Sir Liam Donaldson, the chief medical officer, said the death toll could be "a lot higher", depending on the virulence of the flu virus involved. Estimates that as many as 750,000 could die in a future pandemic were "not impossible," but he added: "It is more realistic that the figure will be a lot lower than that."
Speaking yesterday on the BBC's Sunday AM programme, Sir Liam said that a flu pandemic was inevitable, but it was "less likely" that an outbreak would occur this winter.
Flu kills more than 12,000 people in Britain every winter, but the number could soar if the bird flu virus which has been traced in China, Vietnam, Thailand, South Korea, Cambodia, the Philippines, Russia, Turkey and Romania mutates into a form which passes easily between humans.
"If we had a pandemic, the problem would be that our existing vaccines don't work against it, we would have to develop a new vaccine, and people don't have natural immunity because it hasn't be around before," Sir Liam said.
Tests in a British laboratory confirmed on Saturday that a virus detected in wild birds in Romania was the deadly H5N1 strain, identical to that found among chickens in Turkey last week.
H5N1, which is highly contagious among birds, is harder for humans to contract. But it has killed about 60 people in Asia, mostly poultry farmers who were infected directly by their birds.
Previous flu pandemics have occurred when bird flu combines with a human virus for which people have no natural immunity.
"We can't make this pandemic go away, because it is a natural phenomenon. It will come. But what we can do is to limit its impact," said Sir Liam. Britain was one of the few countries to start preparations at "a very early stage," he said.
In the event of a pandemic, the government's response would concentrate on developing a vaccine and deploying anti-viral drugs, he said.
The Government has ordered 14 million doses of the antiviral drug Tamiflu, enough to treat 25 per cent of the population. The figure is based on the Department of Health's prediction that bird flu would affect one in four people.
Sir Liam admitted that, so far, only 2.5 million doses have been stockpiled, with 800,000 new doses arriving every month. But an effective vaccine cannot be developed until scientists identify the strain of flu virus that reaches the UK.
The priorities for a vaccination programme would include essential workers such as National Health Service employees and high-risk groups such as the elderly and those with chronic diseases.
Authorities in Turkey said yesterday that they had contained an outbreak in a village in the west of the country, but they warned that there was still a risk that migratory birds could spread the virus elsewhere.
Experts believe the disease was brought into Turkey by wild birds migrating through the country from Russia to Africa.
The Romanian government culled thousands of domestic birds yesterday after placing the eastern Dobrogea region under quarantine.
Despite the recent bird flu outbreaks in Turkey and Romania, the epicentre of an eventual pandemic was still likely to be in the Far East, Sir Liam said.
"The attention is focused in Europe because of these outbreaks," he said. "That doesn't mean that the pandemic flu is creeping closer to the UK, it simply means that bird flu is occurring in other parts of the world."
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