The 18 local community cases were recorded in Hanoi capital (15), Dong Nai province (1) and Hai Duong province (1).
Notably, 14 of the cases in Hanoi were detected at the National Hospital for Tropical Diseases, becoming a new coronavirus hotspot in Vietnam.
The hospital was cordoned off during the day for epidemiological investigation, contact tracing and testing.
Under a decision signed by Dr Dao Ngoc Thach, director of the hospital, the hospital has been closed to visitors, starting on May 5, until further notice.
The same day, the Ministry of Health also announced an additional eight imported cases that are Vietnamese citizens entering the country via air and land borders.
As of 18.00hrs on May 5 Vietnam has confirmed 3,022 coronavirus cases, including 2,560 recoveries.
More than 40,000 people who had close contact with COVID-19 patients and entered from epidemic hit countries are being placed in quarantine for medical surveillance.
The number of fatalities caused by COVID-19 and other related illnesses remains at 35.