If the H5N1 virus now circulating among birds in Asia evolved the ability to pass easily from person to person, health authorities would have three weeks to contain it with drugs before it became a global threat, according to two sophisticated computer models.

Should this small window of opportunity be missed, the result could be a pandemic that infects half of the world’s population, killing even more than the estimated 20 to 50 million who died in the “Spanish flu” of 1918-19. Countries in other parts of the world, including Britain, would be powerless to protect themselves, though judicious use of drugs might reduce the death toll.

The findings, from two independent international research teams, show the urgency of building up a “mobile stockpile” of three million courses of Tamiflu, the antiviral drug, that could be deployed anywhere in the world within three days.

The World Health Organisation (WHO) has 120,000 in stock. The models suggest that swift preventive use of the drug to treat people who come into contact with sick patients could contain an outbreak. They also point to the importance of disease surveillance in the Asian nations where an outbreak is most likely. If the first cases were missed, containment efforts could become futile.

“Control of a human outbreak is potentially possible but only when it is in its early stages,” Neil Ferguson, Professor of Mathematical Biology at Imperial College, London, who led the first study, said. “Once it is beyond this, once it reaches the UK, there is no chance of stopping it, only mitigating its impact. We need to act quickly, detect cases quickly and treat people quickly. The challenges are great, but the potential benefits are saving millions of lives.

“This is our only option for making a big difference to a pandemic outcome. If we let it spread we will at best be able to prevent the deaths of perhaps half those infected in the UK.”

Ira Longini, Professor of Biostatistics at Emory University in Atlanta and leader of the second team, said: “If detection and containment starts within three weeks or so, we have a good chance. Once the response takes more than a month there is a very poor chance of containment.”

Both teams said that the WHO should keep Tamiflu in reserve. It is negotiating over a large stockpile with Roche, the manufacturer, which is understood to be considering donating the drugs. While there is an experimental vaccine against H5N1, a pandemic strain is likely to be genetically different from the present one, making it less effective. Tamiflu is considered the best option, as it can be used to prevent infection.

H5N1 flu has caused 109 confirmed human infections and 55 deaths in Thailand, Vietnam, China and Indonesia, and the disease has also been found in birds in Russia and Kazakhstan. While the existing strain is virulent in human beings, it is not readily passed between people — a key step in the development of a pandemic.

Professor Ferguson’s study, published in the journal Nature, examined the likely impact on Thailand of an outbreak of H5N1 flu that can be transmitted in this way. Data about the population, such as number of households, schools and businesses and travel patterns, were used to make predictions as accurate as possible.

In the model, one person in a rural village was infected with the mutant strain and began to pass it on. When nothing was done to contain this outbreak, it spread rapidly throughout the country, reaching Bangkok, the capital, within two months and then spreading abroad.

A year after the first case, about 50 per cent of Thailand’s 85 million inhabitants had been infected. The model did not estimate deaths as it is impossible to predict how lethal a transmittable strain will be, but even if it is ten times less deadly than the present virus, it would kill millions of Thais.

Professor Ferguson then considered what would happen if Tamiflu were given rapidly to everybody within a 5km (3.1m) or 10km radius of an infected person, and measures were taken to reduce contact by closing schools and workplaces.

These approaches will contain an outbreak, but only if Tamiflu is given swiftly, preferably within 48 hours of a case being diagnosed. Prevention must begin before more than 30 to 40 people are infected, and 90 per cent must take the drugs they are given.

Professor Longini’s study, published in Science, analysed a smaller population of about 500,000 people, but reached similar conclusions. Antivirals contained the outbreak, so long as comprehensive action was taken within 21 days of the first case and 80 per cent of the potentially vulnerable population took the drugs. Both models suggest that antivirals will be effective only if the number of people infected by each sick patient is no higher than 1.8.

THE PLANET IN PERIL

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