If schools were closed every time a child developed a sniffle, the education system would break down. Why, then, have half a dozen shut their gates in the past week in response to a few cases of a virus that some recovering patients have described as no worse than a heavy cold?

While it is true that swine flu has generally had mild symptoms so far, the public health authorities cannot afford to treat it like seasonal influenza. That is because it is a new strain that the human immune system has not seen before.

Even if symptoms are comparable to those of seasonal flu, its novelty means the impact could be far worse because more people would be infected. A “mild pandemic” of H1N1 would be expected to cause at least three times more deaths and illness than typical seasonal viruses, which themselves kill 6,000 people in Britain in an average year.

New viruses like H1N1 are also dangerous because of the speed at which they spread. With little immunity in the population, very large numbers of people could fall ill at much the same time, overwhelming the health service’s capacity to cope.

Public health interventions aim to mitigate this by slowing the rate at which a virus spreads. While they cannot usually stop transmission completely, they can delay it, flattening out the infection curve.

This has several advantages: it buys time for the development of a vaccine, which would take at least four to six months; the NHS gets breathing space to manage the pandemic more effectively, because fewer people would become ill at once; and the total number of cases are also reduced.

Viruses spread fastest in communities where large numbers of people are in close contact with one another. Schools are especially vulnerable because children are less likely than adults to wash their hands regularly or to sneeze into disposable tissues.

Computer models and historical examples show that school closures are among the best ways of achieving these goals.

A 2008 study , led by Simon Cauchemez and Neil Ferguson, of Imperial College, London, used data on how school holidays influence seasonal flu transmission in France to simulate the effect of school closures in a pandemic. It found that they could significantly slow the spread of the virus, reducing the numbers falling ill at the pandemic’s peak by as much as 45 per cent. A school closure policy could also prevent up to one in seven cases.

Historical examples are also instructive. A 2007 study led by Marc Lipsitch, of the Harvard School of Public Health, compared the progress of the 1918-19 Spanish flu pandemic in American cities that adopted different public health policies. Cities such as St Louis, which rapidly closed schools and banned many public gatherings, experienced a relatively flat pandemic, with deaths peaking at 31 per 100,000 people. Philadelphia, which waited three weeks before imposing controls, suffered peak deaths of 257 per 100,000.

The overall death rate for Philadelphia was 719 per 100,000, more than double the rate of 347 per 100,000 for the same period in St Louis.

The 1918-19 Spanish flu was caused by a more virulent virus than today’s H1N1 strain, but its lessons are still relevant to the present situation. School closures may be highly disruptive to pupils preparing for exams, and to working parents without childcare arrangements, but they are among the most powerful instruments that exist for containing a pandemic’s impact.

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