Vietnamese health authorities said nearly 30 people have been tested positive for rubella, warning that there are many more showing symptoms of the contagious disease.
The outbreak was first recorded early this month at a US-invested furniture company in an industrial park in Binh Duong Province, which is next to Ho Chi Minh City.
News website VietNamNet cited the General Department of Preventive Medicine as saying that up to 138 people at the company showed symptoms of rubella.
As of January 23, 29 of them have been tested positive for the virus.
Rubella, also known as German measles or three-day measles, is a disease caused by a virus. The infection is usually mild with fever and rash.
The virus is spread by contact with an infected person, through coughing and sneezing.
Over the past weeks health authorities in Ho Chi Minh City have performed testing, kept the infected persons in quarantine and implemented measures to contain the outbreak.
Around 23 million children aged between 1 and 14 are supposed to receive free measles and rubella vaccines in 63 cities and provinces across Vietnam under a campaign starting last September.
At last count more than 17 million of them have been vaccinated.