May 11 (Bloomberg) -- Swine flu extended its reach to China and killed a third person in the U.S. even as scientists said the virus so far lacks the lethal traits of the 1918 Spanish pandemic or the bird flu fatal to half those infected.

A 30-year-old student developed symptoms after his return from the U.S., China’s health ministry said yesterday. A man in his 30s in Snohomish County, Washington, with “underlying heart conditions” died last week from what appeared to be swine flu- related complications, the state’s health department said.

The genetic blueprint of the new H1N1 virus sweeping the globe is “good news,” said Anne Schuchat, interim deputy director for science and public health of the U.S. Centers for Disease Control and Prevention. Schuchat told reporters in a call from Atlanta that swine flu may yet exchange genetic material with other viruses and mutate into something worse.

“The good news so far is that the virulence markers for the 1918 and H5N1 influenza viruses do not appear in the H1N1 strain,” Schuchat said. “What we don’t know is whether there may be other virulence markers. Remember the first wave of the 1918 virus was mild and the next wave was devastating.”

China’s first swine flu patient, a student surnamed Bao at the University of Missouri, developed a fever and cough on a May 9 flight from Beijing to Chengdu city and checked himself into a hospital because he felt unwell, the country’s health ministry said on its Web site. He flew to Beijing from St. Louis, Missouri, via Tokyo, the ministry said.

China Quarantine

The man’s father and girlfriend have been quarantined along with other people who have been in contact with him since his arrival in Chengdu, China’s southwest Sichuan province said. Taiwan issued a yellow alert on travel to China today.

Swine flu has sickened at least 4,379 people in 29 countries, including 49 who died, according to figures from the World Health Organization yesterday. The numbers exclude the Washington and China cases.

As swine flu spreads, its symptoms have been less severe than suggested by the first fatalities in Mexico, where most of the deaths occurred.

How the virus behaves as the Southern Hemisphere flu season begins, and whether it comes roaring back in the U.S. in a nastier form later, depends in part on whether its traits hold steady, mutate, or mingle with the deadly H5N1 bird flu circulating in Asia, Schuchat said.

Costa Rican Case

A 53-year-old Costa Rican with pneumonia died from complications associated with the flu on May 9, according to that country’s Ministry of Health, becoming the first fatality outside the U.S., Mexico and Canada.

The U.S. had 2,532 cases, including three deaths, in 44 states as of 11 a.m. on May 10, according to the CDC.

“It’s too soon to say things are getting better,” Schuchat said. “We’re still accelerating.”

Mexico confirmed 1,626 cases with 48 deaths, Health Ministry spokesman Carlos Olmos said. Stringent measures beginning with the national closing of schools saved more than 8,000 lives, Mexican officials said.

The U.K. has 55 confirmed cases and is testing another 384 people, the Health Protection Agency said. Spain has 93 confirmed flu infections, the most outside North America.

Sweden reported its second confirmed case, a man in his mid-20s who contracted the virus after a trip to New York.

Norway’s Institute of Public Health said on May 9 that two people aged in their 20s who returned home recently from Mexico were infected.

Japan, Australia

Japan confirmed a fourth case and Australia’s Chief Medical Officer, Jim Bishop, told Nine Network television that more infections are inevitable in that country. Both nations confirmed their first cases on May 9. Hong Kong said it was holding two travelers for observation, after it released 351 people from a week-long quarantine.

Singapore today lowered its outbreak alert level to yellow from orange, saying the virus “seems milder than originally feared.”

The WHO hasn’t seen sustained, person-to-person spread of the disease outside North America, so the agency’s pandemic alert will remain at phase 5, the second-highest level, Sylvie Briand, acting director of the WHO’s global influenza program, said on May 8. The alert won’t be raised unless there’s evidence of community spread in another part of the world, she said.

Swine flu investigators in Mexico are trying to understand why some young adults died rapidly from influenza infections that usually kill people who are very young or very old, Briand said during a conference call with reporters from Geneva.

Young Adults

While most of Mexico’s confirmed deaths occurred in people with other health conditions that made them vulnerable, swine flu also killed a “limited number” of young, previously healthy adults and needs more investigation, she said.

The Spanish flu of 1918 that killed an estimated 50 million people in the world’s deadliest recorded pandemic also hit healthy, young adults with serious consequences, experts have said. Scientists are reviewing records of the young, fit patients in Mexico to see whether their care or other health conditions might have factored into the deaths, Briand said.

Twenty-six of Mexico’s swine flu deaths were among people aged 20 to 39, Mexican health authorities said.

About 11 percent of those who died in that country had heart disease, and 24 percent had diabetes or were obese, conditions that might predispose patients to complications, according to the Ministry of Health. The ministry didn’t give the ages of the people with the pre-existing health conditions.

Cytokine Storm

Scientists have proposed that some deaths from flu are brought on in part by an extreme immune response, often called a “cytokine storm,” that occurs when the immune systems of young, robust individuals overreact to infections. About 56 percent of people who died from flu in Mexico showed signs of a “hyper-immune reaction” according to the health ministry, which didn’t give details.

The virus affects youth more than seasonal influenza, and younger patients are entering hospitals, said Richard Besser, the CDC’s acting director. Few with swine flu are older than 60, and the median age is 14. It’s possible that older people have greater immunity or that younger people spread the disease on spring break vacation trips to Mexico, he said.

The spread of the disease and its severity, particularly in healthy people, will play a role in determining the need for a vaccine, WHO officials have said. A WHO panel will meet May 14 to decide whether drugmakers should begin producing hundreds of millions of doses of a vaccine against the new illness.

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 won’t slow the spread of the flu.

To contact the reporters on this story: John Lauerman in Boston at jlauerman@bloomberg.net; Marilyn Chase in San Francisco at Mchase6@bloomberg.net.

Last Updated: May 11, 2009 04:57 EDT