Those in quarantine are members of 71 families in Quang Hoa Commune, Dak Glong District, said Ha Van Hung, deputy director of Dak Nong Health Department. Quang Hoa authorities have also formed two teams to quarantine families across the outbreak area, he added.

"A total 550 people, including those in the quarantined zone and those who have made contact with suspected diphtheria patients, would continue to receive diphtheria vaccines over the next four days to ensure the outbreak is extinguished," said Hung.

A 9-year-old girl in Quang Hoa was taken to Dak Nong General Hospital with symptoms including coughing, sore throat and breathing difficulty on June 19. She died a day later from heart complications induced by diphtheria.

Another 9-year-old boy, a neighbor of the girl, has also been taken to hospital with the same symptoms. He's currently in a critical condition and his prognosis is bad, doctors said.

Four others, aged 9 to 15, in Krong No District were also diagnosed with diphtheria earlier this month. They have thrice tested negative for the bacteria since.

The aforementioned cases were victims of Dak Nong’s first two diphtheria outbreaks as of this moment, according to Hung.

Last year, a 7-year-old girl in the neighboring Dak Lak Province also succumbed to the disease. In 2018 and 2017, six others in the nearby Kon Tum and Quang Nam Provinces died of diphtheria.

Diphtheria is an infection caused by the bacterium Corynebacterium diphtheria. The disease spreads through either the air or direct contact. Symptoms include fever, a sore throat, swallowing difficulty and development of white patches in the throat, which could block the airway. Complications include myocarditis, kidney problems, respiratory failure, inflammation of nerves, coma and death.

Diphtheria vaccination is part of Vietnam's national immunization program.