Two more Vietnamese people are thought to have died of bird flu.
If confirmed, the deaths of the sisters, aged 23 and 30, would bring the death toll from bird flu to 10, of which eight have been in Vietnam. Two boys have died in Thailand.
The sisters died on January 23rd, said Mr Hoang Thuy Long, director of the National Institute of Hygiene and Epidemiology."The two sisters are the latest victims of the bird flu virus. We have repeatedly tested their lung fluid samples and the results were all positive. I am very confident their deaths are caused by H5N1 virus," he said.
But World Health Organisation (WHO) said the patients had been tested before, and showed negative results.
WHO experts meeting in Thailand today to discuss the threat of the virus spreading further heard Thai Prime Minister Mr Thaksin Shinawatra admit "mistakes and human errors" prevented earlier reporting of the outbreak.Chief government spokesman Mr Jakrapob Penkair said provincial officials were to blame for what appeared to be a cover-up."What looks like a cover-up was a misinterpretation of procedures," Mr Jakrapob told a news conference. "The most appropriate word is screw up. Some agencies screwed up.Thirteen countries are attending the bird flu conference in Bangkok, along with representatives of the European Union, the World Health Organisation, the Food and Agriculture Organisation and the world animal health organisation.
Containing the virus was made more difficult when China declared its poultry flocks also had the bird flu. The country is still trying to fully eliminate the SARS virus.
The main response to date has been to slaughter tens of millions of birds but international organisations have launched an urgent appeal for money and expertise.
"This is a serious global threat to human health," said Mr Lee Jong-Wook of the WHO. "We must begin this hard, costly work now."
The great fear is that the H5N1 avian flu virus might mate with human influenza and unleash a pandemic among people with no immunity to it.
So far, there is no evidence of people-to-people transmission. Humans infected so far are believed to have caught the virus directly from birds.