Swine flu cases reached 1,085 worldwide, spreading to 21 countries and every major region of the U.S., health officials said. The symptoms, no more serious than those of seasonal flu, may worsen as seasons change.

In the U.S., the H1N1 virus has been confirmed by laboratory tests in 286 patients in each of nine regions tracked by the U.S. census, according to the U.S. Centers for Disease Control and Prevention in Atlanta. The disease, most active in Canada, Mexico and the U.S., has been confirmed in Central America, South America, Europe, the Middle East, Asia and New Zealand.

The World Health Organization said it’s bracing for the possibility the disease will worsen despite signs the virus may be no worse than seasonal flu. As the outbreak eases in Mexico, where it struck hardest, health officials are focusing on the Southern Hemisphere, where flu season will likely begin before a swine flu vaccine is available.

“That part of the world will be heading into the winter months,” Keiji Fukuda, the World Health Organization’s assistant director-general for health, security and environment, said today at a news conference in Geneva. “We’re really trying to get a handle on how far has this virus spread out and how much has it established itself in different parts of the world.”

The U.S. CDC reported 286 cases in 36 U.S. states, with one death. The number of people with flu in the U.S. is increasing when the normal flu season would be ending, the CDC said. At least 533 schools in 24 states were closed today, shutting out about 330,000 students, according to the U.S. Department of Education.

Mexico Peaks

Mexico’s outbreak probably peaked last week and patients are responding well to antiviral treatments, Health Minister Jose Cordova said yesterday. The virus has been confirmed in 23 of Mexico’s 31 states and the capital district.

“Mexico is trying to return to normalcy as soon as possible,” President Felipe Calderon said yesterday, adding that it was too soon to tally the economic cost. “We are going to win this battle.”

St. Francis Preparatory School in New York, where more than 1,000 students were likely infected, reopened today after being closed since April 27. New York has 73 confirmed cases of swine flu, though it stopped testing everyone suspected of having the illness, said Thomas Frieden, the city health commissioner, at a news conference today.

More Cases

“It is no more severe, from everything we have seen, than seasonal flu.” Frieden told reporters at St. Francis, in the New York borough of Queens.

Declaration of a pandemic is imminent, the World Health Organization said over the weekend. The WHO raised its six-tier alert to 5 on April 29 and a further elevation would signal a pandemic, alerting governments to carry out plans to curb the disease.

“While we’re not out of the woods yet, we are seeing some encouraging signs,” said Richard Besser, acting chief of the U.S. Centers for Disease Control and Prevention in Atlanta, in a conference call today. “Here in the United States, we’re at the very end of our flu season. There aren’t a lot of other flu viruses around that it’s competing with. What happens when it goes into countries where there are other viruses circulating?”

Besser said the U.S. strategy to close schools with suspected cases doesn’t effectively halt the virus’ spread once it becomes established in an area. The CDC is evaluating whether to change its advice to schools, he said.

New Cases

The WHO, a Geneva-based agency of the United Nations, today added Colombia and El Salvador to the list of countries with confirmed cases. The other nations are Austria, Canada, Costa Rica, Denmark, France, Germany, China (Hong Kong), Ireland, Israel, Italy, Mexico, the Netherlands, New Zealand, South Korea, Spain, Switzerland, the U.K., and the U.S.

Canadian health officials on May 2 reported the world’s first known case of swine flu jumping to pigs from a human, probably after a farm worker in the province of Alberta became ill during a trip to Mexico. Hundreds of pigs on the farm showed symptoms of the same H1N1 strain in humans and were recovering, according to the Canadian Food Inspection Agency.

Pigs are an ideal breeding location for new forms of the flu, and further genetic scrambling can result in deadlier forms of the new swine flu, said Nancy Cox, chief of the flu division at the Center for the Immunization and Respiratory Disease at the Atlanta-based CDC.

Mixing in Pigs

The animals serve as a “wonderful mixing vessel” for bird, human and swine viruses, Cox said. The process of two viruses merging to form a new virus can also take place in humans.

Even if swine-flu symptoms are mild, the ease with which the new virus spreads among a world population makes it a threat, health officials said.

Data so far suggest the virus is striking younger patients than is typical for influenza, and younger patients than usual are entering hospitals, said Anne Schuchat, a scientist at the CDC. “Very few” patients with swine flu are older than 50, and the median age is 17. It’s possible that older people have greater immunity, she said.

The three main seasonal flu strains — H3N2, another form of H1N1, and type-B — cause 250,000 to 500,000 deaths a year globally, according to the WHO. The new flu’s symptoms are similar: aches, coughing, and fever. The CDC says people with the swine flu are more likely to have diarrhea.

Pandemic Threat

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, world health experts said. The WHO hasn’t had a phase 6 alert since it introduced the six-level system in 2005. Before last week, the warning had been at phase 3 since 2007, when it was elevated for an outbreak of avian influenza, according to the WHO Web site.

The new flu strain has now struck more people than the H5N1 avian influenza that emerged in 2003. That illness killed more than half of the 421 who contracted the malady worldwide. Unlike swine flu, it didn’t spread from person to person.

The Spanish flu of 1918, another version of bird flu, killed as many as 50 million people in one of history’s deadliest outbreaks.

The U.S. is hastening production of its annual flu shots based on strains identified before the H1N1 outbreak, said Kathleen Sebelius, who was confirmed as the U.S. Health and Human Services secretary last week. That will make capacity available if vaccines are needed for swine flu, she said.

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

Authorities advised hand-washing, hygiene and staying home if sick as the most effective ways to control the outbreak. The WHO and CDC said closing borders or killing animals are costly steps that wouldn’t slow the spread of flu.

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