Pham Duy Tuyen, director of the Center for Disease Control in the northern province of Hai Duong from where the woman hails, told VnExpress on Thursday that she had been tested using the real-time polymerase chain reaction method, which is widely considered the most reliable.

She works for Vietnam Poyun Electronics Co., Ltd. in Hai Duong Town and went to Japan to work on January 17.

The virus strain she has was first found in the U.K. last November, and is said to be 70 percent more transmissible and 30 percent more deadly.

On January 15 she went to Hanoi to complete pre-flight procedures and tested negative for Covid-19.

She returned home and was in close contact with the 34-year-old woman confirmed by the Health Ministry on Thursday as "patient 1553."

But it does not know the source of their infection and provincial health authorities are currently contact tracing to find it.

They have sent over 230 people who came in close contact with patient 1553 and a 31-year-old man working at Van Don International Airport in the northern Quang Ninh Province, also diagnosed with Covid, to central quarantine facilities and taken their samples for testing.

Vietnam now has recorded 1,635 cases, 167 of them active.