By Stephanie Nebehay

GENEVA (Reuters) - Closing schools at the start of an H1N1 flu outbreak can greatly slow its spread and buy time to build up drug stocks, the World Health Organization said on Friday.

Classrooms have played a role in the fast transmission of swine flu in New York and other locations. As educational institutions welcome back students across the northern hemisphere, many are considering how to reduce infection risks.

Certain steps can slow the spread of H1N1, the United Nations agency said. The greatest benefits come "when schools are closed very early in an outbreak, ideally before 1 percent of the population falls ill," it said.

"Under ideal conditions, school closure can reduce the demand for health care by an estimated 30-50 percent at the peak of the pandemic."

It added: "However, if schools close too late in the course of a community-wide outbreak, the resulting reduction in transmission is likely to be very limited."

Reducing the number of people needing medical care is especially important because clinics risk being overrun.

The H1N1 virus has killed at least 3,205 people worldwide since emerging last April in North America and is the predominant flu virus circulating in both hemispheres, according to the WHO's latest weekly update issued separately on Friday.

Bolivia, Ecuador, Venezuela and other South American states are reporting higher levels of respiratory disease. The flu is on the rise in India, Bangladesh, Cambodia and eastern Europe.

H1N1 usually causes mild symptoms, but pregnant women and people with conditions such as asthma are at higher risk.

School closures can provide extra time for authorities to build up supplies of antiviral drugs such as Roche's Tamiflu or vaccines being developed by companies like Novartis, GlaxoSmithKline and Sanofi-Aventis.

Students, teachers and other staff should stay at home if they are feeling sick, and schools should set aside space to isolate anyone who takes ill on the premises, it added.

School closures carry a hidden economic cost, as parents stay at home to take care of children. Studies estimate closures can lead to the absence of 16 percent of the workforce, in addition to normal levels of absenteeism and absenteeism due to illness, the WHO said.

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