Pham Quang Thai, deputy head of the National Institute of Hygiene and Epidemiology, said the current Covid-19 hotspot in Hai Duong Town in the eponymous northern province was not caused by a new virus strain.

In fact, it is of the same strain responsible for the outbreak in central Da Nang City since late July, when community transmissions returned to Vietnam after more than three months, studies from the institute showed.

On August 12, a 63-year-old man from Hai Duong Town visited Hanoi for a health check. Employed as a security guard at a beef restaurant in Hai Duong, he was later diagnosed with Covid-19. His workplace subsequently became a hotspot with 11 other related cases, including staff and diners.

The hygiene institute had said on Sunday that the virus that sickened him was of the same strain as the one in Da Nang, but did not affirm any connection.

Doctor Nguyen Viet Nhung, director of the National Lung Hospital, said Tuesday it could now be confirmed the source of transmission in Hai Duong stemmed from Da Nang.

It is possible the restaurant guard had come into contact with someone returning from Da Nang before developing Covid-19. The man did not visit Da Nang himself.

This conclusion will allow the pandemic prevention task force to locate stricken areas and trace suspected cases more easily and precisely, she said.

However, Nhung warned the situation in Hai Duong remains complicated with the source yet to be identified and possibly still making community rounds.

She warned local residents to keep their guard up as much as possible.

Hai Duong Town employed a 15-day social distancing period on August 14.

Thai said what needs to be done now is to speed up work to track down all those that had direct contact with infectees at the Hai Duong hotspot and test everyone returning from Da Nang within July.

"Leaving just one Da Nang returnee unchecked could create another Covid-19 hotspot," he said.

In late July the health ministry had stated the virus responsible for resurging local transmissions in Da Nang was a new, more contagious strain of foreign origin.

For now, it has yet to identify the source for the Da Nang outbreak.

After community transmission returned to Vietnam, Da Nang has recorded 350 new cases.

Locally transmitted infections have spread to 13 other cities and provinces, including Hanoi with 11 and HCMC with eight.

Vietnam has so far recorded 983 Covid-19 cases, 478 recoveries, and 25 deaths. The rest remain under treatment.