Proper cleaning, ventilation and measures to reduce crowding are recommended, along with hand hygiene and repiratory etiquette, WHO said. (LM Otero/Associated Press)

It's only worth closing schools to slow the spread of the swine flu pandemic virus during the earliest stages of an outbreak, the World Health Organization said Friday.

"[S]chool closure has its greatest benefits when schools are closed very early in an outbreak, ideally before one per cent of the population falls ill," the UN health agency said in a statement advising its 193 member states.

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

Under the recommendation, a school with 700 students could close if seven students fell ill.

Health officials are concerned that hospitals and health facilities could be overwhelmed by people needing medical care during the peak of the global outbreak.

The United States and Europe also include school closures as an option to stem the spread of swine flu in schools early in an outbreak. So far, only WHO has set a suggested level of sickness to trigger schools to close their doors.

But last month, the Public Health Agency of Canada said there is no point closing schools unless the severity of swine flu cases increases, if there aren't enough teachers coming to work or many students are sick.

Closing schools could also mean that parents, including doctors and nurses, would have to stay home, which could also stress the health-care system.

Isolate the sick

School closures do not help reduce the number of cases but spread them out over a longer time while keeping parents, including health care workers, away from work.

Like everyone else, students, teachers and other staff should stay home if they are feeling sick, the WHO said. Schools should also designate spaces to isolate those who fall ill while at school.

During school closures, officials should also try to limit students from congregating elsewhere socially, such as malls.

"If students congregate in a setting other than a school, they will continue to spread the virus, and the benefits of school closure will be greatly reduced, if not negated," the statement said.

Also on Friday, U.S. health officials reported that U.S. trials of two vaccines against H1N1 confirmed that only one dose is needed to protect adults.

"I am very pleased today to be able to tell you that the initial results from the NIH (National Institutes of Health)-sponsored trial corroborate and reinforce the findings from the companies," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told a news conference.

The vaccines tested were made by Sanofi-Pasteur and CSL. GlaxoSmithKline is producing Canada's swine flu vaccine.

On Thursday, Australian and U.S. authorities first reported that one dose of the vaccine looks strong enough to protect adults. No results are available yet on children and teens, seniors and people with weakened immune systems.

The virus is now active in all 50 U.S. states, Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention said.

Tamiflu resistance

The H1N1 virus has killed nearly 3,000 people since emerging last April. It is now the main flu strain circulating in both hemispheres, according to the WHO. It usually causes mild symptoms, but pregnant women and those having underlying medical conditions are at higher risk.

WHO also said Friday that swine flu has claimed at least 3,205 people worldwide.

The number of reported cases resistant to the antiviral Tamiflu or oseltamivir is 21, the agency said in its weekly report.

Australia on Friday reported its first case of drug-resistant H1N1, which would raise the tally to 22.

Most resistant viruses were found in people who had been taking the antiviral to prevent infection or treat it.

On Thursday, the U.S. Centers for Disease Control and Prevention reported what could be Tamiflu-resistant flu virus spreading from person to person.

Health officials are watching for signs that the virus may be mutating in a way to make the antiviral ineffective. So far, Tamiflu and Relenza, another antiviral, are considered effective against the H1N1 virus.