In Ho Chi Minh City, Vietnam's largest metropolis with a population of nine million, at least 85 people have tested positive as part of a cluster at a Protestant church, the health ministry said.
It is possible the church is Revival Ekklesia Mission in Ho Chi Minh City, although it is not clear.
Worshippers reportedly sang and chanted while sitting close together without wearing proper masks or taking other precautions.
Vietnam has since ordered a nationwide ban on all religious events.
In major cities, authorities have banned large gatherings, closed public parks and non-essential business including in-person restaurants, bars, clubs and spas.
Scientists in Vietnam have discovered a new coronavirus variant that is a hybrid of strains first found in India and the UK, the country’s health minister has said.
Nguyen Thanh Long said scientists examined the genetic make-up of the virus that had infected some recent patients, and found the new version of the virus.
Viruses often develop small genetic changes as they reproduce, and new variants of the coronavirus have been seen almost since it was first detected in China in late 2019.
Mr Long said the new variant could be responsible for a recent surge in Vietnam, which has spread to 30 of the country’s 63 municipalities and provinces.
Vietnam was initially a standout success in battling the virus - in early May, it had recorded just over 3,100 confirmed cases and 35 deaths since the start of the pandemic.
But in the last few weeks, Vietnam has confirmed more than 3,500 new cases and 12 deaths, increasing the country’s total death toll to 47.