Scientists work on a COVID-19 vaccine in a lab in Vietnam. (Photo: Duyen Phan/Tuoi Tre)

More than 860 direct contacts and 1,400 indirect ones were tested and not a single result returned positive for COVID-19, the center affirmed.

Those people had been exposed to the patients, who were registered  between November 29 and December 1, after Vietnam had been free from community transmission for nearly three months. The city is quarantining more than 5,400 people either at home, centralized centers, or other accommodations.

The municipal Center for Disease Control advises that people don face masks in public, frequently sanitize hands, keep a distance in social interaction, avoid large crowds, and declare health conditions in order to prevent COVID-19 infection.

Local authorities have been beefing up their crackdown on anyone failing to wear face masks in public, with fines of up to 3 million VND (130 USD).

Vietnam reported 1,366 coronavirus cases as of Monday morning, with 1,220 recoveries and 35 virus-related deaths, according to the Ministry of Health.