Earlier Wednesday, officials reported that an 80-year-old man — who had two infected grandchildren — also had the virus but did not fall ill.
The two cases raised concerns that avian influenza may be more widespread than originally believed.
"It's quite possible that some people are falling sick and their symptoms are very light and they don't end up in hospital," WHO regional spokesman Peter Cordingley said earlier this week.
Nguyen Tran Hihai Binh province had contracted the H5N1 virus.
She remains in good health but has been isolated at her home, said Nguyen Van Thieu, director of the medical center of Kien Xuong District in Thai Binh.
Her 69-year-old husband had developed bird flu symptoms on Feb. 19 and died Feb. 23, he said.
Vietnam has had a total of 33 deaths from the deadly disease, 13 of them in the latest outbreak which began last December. A woman sickened in Cambodia also died this year in a Vietnamese hospital. Twelve people have died in Thailand.