Vietnamese authority updated the number of its A/H1N1 flu patients to 13 on Sunday evening, the Vietnam News Agency reported.

According to the Ho Chi Minh City Health Department, the mother and younger sister of the 11th case has been confirmed to be infected with the virus.

The family returned to Vietnam from the United States on June 6.They were now isolated and treated at the Nhi Dong No.1 Hospital.

Nguyen Huy Nga, head of the Vietnamese Health Ministry's Department of Preventive Medicine said on Sunday that some people show no obvious symptoms after being infected with the virus, posing a big difficulty to efforts to prevent the spread of the disease.

(Xinhua News Agency June 8, 2009)