Dr Nguyen The Dzung, director of the city’s health department, said further tests were being carried out on the children’s blood samples to discover the nature of the toxins, how they got there, and how they caused shock.

The tests would take some more days, he said after a meeting with local medical experts and representatives of the vaccine importer.

Vietnam has temporarily banned the use of the measles, mumps, and rubella vaccine, Priorix, produced by the UK’s GlaxoSmithKline, pending the investigation outcome.

The batch of vaccines administered to the children would be sent to a World Health Organization laboratory for tests, Dzung said.

The six victims were among more than 100 children in Ho Chi Minh City’s district 5 given Priorix.

The other children were not affected.

Of the unfortunate ones, 13-month-old Ng.Thi.B died Wednesday after being admitted to the Pediatrics Hospital No 1 in medical shock and with breathing difficulty.

Another child remained on a respirator while the rest were recovering, Dzung said.

GSK awaits word

GlaxoSmithKline said Friday confirmation was awaited if it was indeed its pediatric vaccine, also known as MMR, which was administered to the children.

A company spokeswoman told Dow Jones Newswires: "Priorix has been on the market since 1997 and no similar event has ever been reported. The safety record of Priorix would lead us to believe it was not our vaccination."

MMR vaccines contain a mixture of live, but weakened viruses, and work by provoking the body's immune response without causing the diseases.

This vaccine is given in two doses as part of a childhood vaccination schedule.

There have been several media reports over a possible link between the vaccine and autism and bowel disease.

The UK’s Medical Research Council, after reviewing current evidence and research, has concluded there is no link between the MMR vaccine and these disorders.