Seven Vietnamese who initially tested negative for bird flu have now been found to have carried the virus, the World Health Organization has said.
The seven patients had tested negative in Vietnam's labs
All seven patients have since recovered from the disease.
A WHO scientist in Hanoi, Peter Horby, said further support for Vietnam's laboratories was needed in order to "make sure quality control is good".
Bird flu has killed 46 people in Vietnam, Thailand and Cambodia since December 2003.
Specimens from the seven patients were originally tested in Vietnamese laboratories, but were then retested in Japan, where the virus was detected.
"There's no doubt. The WHO accepts that we are missing cases. It's quite possible that some people are falling sick and their symptoms are very light and they don't end up in hospital," said WHO regional spokesman Peter Cordingley.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam
"It's also possible that they have a very light viral load, and Vietnamese tests may not be sensitive to pick it up," he added.
His colleague, Peter Horby, said the finding underlined the need for support for Vietnam's laboratories.
But he stressed that there had not been any problems with Vietnamese results before.
"It could just be a problem with one lab," he said.
The news comes a day after another Vietnamese citizen tested positive for bird flu.
The man, a 26-year-old nurse, had looked after a patient suffering from bird flu, but it was not clear whether he had contracted the disease this way, or from infected poultry.
The patient he nursed is thought to have caught bird flu from eating raw duck's blood last month, as did his 14-year-old sister. State media says the pair are recovering.
The resurgence of bird flu has renewed scientists' fears that the virus could mutate into a form which is easily spread from human to human.
The World Health Organization warned last month of "the gravest possible danger of a pandemic".
During an international conference on bird flu in Ho Chi Minh City, Vietnam promised to overhaul its poultry industry, as part of efforts to stamp out bird flu.