The bird flu strain known as avian influenza H7N9 has killed six people in China so far and infected 14. The U.S. Centers for Disease Control and Prevention (CDC) announced Thursday it's working on a vaccine. The agency is keeping a close eye on bird flu developments and is studying the strain's genetic sequence information.

The new strain of bird flu causes fatal respiratory illness and is not transmitted between humans, according to Chinese health officials. Vaccine production in the U.S. won't start until it's known if the virus is transmitted person to person.

"Right now there is no evidence to suggest that is the case," CDC spokesman Tom Skinner said.

The World Health Organization said it's too soon to consider the bird flu cases a pandemic and has confirmed that there is no evidence of person- to-person transmission of the virus. The bird flu H7N9 virus was found in pigeons sold at a Shanghai market and resulted in the slaughter of 20,000 poultry Thursday.

Manufacturers will make the vaccine using a version of the bird flu virus that is genetically modified by the CDC called a "seed" virus. The seed virus will be tested on ferrets first before it's cleared to produce a vaccine. The overall production of the vaccine may take several months. As Fox News reported, production of the vaccine to combat the new strain could take five to six months to begin.

The CDC will also test the new strain of bird flu virus to see if Tamiflu and other drugs used to treat the flu have any effect on it. This is part of routine procedure that happens when a new virus is detected.

Using a process that's been around for 60 years, most vaccines in the U.S. are grown using fertilized chicken eggs.