The one-year-old boy initially tested negative on the first day after arriving, but the test results on May 28 and 29 turned positive.
With the newest case, the number of infected passengers on the May 13 flight from Russia has increased to 34. All have been in quarantine since arrival, posing no risk of transmission to the community.
Vietnam has not recorded any community infection of COVID-19 for 44 consecutive days since 6am April 16 to 6am May 30.
A total of 7,870 people who had close contact with COVID-19 patients or returned from pandemic-hit areas are being quarantined.
Among the 328 patients recorded so far, 278 have been given the all clear, and there has been no death.