* U.S. has not issued any travel advisories
* No bird flu has been found in the United States (Adds details from news conference, facts about bird flu)
By Julie Steenhuysen
CHICAGO, April 5 (Reuters) - The U.S. Centers for Disease Control and Prevention said on Friday it has issued an advisory to U.S. doctors that may help them identify any cases of the new bird flu virus known as H7N9, but stressed that no cases have been found in the United States.
So far, the new strain of bird flu that has infected 16 people in China and killed six has not been shown to be capable of transmission from person to person, CDC's Dr. Thomas Frieden told reporters on a teleconference.
All of the 16 confirmed cases appear to be isolated, with no links between the flu strains that would suggest they were being passed from person to person, Frieden said.
However, two of the 16 had family members who were ill, and Chinese health authorities are assessing those cases to see if there was transmission within the family.
Frieden said the CDC has not issued any formal travel advisories and that there are no steps U.S. citizens need to be taking to protect themselves, though the CDC is putting doctors on alert.
"Today, we issued a health advisory to inform clinicians and public health specialists throughout the U.S. of what is happening and what they should do if they have a suspected case in a traveler who is returning from China in terms of testing, infection control and care," he said.
Frieden said the CDC has developed a test that could be used to screen severely ill travelers returning from China for the new bird flu strain, and he said the United States is developing diagnostic test kits that it will distribute to China and other countries to ensure rapid diagnosis of this strain of flu.
Such kits could help contain the spread of the virus if it does learn to easily pass from person to person.
With flu, Frieden said there are two factors that are important: how virulent the strain is and how readily does it spread among people.
Health authorities won't know the answers to those questions until they are able to test many more people beyond the initial 16 confirmed cases.
"It could be that hundreds of other people had very mild infections and these (16) people were kind of the tip of the iceberg," Frieden said.
Or, he said, it could be a very rare event that passes from animals to people that causes severe illness in the unlucky few who become infected.
Until that becomes clear, the CDC said it will continue working with vaccine makers to develop a candidate virus that could be used to make a vaccine if one should be needed.
Because of recent advances in U.S. vaccine manufacturing capabilities, including the recent approval of Novartis' cell-based flu manufacturing plant in Holly Springs, North Carolina, Frieden said development of a bird flu vaccine should not disrupt the U.S. supply of seasonal flu vaccine.
The first cases of this new strain of bird flu, officially called avian influenza A (H7N9) viruses, were reported by the World Health Organization on April 1.
Type A flu viruses occur naturally among wild birds and can infect domestic poultry and other bird and animal species, but they rarely infect humans and have typically occurred when people have had close contact with poultry, as with the recent H5N1 bird flu outbreak.
According to the CDC, this is the first time H7N9 has infected people. Frieden said most of those who have become sick had direct contact with live poultry. Chinese health authorities are investigating the source of the infections. Frieden said they have reported positive cultures for a similar strain in both chickens and pigeons, and more testing is going on.
Because this is a non-human virus, the worry is that it could cause a flu pandemic if it learned to become easily spread from person to person.
Reporting by Julie Steenhuysen; Editing by Vicki Allen and Philip Barbara