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HANOI, Vietnam - Thought bird flu was gone? Recent human deaths in Asia and Egypt are a reminder that the H5N1 virus is still alive and dangerous, and Vietnam is grappling with a new strain that has outsmarted vaccines used to protect poultry flocks.

Ten people have died in Cambodia, Indonesia, Egypt, China and Vietnam since December during the prime-time flu season when the virus typically flares in poultry.

"We are worried, and we will be very cautious," said To Long Thanh, director of Vietnam's Center for Animal Health Diagnostics in Vietnam.

The H5N1 virus has killed 345 people worldwide since 2003, when it rampaged across large swaths of Asia decimating poultry stocks before later surfacing in parts of Africa, the Middle East and Europe. The number of poultry outbreaks has greatly diminished since then, but the virus remains entrenched in several countries and continues to surface sporadically, resulting in 20 to 30 human deaths globally in recent years.

Bird flu remains hard for people to catch, with most people sickened after being in close contact with infected poultry, but experts have long feared it could spark a pandemic if it mutates into a form that spreads easily among people.

The fresh wave of cases comes amid a controversy involving scientists who created new lab-only versions of the virus that spread more easily among animals, hoping to better understand it. After a loud uproar over whether publishing the research would put the recipe for a bioweapon into the hands of terrorists, the researchers have agreed to temporarily halt their work.

They are set to wrap up a two-day meeting on the issue Friday with international experts at the World Health Organization in Geneva.

Vietnam has long struggled to control the virus, but it has made progress — going 21 months before reporting its two most recent deaths in the past month. It has also experienced a burst of poultry outbreaks in 11 provinces nationwide over the same period. Officials have issued fresh warnings for farmers to beef up surveillance, especially since they can no longer rely on the latest poultry vaccine in the north and central aress where it is weak or useless against a new strain that has emerged in the region.

"We have to increase biosecurity," said Thanh, the animal diagnostics director.

The new strain had earlier been identified in China and was also recently found in Bangladesh and Nepal, where it likely spread via wild birds, said Jan Slingenbergh, a senior animal health officer at the U.N. Food and Agriculture Organization in Rome.

Viruses are constantly mutating and require new vaccines to protect against infection — the same reason new human seasonal flu shots are developed each year. A new vaccine is in the works, and scientists have stressed that the new strain has not changed in any way that suggests the virus is becoming more dangerous.

The U.N. agriculture agency warned of a possible resurgence in bird flu outbreaks after the new strain was identified, potentially increasing risk to humans. However, Vietnam's two recent deaths occurred in the southern Mekong Delta, where the vaccine remains effective. WHO stressed it is also normal to see a spike in cases and poultry outbreaks during this time of year.

"WHO has always said that as long as the virus is entrenched in poultry, which it is, there continues to be the risk of bird-to-human transmission," spokesman Gregory Hartl said in Geneva. "That risk means that you cannot predict exactly if the transmission will happen and if it will be regular, but there is the risk so that's why it's not surprising to see cases."

Vietnam buys most of its poultry vaccine from China, which has continued with its robust vaccination campaign of some 15 billion doses despite the emergence of the new strain. Researchers there have developed a new version that works against the strain, but it's unclear when it might be ready for distribution, said Keith Hamilton, an animal influenza expert at the Paris-based World Organization of Animal Health.

"We emphasize that vaccination is a complementary tool," he said. "It has to be used in conjunction with other control measures — biosecurity on farms, early detection is essential, so is a rapid response to contain and eliminate sources of disease."

But in Ha Nam province, on the outskirts of Vietnam's capital, Hanoi, animal health officials are desperate to contain a poultry outbreak that hit last week. They have ordered 1 million doses of vaccine, hoping it will provide at least some protection.

"I wish to have an effective vaccine against bird flu as soon as possible, so I can go on raising ducks," said farmer Nguyen Van Duong, whose entire flock was slaughtered after the virus was detected.

"I am devastated at losing my investment on the ducks, but we will do anything to work with the authorities to stop it from spreading. The last thing we want is an outbreak to happen again."