May 3 (Bloomberg) -- Swine flu is killing fewer people and producing milder symptoms than the world's previous influenza outbreaks, an "encouraging" sign for an illness that is spreading easily, the U.S. chief for disease fighting said.

The official, Richard Besser, spoke as the newest tallies confirmed the disease in at 19 least countries. Just nine days ago, on April 24, the World Health Organization warned of cases of "influenza-like illness" in the U.S. and Mexico. The Geneva-based international agency said yesterday it may declare a pandemic, a measure of the flu's geographical reach and ability to spread.

Swine flu, formally H1N1, contains genes from swine, bird and human viruses. It has been confirmed in at least five countries in people who didn't travel to Mexico, a sign the illness is taking root. The World Health Organization is seeking proof of a sustained outbreak outside the U.S. and Mexico before raising its disease-alert system to the sixth and highest stage.

"What we've found is that we're not seeing the factors that were associated with the 1918 pandemic, we're not seeing the factors that were associated with other H1N1 viruses, and that's encouraging," said Besser, acting chief of the U.S. Centers for Disease Control and Prevention, on the ABC News program "This Week." Because the virus is new and possibly evolving, "I don't think it's time to let our guard down."

Mostly North America

The WHO, a Geneva-based agency of the United Nations, today reported 787 laboratory-confirmed cases of swine flu, with 20 deaths. Mexico has the biggest outbreak, with 506 cases, including 19 dead. The U.S. reported 226 cases in 30 states, with one death, a 22-month-old child who had traveled from Mexico and died April 27 at a Houston hospital.

Canada had 70 cases, reinforcing that the outbreak is most severe in North America. Canada also reported the world's first case of the 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. The animals are recovering and no need exists to cull herds worldwide, said Peter Ben Embarek, a food safety science with the WHO, on a conference call today.

Spreads ‘Easily'

Italy today confirmed its first two cases, and Colombia confirmed its first. The WHO hasn't added those to its confirmed list, which has 17 nations and typically lags behind country and local reporting.

The agency has confirmed the disease in these countries in addition to Canada, Mexico and the U.S.: Austria, China, Costa Rica, Denmark, France, Germany, Hong Kong, Ireland, Israel, the Netherlands, New Zealand, South Korea, Spain, and Switzerland.

Even if symptoms remain mild, the ease with which the illness can spread among a world population with no natural immunity still makes it a threat, health officials said.

"It does spread very easily," Besser said. "The word out of New York City where they had a school cluster is it spread very rapidly through that school. But what they were seeing was disease that was not that severe, and when it transmitted to people in the families, they were seeing disease that was not that severe, and that's encouraging."

Besser said it's important to remain "aggressive" this early, and he said there's no evidence that health officials have overreacted.

No Room for Error

"With new infectious diseases, there's a lot of unknown, a lot of uncertainty," he said. "You basically get one shot."

Gregory Hartl, a spokesman for the WHO, said on a conference call today that this outbreak of H1N1 "could be the first wave" and the real test will be after autumn starts in the Northern Hemisphere. That would be late September.

Readiness includes vaccine preparations. Kathleen Sebelius, who was confirmed as the U.S. Health and Human Services secretary last week, said production of the vaccines for the strains already expected for the season is being hastened to add capacity to produce swine flu vaccine if necessary.

"We are ramping up and accelerating the production of seasonal flu vaccine to make sure that we kind of clear the decks," she said on the ABC program. "Ultimately the scientists will tell us whether or not production of that vaccine makes sense."

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.

Past Pandemics

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 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 flu, according to the WHO Web site.

"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 agency's director of global alert and response, said at a news conference yesterday. "I would still propose that a pandemic is imminent."

The new influenza 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.

Seed Virus

The U.K. has confirmed 18 cases of the disease with more than 700 test results still to come, the Health Protection Agency said on its Web site today.

Batches of seed virus are being developed for potential vaccine production, according to the World Health Organization. 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.

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 U.S. disease control centers, which is based in Atlanta.

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

Border-Closing Rejected

U.S. officials reasserted their argument that no scientific justification exists for closing borders, and that "mitigation" counts most.

"You also have to understand the inordinate costs associated with closing a border, the number of jobs associated with that, the trade that goes back and forth," the U.S. secretary for Homeland Security, Janet Napolitano, told "Fox News Sunday." "If it had a benefit to it, you would make that calculation perhaps a little differently."

Napolitano and Sebelius also said only the sick should avoid jetliners, subways and trains, remarks following comments last week by Vice President Joe Biden that he would tell his family not to travel in "confined places now."

"Each member of our country makes decisions about themselves and their family and about safety and security," Sebelius said on Fox News. "What we're telling you is what the science says."

She said her "elderly father and aunt" were planning to fly today. Napolitano said, "The vice president did take the train home from work yesterday."