The director of the hospital where she was treated said she probably also had avian flu.
"We suspect that it was bird flu that caused her death," said Truong Thi Mau, director of the Ba Thuoc district hospital. "But we can't be 100% sure because there was no sample for tests. At the time, we thought her illness was something else," Mau said by telephone from Ba Thuoc, about 150 km (95 miles) south of Hanoi in Thanh Hoa province.
It was reported that both girls had eaten duck and chicken raised on the family farm.
The 13-year-old developed a high fever and severe cough on December 25 and died in the district hospital on January 2, Mau said. After her 8-year-old sister was confirmed to be infected with H5N1, about 50 children with flu symptoms were brought in by nervous parents in the area, the article said, but tests for avian influenza were negative on all of them.