SWINE flu outbreaks shouldn't close US schools unless so many students or teachers get sick that it interferes with teaching or puts the students at risk, health officials said yesterday.

The pandemic flu, also known as H1N1, can no longer be contained by closing schools and the disruptions caused by closures outweigh the public health benefits, according to guidelines posted on the Health and Human Services Department's website.

The guidelines cover the nation's 55 million students and 7 million staff at 130,000 public and private schools.

Health officials around the world are completing plans to deal with widespread outbreaks in the northern hemisphere, beginning in September when cooler temperatures and the return to schools may fuel the biggest flu epidemic in decades.

The recommendations say sick students should not return to school until 24 hours after their fever subsides.

''We know now that closing schools is not the best option in most cases,'' said Thomas Frieden, the director of the US Centres for Disease Control and Prevention. ''If flu comes back in the [autumn] as it was in the spring, the key messages would be to stay home when you're sick, to wash your hands and to cover your cough and your sneeze.''

The initial spread of swine flu in April kept almost 500,000 students out of class by May.

Health Secretary Kathleen Sebelius said vaccination was the best way to prevent the spread of swine flu.

The swine flu vaccine will probably require two shots, administered three weeks apart, according to yesterday's report. The body won't be protected for two weeks after the second shot.