Nguyen Chi Hanh, deputy director of the northern province's Department of Health, on Tuesday said the seven cases were workers who returned to the Chinese electronic component firm AAC Technologies at the Que Vo industrial park on June 2, following shutdown due to the coronavirus.

Before returning to work, the staff had tested negative for the coronavirus on May 30 and June 1. The firm, which has around 6,500 workers, is divided into three different workshops called A, B and C, said Hanh. Each workshop is isolated from the others, he added.

On June 3, 450 staff in workshop A were tested, revealing the seven coronavirus cases. Standard coronavirus control protocols were deployed, including disinfection, isolation and other measures. B and C workshops were tested too, with all results returning negative and workers resuming operations.

By Wednesday however, 66 cases in total were recorded at the firm, turning it into a new coronavirus hotspot.

"The seven initial coronavirus cases at the AAC firm might have incubated the virus. At the point of testing, they had no symptoms and their results returned negative," said Hanh.

To prevent such an incident from reoccurring, Bac Ninh has ordered increased Covid-19 testing, done at least twice within a week before workers return to their factories. Moreover, at least 20 percent of factory workers must be screened to ensure no case slips by and that their health remains tip-top.

So far, Bac Ninh has recorded 1,170 Covid-19 cases in Vietnam's latest coronavirus wave since late April, second only to neighboring Bac Giang with 3,586 cases.