Vietnam Health Ministry on Monday asked epidemic institutes to prepare for a new subtype of "swine flu" virus that can enter country.
The statement was made by the ministry's Preventive Health Department after the US Centers for Disease Control and Prevention, or CDC, on November 23 announced three more cases infected with the new subtype.
The virus has been named S-OtrH3N2, which is a combination of the two subtypes of influenza A virus H1N1 and H3N2, the latter found in pigs.
CDC has reported ten US people infected with the virus. The first seven people had contacted with pigs before the infection while the recent three did not.
Influenza A reached pandemic level in Vietnam in 2009.
Since the virus was first detected in Vietnam in May 2009, more than 11,000 people have been inflicted. Around 65 have been killed including 13 this year.
With CDC warning, epidemic centers including Pasteur Institutes nationwide have been asked to keep tight surveillance at epidemic quarantine at borders and people with flu symptoms coming from the US.
People were recommended to have flu vaccination, which would prevent them from influenza virus infection, or at least make them infected with less severe symptoms, news website VnExpress reported.
Nguyen Tran Hien, head of the National Institute of Hygiene and Epidemiology, said the institute's flu control centers around the country is taking samples of flu patients every day to check for the new virus subtype.
Hien said local studies have not found any new subtype of influenza A.