A health worker prepares to take swab samples from ducks at Meijiang poultry wholesale market, April 8, 2013, in Changde, China. (Credit: ChinaFotoPress/Getty Images)

Three new cases of the new bird flu strain, H7N9, have been reported in China, bringing the total number of cases to 21, according to the World Health Organization. Six of those who were infected have died, but no new deaths have been reported since Friday.

The new patients include a 59-syear-old Shanghai man who is in critical condition, and a 55-year-old Anhui man who is in stable condition. Another new patient, a 67-year-old Shanghai man, has a mild case, according to the WHO.

"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." "The big concern is could this become the next pandemic strain?"

More than 530 close contacts of the H7N9 patients have been monitored, according to the WHO.

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 the original bird flu strain, 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.

On Friday, a Shanghai market where the virus was detected in pigeons halted live bird sales and slaughtered all poultry, amounting to more than 20,500 chickens, ducks, geese and pigeons, according to China's Xinhua News Agency.

"The key to controlling the number of H7N9 patients depends on whether the virus can spread among human beings," said Wu Fan, director of the Shanghai Municipal Center for Disease Control and Prevention, at a news conference last Friday, according to Xinhua. "So far we haven't found any cases that show this kind of virus can spread from people to people."