Mass immunisation against H5N1 flu could reduce the impact of a pandemic to normal seasonal levels if combined with extensive use of antiviral drugs, government scientific advisers have concluded.

Research presented to the Department of Health has suggested that inoculating the entire population with a “pre-pandemic” vaccine that does not precisely match the pandemic strain could confer sufficient immunity to cut infection dramatically. Such a strategy would have to be combined with stockpiling enough of the antiviral drug Tamiflu to treat 75 per cent of the population, the department’s Pandemic Influenza Scientific Advisory Group (SAG) said. This would allow every infected person to be treated while the drug was given preventively to those in infected households.

This would minimise death and illness in a pandemic that could affect up to half the population, but would be hugely expensive and require a massive revision of pandemic plans. The Government has ordered 14.6 million doses of Tamiflu, enough to treat 25 per cent of the population. The SAG has recommended raising this to 50 per cent, at a cost of about £150 million. Health ministers are understood to have accepted this advice but it has yet to be approved by the Treasury.

The more drastic pandemic plan would require a further 15 million doses of Tamiflu, as well as spending hundreds of millions of pounds on prepandemic vaccine. Only 3.3 million doses of pre-pandemic H5N1 vaccine have been ordered, for essential workers. The pre-pandemic vaccine that has looked most promising in early trials, which is made by GlaxoSmithKline, has not yet received regulatory approval. Also the vaccines have a shelf life of only three years.

Pre-pandemic vaccines use the strain of flu that is predicted to have the biggest risk of mutating so that it passes from person to person. The vaccines would not be a precise match to the pandemic strain but are forecast to offer some protection.

Professor Neil Ferguson, of Imperial College, London, a leading flu epidemiologist and a member of the SAG, said: “I am not yet sure what the best course of action is, but the Department of Health should certainly do the cost-benefit analysis.”

The Department of Health said: “We are continuing to keep the situation under review.”

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