New U.S. guidelines for the pandemic of H1N1 swine flu released on Friday discourage the early closure of schools, unless the virus becomes worse.

But patients who get sick, including children, can return to school or work 24 hours after their fever breaks, instead of the full seven days previously recommended.

We know from the spring that where there was H1N1 there were very large explosive outbreaks in schools, Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, told a news conference. But he said experts had also learned more about the way H1N1 spreads.

We know now that closing schools is not the best option in most cases, said Frieden, who was New York City Health Commissioner until June. He said as many as 800,000 people were infected in New York alone at the peak of the outbreak there.

We can't stop the tide of flu from coming in, but we can help reduce the number of people who become ill with it.

If the virus starts to spread faster when school starts, officials should try other means to slow it, such as keeping students more widely separated and stressing hand hygiene, the guidelines recommend.

The benefits of closing schools are outweighed by social costs such as unsupervised children, health workers stuck at home to care for children, missed meals and missed education, according to the guidelines, available at www.flu.gov.

Schools for pregnant mothers and disabled children are an exception, as such people are at high risk from flu.

About 55 million students attend 130,000 U.S. public and private schools, and 7 million staff work there, according to the Department of Education.

Sick students should be sent home promptly, the guidelines stress, parents need to take responsibility for not sending feverish children to school and schools need to designate a room where sick students can stay safely until picked up.

NO IMMUNITY

H1N1 swine flu is unstoppable, according to the World Health Organization, which has given up on trying to get a precise count of cases.

Experts consider the pandemic to be moderate at this point, meaning it can kill people and can put many into the hospital, sometimes for weeks at a time with severe illness. But most people get a mild illness and get better with little or no treatment.

However, most people have no immunity to this virus, which means that more are likely to become infected than with a seasonal strain of influenza. And the strain could change at any moment into a more serious form.

Vaccine makers have started their first batches of vaccine but U.S. officials do not expect any public vaccination until mid-October and they do not expect those most at risk from H1N1 -- pregnant women, people with chronic health conditions, children and young adults -- to be fully vaccinated until the first week of December.

Education Secretary Arne Duncan said school officials and teachers unions met with government officials earlier this week and endorsed school-based vaccination programs.

Health and Human Services Secretary Kathleen Sebelius said separate guidelines would be issued soon for preschools, day-care centers and colleges and universities.

It is really the dorm environment that presents some challenges, Sebelius said.

Medicine