Vietnam's latest outbreak of bird flu poses a huge threat to public health, the head of the World Health Organization (WHO) in the country says.
Vietnam has won praise for its handling of the bird flu crisis
Dr Hans Troedsson said the cooler weather and forthcoming Tet festival heightened the risk of the disease spreading to the human population.
An outbreak of the lethal H5N1 strain was found on a poultry farm in December, the first in almost a year.
New WHO chief Margaret Chan has warned that bird flu remains a global threat.
Dr Chan, a bird flu expert from Hong Kong, said reports of bird flu had started to surface in recent weeks after a lull and that the danger was particularly severe in poor countries.
"We must not let our guard down. We must maintain our vigilance," she said, after becoming the first Chinese citizen to head the UN agency.
Vietnam has been widely praised by international health experts for its success in tackling the highly pathogenic avian influenza.
The country had been free of human cases of bird flu for a year - thanks to a mass culling and vaccination drive.
Margaret Chan has vowed to work hard to combat the bird flu threat
Dr Troedsson, the WHO's Vietnam Country Director, said last month's confirmed outbreak did not mean Vietnam's current policies had failed.
But he warned that the onset of the colder winter months - when the virus survives for longer - and February's lunar new year festivities "could have a negative impact".
"Not only the movement of people will increase but the consumption of poultry, so there will be more transport and handling and slaughtering of poultry," he told the BBC's Bill Hayton in Hanoi.
Dr Troedsson said Vietnam's government should maintain its strategy of openness, good surveillance and the vaccination of hundreds of millions of birds.
But he warned that it could take several years for the bird flu virus to be completely eradicated from the country.
Margaret Chan has pledged to take a hard line on countries that do not comply with requirements to carry out checks against bird flu or hinder global efforts to develop vaccines.
Her Chinese origin, she said, would help her in any dealing with the authorities in China, the country where the lethal H5N1 strain of bird flu first emerged.
"I think of all people I would be in a better position to work with the Chinese government," she told the BBC.