May 9 (Bloomberg) -- The swine flu virus so far lacks the killer traits of the 1918 Spanish pandemic or the bird flu fatal to half those it infects, American scientists said.

The genetic blueprint of the new H1N1 virus sweeping the globe is “good news,” Anne Schuchat, interim deputy director for science and public health of the U.S. Centers for Disease Control and Prevention, said today. Swine flu, now in 29 countries, may yet exchange genetic material with other viruses and mutate into something worse, Schuchat told reporters in a conference call from Atlanta.

As swine flu spreads, its symptoms have been less severe than Mexico’s first fatalities suggested. That could change, Schuchat said. 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.

“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.”

The flu has sickened at least 3,440 in 29 countries, the World Health Organization said today on its Web site, and killed at least 49. Costa Rica said a 53-year-old man who had diabetes and lung disease died, the first fatality outside North America, the Associated Press said. Canada reported its first death yesterday.

U.S. Cases

The U.S. had nearly 3,000 cases as of today, about 20 percent more than the day before, and “the numbers are likely a very great understatement,” Schuchat said.

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

The U.S. confirmed 2,254 cases and two deaths in 45 states, and had 720 probable cases, Schuchat said. The increase came as test kits reached all states and laboratories processed backlogged samples from suspected patients, the CDC said.

Mexico, where the flu has hit hardest, confirmed 1,626 cases and 48 deaths, Health Minister Carlos Olmos told reporters today in Mexico City. Stringent measures beginning with the national closing of schools saved more than 8,000 lives, Mexico officials said yesterday.

Japan and Australia confirmed their first cases of swine flu today as Hong Kong released 351 people from a week-long quarantine.

WHO Alert

Asian health ministers called on WHO to review criteria for declaring a flu pandemic. The Association of Southeast Asian Nations said WHO should consider the severity of illness in addition to person-to-person spread, the association said in a statement distributed in Bangkok yesterday.

“We are at level 5, but there is some evidence this may be just like seasonal influenza,” Kamnuan Ungchoosak, an official at the Thai Health Ministry’s Department of Disease Control, said in an interview yesterday. “The perception is that once you raise from one level to another level, it means there is more danger, more severity, a greater threat,” he said.

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 WHO’s global influenza program, said yesterday. The alert won’t be raised unless there’s evidence of community spread in another part of the world, she said.

Young, Healthy Adults

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.

While most of Mexico’s 48 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, said Briand.

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.

“We’re still trying to understand who are the high-risk groups for this disease,” Briand said. “The more we know about this disease, the more we’ll be able to have better control measures.”

Twenty-six, or 58 percent, of Mexico’s swine flu deaths were among people ages 20 to 39, Mexican health authorities said yesterday.

Health Conditions

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 government’s Ministry of Health. The ministry didn’t give the ages of the people with the pre-existing health conditions.

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 health ministry. The ministry didn’t give details.

The virus affects youth more than seasonal influenza, and younger patients are entering hospitals, the CDC’s acting director, Richard Besser, said. 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.

Vaccine Preparation

The spread of the disease and its severity, particularly in healthy people, will play a role in determining the need for 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.

Hong Kong yesterday freed 286 people who had been quarantined in a downtown hotel, the site of the city’s first confirmed swine flu case. A further 61 were allowed to leave a holiday village and four were discharged from a local hospital.

Guests and staff at the Metropark Hotel in the Wanchai district were permitted after showing no flu-like symptoms, said York Chow, secretary for food and health.

“They sacrificed freedom for seven days to give confidence to Hong Kong people,” Chow told reporters outside the hotel. Donald Tsang, the city’s chief executive, spoke to guests and staff before they left the hotel and was seen bowing as he addressed them in the lobby.

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; Andres R. Martinez in Mexico City at amartinez28@bloomberg.net;

Last Updated: May 9, 2009 15:43 EDT