On February 26 the 42-year-old woman and her 18-year-old son had tested negative in Hanoi, and two days later boarded a plane to Australia through Singapore.

They landed on March 1, and the boy and woman were diagnosed with Covid-19 on March 3 and 6, the Hanoi Center for Disease Control, which was informed by Australian authorities, said.

The source of their infection is not known yet.

Hai Phong authorities on Wednesday night locked down their house on Nguyen Cong Tru Street in Le Chan District, and medical authorities have begun tracing people who had come into contact with the duo.

So far 49 who had close contact with them have tested negative for the virus.

Hai Phong is one of the 13 localities where cases have been found in the latest wave that began in late January after a two-month hiatus.

Like many of the other 13, the outbreak has been quelled in the northern port city and many non-essential businesses have been allowed to reopen.