By The Associated Press

BEIJING -- Shanghai has reported two more cases of human infection of a new strain of bird flu, raising the number of cases in eastern China to 20. The death toll among those who contracted the virus remains at six.

Health officials believe people are contracting the H7N9 virus through direct contact with infected fowl and say there's no evidence the virus is spreading easily between people.

China's official Xinhua News Agency reported the two new Shanghai cases Sunday, citing local authorities.

Shanghai has been ordered by the agriculture ministry to halt its live poultry trade and slaughter all fowl in markets where the virus has been found.

The capital cities of the neighboring provinces of Zhejiang and Jiangsu also have suspended sales of live poultry. Both provinces have reported H7N9 cases.

On Friday, Dr. Tom Frieden, the director of the Centers for Disease Control and Prevention, said that the CDC has been in close contact with Chinese official and cautioned that there is no cause for widespread alarm.

“At this point, there are several things that give us confidence that this is not spreading widely from person to person,” Frieden said.

For example, Chinese authorities have tracked 100 close contacts of people who got sick, and none of them became ill. With typical influenza, perhaps 20 percent to 30 percent of family members could be expected to develop the flu, Frieden said.

CDC is working with vaccine manufacturers to develop a seed strain to produce a vaccine to protect against the H7N9 virus, but that would only occur if there appeared to be widespread transmission. If that were necessary, it would not disrupt production of the seasonal vaccine, CDC officials said.

There have been no cases reported in the United States.

NBC News' senior health reporter JoNel Aleccia contributed to this report.

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