic, alerting governments to enact plans to curb the disease. Ireland became the 17th country yesterday to confirm swine flu and the new virus may be spreading in five nations among people unconnected to Mexico, where cases were first reported.

“At this stage we have to expect that phase 6 will be reached; we have to hope that it won’t be,” Michael Ryan, the Geneva-based agency’s director of global alert and response, said at a news conference yesterday. “I would still propose that a pandemic is imminent.”

Still, the WHO isn’t seeing sustained community transmission outside of North America for the virus, he said.

International health experts said the world is now closer to another influenza pandemic than at any time since 1968, when the last of the previous century’s three pandemics occurred. The WHO hasn’t had a single phase 6 alert since it introduced the six-level system in 2005. Before this week, the warning had been at phase 3 since 2007, when it was elevated for an outbreak of avian flu, according to the WHO Web site.

Tracked One Week

In little more than a week, world health authorities have tracked the emergence of the flu from an outbreak in Mexico and a few cases in Texas and California to more than 780 confirmed illness in 17 countries across the globe. Germany and the U.K. each confirmed two more cases, while Spain confirmed an additional 5 cases.

The virus has now struck more people than H5N1 avian influenza, with 421 confirmed cases, has in the past six years. Still, while bird flu has killed almost two-thirds of its victims, fatalities from swine flu are less than 3 percent of those infected, according to WHO data.

The pathogen has shuttered schools and offices in Mexico and the U.S., the next hardest hit country, stirred governments to use their treatment stockpiles, and spurred a quest for a vaccine before the beginning of the next flu season.

The number of confirmed dead from the H1N1 virus in Mexico is 19, up from 16 on May 1, said Mexico’s health minister Jose Cordova at a news conference in Mexico City. Laboratories in the country work through a backlog of samples to confirm diagnoses.

Campaigning Ban

The government put in place a ban on public campaigning for the July 5 mid-term elections and Mexico City is considering extending its school closures an extra week.

“Mexico has been living up to its duty” to fight the disease, Cordova said.

“We are in a stabilization phase,” he said. “Still, it is too soon to say we are past the most complicated moment.”

The WHO’s statistics, which lag reports by national and local agencies, listed 506 confirmed cases in Mexico and 787 worldwide. Illnesses were confirmed in the U.S., Canada, China, the U.K., Austria, Germany, the Netherlands, Denmark, Switzerland, Spain, Israel, Hong Kong, New Zealand, France, South Korea and Costa Rica.

The virus is already at pandemic level, according to Ira Longini, a researcher at the University of Washington in Seattle who advises the U.S. government on flu.

“The definition of a pandemic is that the new virus has spread to several countries and is transmissible,” Longini said in an interview yesterday. “It’s hard to imagine it’s not going to continue to spread in some form.”

Travel Affected

Baggage handlers in Paris yesterday refused to unload airplanes from Mexico and Spain for fear of contracting swine flu, a spokeswoman for the airport operating company said. The strike began when a flight from Cancun, Mexico, arrived at Paris’s Orly airport, and affected ensuing arrivals from Spain, the spokeswoman for Aeroports de Paris SA said in a telephone interview today. The handlers are working today.

Nestle SA, the world’s largest food company, said it restricted travel to and from Mexico and the U.S. in response to the swine flu outbreak. Nestle will apply limitations to any other countries where human-to-human transmission of the virus is established, the Vevey, Switzerland-based company said in a statement on its Web site May 1.

U.S. President Barack Obama spoke yesterday by telephone with Mexican President Felipe Calderon to share information about their countries’ efforts to limit the spread of the influenza. They stressed the importance of continuing close cooperation between their governments, according to a White House statement.

Pandemic Potential

“This is a new strain of the influenza virus, and because we haven’t developed an immunity to it, it has more potential to cause us harm,” Obama said yesterday in his weekly radio and Internet address. “This creates the potential for a pandemic, which is why we are acting quickly and aggressively.”

Obama has asked U.S. lawmakers for $1.5 billion to battle the outbreak and prepare for it to resurface during flu season.

Still, he said the virus hasn’t been as virulent in the U.S. as in Mexico and antiviral treatments have shown to be effective.

The U.K., U.S., Germany, Canada and Spain each confirmed cases in people who didn’t travel to Mexico.

German Cases

Germany has two new cases of swine flu after passengers on the same flight from Mexico as another victim contracted the illness, the Berlin-based Robert Koch institute said today. The institute is coordinating efforts to manage the outbreak. Germany has now eight confirmed cases of swine flu.

The U.K. has confirmed 15 cases of the disease with more than 600 test results still to come, the Health Protection Agency said on its Web site yesterday.

Ireland yesterday confirmed its first case, a man who had recently returned from a trip to Mexico, according to a statement last night from Tony Holohan, chief medical officer of the Department of Heath and Children.

The expanding wave of sickness has been similar to seasonal flu, though health authorities are taking no chances with a virus that may flash across the globe, infecting a population with no natural immunity, said the U.S. Centers for Disease Control and Prevention.

New York officials said they suspect more than 1,000 cases, so many that the government has stopped testing all but the sickest there.

Schools Close

Evidence suggests “transmission is widespread, and that less severe illness is common,” the Atlanta-based CDC said in a report May 1. In Mexico “a large number of undetected cases of illness might exist in persons seeking care in primary-care settings or not seeking care at all,” the CDC report said.

In the U.S., at least 433 schools had closed in 17 states, leaving parents to find other arrangements for 245,449 students, according to the Education Department. Five colleges closed, the department said in an e-mail.

The CDC raised its flu count yesterday to 160 cases in 21 states, including the only U.S. fatality, a 22-month-old child who died April 27 at a Houston hospital. The Boston Globe reported that New Hampshire became the 22nd U.S. state with an illness after authorities confirmed its first H1N1 infection, which had been reported April 30 as probable.

Batches of seed virus are being developed for potential vaccine production, according to WHO. Paris-based Sanofi-Aventis SA, Baxter International Inc. of Deerfield, Illinois, and GlaxoSmithKline Plc of London are talking with world health authorities about producing shots, the agency said.

Vaccine Production

Production of vaccines against the new H1N1 influenza will be completed “in parallel with or after the seasonal vaccine is produced,” Nancy Cox, chief of the flu division at the CDC’s Center for Immunization and Respiratory Disease, said at a news conference in Atlanta on May 1.

The three main seasonal flu strains -- H3N2, H1N1 and type- B -- cause 250,000 to 500,000 deaths a year globally, according to WHO. The new flu’s symptoms are similar, including fever and coughing, nausea and vomiting, according to the CDC.

Authorities advised hand-washing, hygiene and staying home if sick as the most effective ways to control the outbreak.

To contact the reporters on this story: Tom Randall in New York at trandall6@bloomberg.net. Dermot Doherty in Geneva at ddoherty9@bloomberg.net

Last Updated: May 3, 2009 07:30 EDT