The death toll from the influenza pandemic of 2009 is not expected to approach the 50 million killed worldwide by Spanish flu in 1918-19. Public health officials and scientists have nonetheless drawn valuable lessons from the outbreak 90 years ago, which are helping to inform the strategy for containing the present one.

Mass school closures are being actively considered by the Government should the number of cases surge in the autumn as predicted, in part because of the effect they had in 1918-19. Professor Neil Ferguson, of the MRC Centre for Outbreak Analysis and Modelling at Imperial College London, told The Times Cheltenham Science Festival last week that school closures could be a powerful tool. The benefits needed to be balanced against the social and economic costs, though. Working parents would have to make alternative childcare arrangements for weeks, adding to mass absenteeism because of illness.

The critical evidence comes from a study led by Marc Lipsitch, of the Harvard School of Public Health, comparing the course of the Spanish flu pandemic in American cities that adopted different policies. In St Louis, where schools were rapidly closed when the virus arrived, the death rate peaked at 31 per 100,000 people and infections were spread over several months.

In Philadelphia, the authorities waited three weeks before taking similar measures. The city had a peak death-rate of 257 per 100,000, and the overall rate was double that of St Louis.

The rationale for closing schools is not confined to the past. More recent mathematical models have also suggested that the strategy could play a big part in reducing deaths in a pandemic, and in spreading out infections over a longer period so that fewer people become sick at the same time.

A 2008 study led by Simon Cauchemez and Professor Ferguson used data on how the French school holidays influence the transmission of seasonal flu to create a computer model of the effect of school closures on a pandemic. It showed that the measure would significantly slow the spread of the virus, reducing the numbers falling ill at the pandemic’s peak by 39 to 45 per cent. A school-closure policy would also prevent about one in seven pandemic flu cases, and about one in five cases among children. The policy’s ability to prevent child cases could prove particularly important, as children make up a disproportionate number of those infected so far.

Margaret Chan, the World Health Organisation’s Director-General, said yesterday that the majority of cases reported so far have been in people under the age of 25, mostly children.

It is thought that older adults may have some residual immunity to the H1N1 strain, because they were alive when related viruses were circulating.

More than 20 schools in Britain have already been closed in the early phase of the swine flu outbreak to slow the spread of the disease. The approach was designed to buy time until summer, when flu viruses are not generally transmitted as easily.

Public health officials will closely monitor the spread of the pandemic in countries such as Australia, Chile and Argentina over the southern hemisphere’s winter to judge whether school closures are likely to be effective. Scientists will also use the southern flu season to identify the population groups at highest risk, who will be given priority access to vaccines when supplies become available.

Though the UK has ordered 60 million doses of pandemic flu vaccine — enough for the entire population — not all will be delivered at once. Production is not expected to begin until next month, after manufacture of seasonal flu vaccine for the northern hemisphere winter has been completed.The first batches will reach Britain in October at the earliest, and will be given to frontline NHS staff and other essential workers. Vaccination will then be targeted at vulnerable groups, such as children.

The Government has stockpiled 50 million courses of the antiviral drugs Tamiflu and Relenza to treat people with pandemic flu. Drugs will be delivered directly to patients, or picked up by a friend or relative.

The decision to raise the pandemic threat level to the maximum of six will not have an immediate impact on containment policies in the UK, the Department of Health said. If the outbreak worsens in the autumn as predicted, several measures set out in the Government’s pandemic plan could be activated, depending on its severity.

Sir Liam Donaldson, the Chief Medical Officer for England, said that officials were still operating a containment approach in Britain. This involves isolating people with flu, giving them anti-virals to lessen the symptoms and shorten the length of the illness as well as tracing people with whom they have come into contact and giving them drugs in an attempt to stop the disease developing.

Sir Liam said: “At the moment if we continue to get a steady number of cases we can continue this for quite a bit longer.” He added that a surge in the number of cases could probably be expected in the autumn, with more severe illness and deaths if the virus significantly mutated, though this was “still the least likely scenario”.

Under the flu contingency plans, the Government can restrict movement and public gathering, such as at concerts. If there was a significant rise in the number of cases ministers could also stop giving anti-virals preventatively and distribute them to people who develop symptoms of illness.

However, the move to a pandemic phase may make it slightly more difficult to get a supply of vaccines. The Government has a pre-pandemic contract for vaccines, but the move to highest level of alert triggers deals other countries had with suppliers.

Professor Ferguson’s computer models suggest that between 25 per cent and 35 per cent of the population is likely to fall ill with the virus within a period of 12 to 16 weeks, placing severe strain on the NHS.

Energy and telecommunications infrastructure, fuel supplies and cash and bank clearing services are not expected to be seriously affected, though non-essential repairs may be postponed and there may be temporary, short-lived local shortages if there is a high degree of illness and absenteeism.

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