CDC officials say they "have the public's back" on this and are already working on a possible vaccine for China's new bird flu variant, though so far only people exposed to live poultry in China have gotten it.

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Federal health officials are already working on seed strains of China's new bird flu so they can make vaccines quickly should they be necessary.

The move is in response to a new strain of bird flu that has emerged in China in the last two months. It has already killed six of the 16 people who've gotten it, a 37% mortality rate that keeps public health officials up at night. All the known victims got sick between Feb. 19 and March 31, said Thomas Frieden, director of the Centers for Disease Control and Prevention. The strain, named H7N9, appears to be transmitted from poultry.

This week,Chinese officials began slaughtering more than 20,000 birds in a Shanghai poultry market to help stop the spread of the strain, which has been found in four provinces along China's eastern seaboard. The United States has not issued any travel advisories for China, but for a decade has advised Americans traveling to China to avoid contact with birds and other animals.

The CDC's on this, said Frieden. "We work to have the public's back, it's our job to be concerned and to move quickly whenever there's a potential problem."

But though public health officials are on alert, the public doesn't need to worry, he said. "There's no evidence that the virus is being transmitted between people or that it's present in the United States."

Public health officials are engaged in highly aggressive surveillance, said William Schaffner, an influenza expert who chairs Vanderbilt University's department of preventive medicine in Nashville, Tenn. Any out-of-the ordinary pneumonia cases in the United States are being looked at carefully and specimens sent to ultra-fast specialty labs, with the results communicated rapidly to federal health officials, Schaffner said.

The good news is that compared with the severe acute respiratory syndrome (SARS) outbreak in 2003, China has made huge changes to how it responds to this kind of outbreak, Schaffner said.

"Ten years ago, China was very secretive and the world's health community was frustrated," he said. "This time around, China is being very open and aggressive in dealing with the outbreak."

The H7N9 bird flu in China is of concern because it's a new strain that hasn't been seen in humans before, said Joseph Bresee, chief of CDC's epidemiology and prevention branch, influenza division. However, so far it seems the only way to get it is to be in close contact with sick poultry.

Chinese health officials quickly developed a test for exposure to the virus and then took blood samples from more than 100 people who'd had close physical contact with the first three people who got this flu strain. None of them had any evidence of immunological response, meaning their bodies hadn't been exposed to it.

"With influenza you'd expect at least 20% to 30% of family members to develop illness," said Frieden. "So that's good news."

Bird flu strains regularly emerge in southeast Asia, where there are large poultry flocks. Every few years, one appears that humans can get, but usually only from coming into contact with sick birds. Health officials worry when a strain emerges that can easily pass from human to human.

That hasn't happened in this case, health officials emphasize.

Or at least it hasn't happened yet, Schaffner said. Flu viruses are notoriously changeable; virologists talk about their "constant genetic turbulence." That's why "we have to watch this outbreak like a hawk, because if it does mutate we could have a worldwide flu pandemic on our hands," Schaffner said.

To be on the safe side, health officials and vaccine manufactures are creating the strains now, said Frieden. "It would only be produced if there was evidence of widespread human to human transmission," he said.

The U.S. effort was "started on that within a matter of days" of first word of the new variant, Schaffner said.

While the news out of China might sound scary, the global response is immensely reassuring, Schaffner said.

"The average person should say 'Wow. This is an international, integrated public health prevention structure that's really working!' This is exactly how the international community ought to work," he said.

By USA Today