Federal health officials have begun to make a seed vaccine against the mysterious new H7N9 flu circulating in China, they said Thursday.
While it is being made "only as a precaution," a spokesman for the Centers for Disease Control and Prevention emphasized, an agency virologist said the agency was "fairly worried" about the new virus.
China reported that it had confirmed 14 cases of the new flu, and on Friday it reported that six cases were fatal.
No cases of human-to-human transmission have been confirmed, even though China's disease control agency has traced hundreds of people who had contact with the 14 known cases, the news agency Xinhua reported.
The flu, previously found only in wild birds, was isolated for the first time in domesticated birds on Thursday; after it was found in pigeons in a Shanghai live bird market, the authorities began culling every bird there.
It will take at least a month to create the seed vaccine, even though the agency is speeding the process by building it from synthetic DNA rather than waiting for a virus sample to arrive from China, said Michael Shaw, associate laboratory director for the C.D.C.'s influenza division.
Because China has posted the genetic sequences of the virus on public databanks, it is possible to build the genes for the virus's outer spikes in a laboratory and attach them to a viral "backbone" that has already been proven to grow well in labs and in the sterile chicken eggs in which flu vaccines are made.
Then the seed vaccine must be tested in ferrets. They will be vaccinated and given some time to grow antibodies, then a solution of the H7N9 flu will be squirted into their noses. Doctors will then have to wait a few days to see if they get sick.
"If everything works smoothly the first time, we could theoretically have it ready to send to manufacturers within four weeks," Dr. Shaw said. "But some things, like ferrets, you can't speed up."
By that time, said Tom Skinner, a spokesman for the agency, it would have a clearer idea how dangerous the new flu is.
It is still not clear how lethal it is, Dr. Shaw said, "because we may be seeing only the serious cases, the ones who go to hospitals." How many mild cases went undetected will be known only by testing hundreds of blood samples for antibodies.
It is also not known how people are getting infected, he said. The few known cases are spread out in a wide area around Shanghai. Exposure to infected poultry is an obvious suspicion. That is the risk factor for H5N1, another bird flu that has killed 371 people since 2003 but recently has been found only in Cambodia, China and Egypt. With the new flu, only two of the 14 known cases are poultry workers; a third was a cook.
Finding it in pigeons is novel, Dr. Shaw said. "It's clearly not making the pigeons ill, since no one's seen large numbers of pigeons dying. Pigeons usually aren't tested. And this could make control harder. Chickens are easy to round up."
According to the World Health Organization, preliminary tests in China suggest the new virus is susceptible to the antiviral drugs Tamiflu and Relenza.
Several countries have acted against the flu. Vietnam banned imports of Chinese poultry. Japan posted airport notices urging people to report flu symptoms. Hong Kong banned the import of live birds from the mainland.
Donald G. McNeil Jr. reported from New York, and Andrew Jacobs from Beijing.
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