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New bird flu kills more than a third -- but less deadly than earlier virus

The new strain of bird flu, H7N9, has killed more than a third of people hospitalized with the virus, Chinese researchers reported Monday.

There have been 132 laboratory-confirmed cases of H7N9 since late March, with 123 patients hospitalized and 37 deaths. While the new virus has a 36 percent fatality rate among people sick enough to land in the hospital, it is less lethal than the previous bird flu strain that burst on the scene in 2003. That strain, H5N1, killed more than 70 percent of those who were hospitalized, according to research published in the Lancet.

Another difference between the two strains of bird flu : The new one appears to hit the elderly harder. The median age of those struck by H5N1 was 26, compared to 62 for those infected by H7N9, the researchers found.

“An estimated case fatality risk of 36 percent for all ages on admission to hospital among H7N9 cases to date is still very high, even when compared to H5N1 (70 percent) numbers,” Darlene Foote, a spokesperson for the Centers for Disease Control and Prevention told NBC News.

The H7N9 strain does not seem to spread easily from person-to- person, the researchers said, noting when China closed many of its live animal markets, the outbreak petered out. However, the World Health Organization warns the virus could mutate to pass between humans.

At this point it’s unlikely that the virus will reach the United States unless it mutates. The only way Americans are likely catch the virus is if they travel to China and come in contact with infected fowl.

“No infections of wild birds, domestic poultry, or humans with this avian influenza A(H7N9) virus have been identified in the U.S.,” Foote said. “This H7N9 virus has only been identified in birds and people in China to-date (including one exported case to Taiwan).”

Meanwhile, the outbreak of H7N9 has slowed in China. “The warm season has now begun in China, and only one new laboratory-confirmed case in human beings has been identified since May 8, 2013,” one of the study authors wrote.

“The good news is that numbers of (H7N9) cases have stalled," Cecile Viboud and Lone Simonsen, two National Institutes of Health scientists concluded in an accompanying commentary.

It’s unclear how many people were infected with the new virus and weren’t sickened enough to be hospitalized. As a result, “a large portion of the ‘clinical iceberg’ might have remained undetected so far,” the Chinese researchers noted.

That doesn’t mean that the threat is over. When winter returns, the virus may start reappearing during traditional flu season. Viboud and Simonsen conclude, “continued monitoring of infections … remains crucial.”