Closing schools at the first sign of a new pandemic might delay the worst so health officials can prepare, but cannot prevent the spread of the disease, British researchers said on Monday.

And while closing schools might spread out demands on hospitals, it could disrupt healthcare services and the rest of the economy in other ways, Neil Ferguson of Imperial College London and colleagues said.

Writing in the journal Lancet Infectious Diseases, they said governments need to come up with plans for when and how to close schools if the pandemic of H1N1 swine flu worsens.

"The H1N1 pandemic could become more severe, and so the current cautious approach of not necessarily recommending school closure in Europe and North America might need reappraisal in the autumn," they wrote.

At the peak of the epidemic in the United States, more than 700 schools closed, according to the U.S. Department of Education. Officials at the U.S. Centers for Disease Control and Prevention estimate that more than a million Americans have been infected with the new H1N1 virus.

The World Health Organization said last week the flu was too widespread to make counting individual cases possible and called the virus unstoppable.

Ferguson and colleagues studied past pandemics of influenza in 1918, 1957 and 1968, as well as patterns of disease spread during French school holidays and a teacher's strike in Israel.

Infections fell when schools closed but rates rose immediately again when children returned to school, they noted.

French school holidays appeared to prevent about one in six seasonal influenza cases, they said.

A teacher's strike that closed elementary schools in Israel during the 1999-2000 flu season reduced doctor visits by 22 percent with a 43 percent drop in the number of respiratory infections diagnosed, including flu.

But healthcare workers are also parents, Ferguson and colleagues noted.

"A concern is that, for many countries, school closure might be particularly disruptive for health-care systems. This is because women often represent an important proportion of this workforce," they wrote.

In Britain, they said, 78 percent of doctors and nurses are women, 50 percent have children under the age of 16 and 21 percent told the Department of Health they would likely be absent from work if schools closed during a pandemic.