The duo from Hai Phong were infected by the South African variant, B.1.351, which was hitherto not found in Vietnam, the northern port city’s Department of Health said after doing a genetic sequencing of the virus.

It had received samples from Australian authorities.

The department said two more passengers on the same flight to Australia as the two also have Covid-19.

The 42-year-old woman and her 18-year-old son had tested negative in Hanoi on February 26. Two days later they boarded Singapore Airlines flight SQ191 to Singapore. On March 1 they flew by SQ211 to Australia.

"Therefore, it is highly possible that the woman and her son had got the virus on the flight from Singapore and not from Vietnam," the department added.

As soon as they received the news from Australia on March 10 Hai Phong authorities locked down the duo’s house on Nguyen Cong Tru Street in Le Chan District, and medical authorities have been contract tracing since then.

So far 16 people who had direct contact with them and 124 who had indirect contact, and 45 others who visited or passed by the area have been tracked down. All of the 16 people and the group of 45 have tested negative.

The city is keeping track of all information coming out of Australia and its CDC is prepared for possible community spread.

Vietnam has had 899 cases since local transmission reemerged on January 28.

But of the 13 affected localities, 10 cities and provinces, including HCMC and Hanoi, have gone almost a month without any cases.