(Repeats to fix typo in headline)

By Ho Binh Minh

HANOI, March 29 (Reuters) - Vietnam plans to vaccinate up to 90 percent of its poultry, fearing a return of bird flu virus in the winter season after a brief lull, a senior agriculture ministry official said on Thursday.

Vietnam has had no human cases of bird flu since November 2005, but the H5N1 virus, which first arrived in late 2003 and has killed 42 people, returned to poultry in the south late last year and struck ducks in early March.

However, the arrival of warmer spring weather in the north and the dry season in the south has slowed down any further outbreak of a virus which seems to thrive best in cooler weather.

"Bird flu outbreaks have declined and will remain so between now and August because the temperature is higher," Deputy Agriculture Minister Bui Ba Bong told Reuters.

But he said the government had stepped up a poultry vaccination drive, fearing a return of the disease later in the year.

"Between August and December, we could face the risk of recurrences of the disease," Bong said in an interview, referring to the autumn and winter periods in northern Vietnam.

"Now we have started the first phase of vaccinating poultry and we will target 70 percent of the country's poultry stock to be vaccinated, while in the areas with a high infection risk, the rate is targeted at 90 percent," he said.

Experts fear that although the virus does not appear to have developed the ability to spread easily from person to person, it could do so and set of a pandemic in which millions could die.

The World Health Organisation says bird flu is known to have killed about 170 people, most of them in Indonesia and Vietnam.

Experts and government officials said poultry vaccinations had helped prevent the H5N1 virus from spreading in Vietnam.

Bong said while Vietnam had started trial production of a bird flu vaccine, it would take a long time before production could meet domestic demand.

"We have been buying the vaccines for poultry from China and Holland and imports will continue for the next two years," Bong said.

The Agriculture Ministry expects to import 500 million doses of anti-bird flu vaccines this year and next.

On Thursday, Indonesia announced two more human bird flu deaths and China said a teenage boy had died from the virus, which has now spread to more than 50 countries.

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