Treating a patient in China. (Credit Image: Liu Junxi/Xinhua/AP Photo)

Another person has died in China of the new bird flu, H7N9, bringing the death toll to eight, according to China's Xinhua News Agency. Twenty others have been infected, all of them in eastern China.

The latest victim was an 83-year-old from Jiangsu who was admitted to a hospital with a fever March 20, according to Reuters, which attributed the information to Xinhua.

"They've already seen some changes that allow it to survive in people," ABC News chief health and medical editor Dr. Richard Besser told "Good Morning America" Monday. "The big concern is could this become the next pandemic strain?"

Close contacts of the H7N9 patients have been monitored, according to the World Health Organization's latest update.

The U.S. Centers for Disease Control and Prevention is already working on a vaccine, using the virus's genetic code rather than the virus itself, a first for the agency, according to Nancy Cox, head of the CDC's influenza division.

Although H7N9 is more easily transmittable from birds to humans than H1N1, Cox said she expects to see limited human-to-human transmission.

Since H7N9 is not as deadly to birds as it is to humans, it will be harder to track because there won't be large bird kill-offs, Cox said.

"That's very concerning because you can't tell where it is without testing the birds directly," Besser said.

A Shanghai market where the virus was detected in pigeons Friday halted live bird sales and slaughtered all of its poultry, amounting to more than 20,500 chickens, ducks, geese and pigeons, according to Xinhua.

"The key to controlling the number of H7N9 patients depends on whether the virus can spread among human beings," Wu Fan, director of the Shanghai Municipal Center for Disease Control and Prevention, said at a news conference Friday, according to Xinhua.

"So far we haven't found any cases that show this kind of virus can spread from people to people."