The biggest concern was the bird flu virus latching on to the human influenza virus, a WHO spokesman in Manila said on Monday, who warned of dire consequences if this occurred.
"There is no evidence at the moment that this is the H5N1 virus, which is the avian virus," Peter Cordingley, a spokesman for the WHO's Western Pacific headquarters in Manila said.
Cordingley said of the 14 people who fell ill with influenza in Hanoi and surrounding provinces, 11 of the 13 children and the mother of one of the children had died.
The H5N1 virus spreads rapidly among poultry, but rarely infects humans. It did so in Hong Kong in 1997 when an outbreak of bird flu killed six people and again last year when it infected a father and son.
"The point of concern is not so much that those tests will show that those kids have got this virus. We're worried that the virus will latch on to a normal human influenza virus, which is extremely contagious, and then we'll have a big problem."
Results of tests on some of the Vietnamese patients should be released by a laboratory in Hong Kong later on Tuesday, but will first be sent to officials in Vietnam, Cordingley said.
"We're worried that the virus (H5N1) will latch on to a normal human influenza virus, which is extremely contagious, and then we'll have a big problem."
Whatever the test results showed, they would not reveal whether all of the cases were caused by the same virus as samples from all the patients had not been sent to the laboratory, he said.
Cordingley stressed there was "no evidence of human-to-human transmission" among the Vietnam flu victims.
"While there are people falling sick inside families, it's more likely that the transmission - if it comes from anywhere - comes from the chickens," he said.
The WHO sent a virologist to Hanoi on Tuesday to help investigate the bird flu and human influenza cases in Vietnam.