The World Health Organization yesterday declared a full-blown pandemic while at the same time saying that most people were in no imminent danger.

With increasing reports of flu in some Philadelphia-area schools, local officials are striking a similar balance - strengthening policies on how to respond to illness and absenteeism while telling parents of healthy children that there is no excessive risk.

Both moves reflect the same reality: The novel H1N1 influenza so far has proven no more severe than seasonal flu; but more people, particularly children, lack immunity and are more likely to get sick.

"This means the world is moving into the early days of its first influenza pandemic in the 21st century," WHO Director General Margaret Chan told a news conference in Geneva, Switzerland, yesterday after the agency decided to go to Phase 6, its highest alert. "We have good reason to believe that this pandemic will be of moderate severity."

At almost the same time in Philadelphia, after a meeting with school principals, City Health Commissioner Donald F. Schwarz said: "This is influenza, period. It is not influenza plus."

About 57 percent of students were absent from Hunter Elementary School in Kensington yesterday and 22 percent from Rowen Elementary in West Oak Lane, but officials said attendance elsewhere in the district was near normal for this time of year.

That, too, appeared to be a microcosm of what experts describe as an expected yet unpredictable "jigsaw" pattern of illness in schools around the region, communities around the country, and nations around the world.

Southeastern Pennsylvania and South Jersey, for example, saw their first cases of swine flu more than a month ago, before it spread to many other parts of the country. Then it seemed to increase elsewhere and decline here. Now the pattern appears to have reversed again.

It is precisely the new strain's ability to spread to different communities - some of which, particularly in developing nations, could be quite vulnerable to even a mild flu - that met WHO's criteria for proclaiming a pandemic.

In a statement sent to member countries, WHO urged nations not to close borders or restrict travel and trade. A WHO spokesman, Gregory Hartl, said Phase 6 "doesn't mean anything concerning severity" but has to do with "geographic spread."

WHO reported nearly 30,000 laboratory-confirmed cases, including 141 deaths, in 74 countries. But with most illness not resulting in lab cultures - in the United States, for example, most doctors are now treating patients for "the flu" and testing only severe cases or people with underlying health conditions - the actual numbers are thought to be well over 10 times the official count.

WHO had been reluctant to move the pandemic alert to its highest level because of fears that doing so would confuse the public and trigger panic. Plus, the new H1N1 strain is far less virulent than most experts expected the next pandemic flu to be. It generally causes an infection that people can recover from at home without specific medical intervention.

In recent weeks, WHO officials acknowledged that they fear people might become complacent if Phase 6 is declared for what is not perceived as a serious health threat. The stepped phases in WHO's index, however, address only where a new virus is spreading, not the severity of the disease it causes.

Most pandemic preparedness plans - and nearly all the public discussion in recent years - have assumed that a Phase 6 pandemic would constitute a global public-health emergency that would close businesses, reduce trade, limit travel, and probably ration medical care.

The model was the 1918 pandemic of "Spanish flu," which killed more than 50 million people worldwide over a two-year period.

In the United States, officials at the Centers for Disease Control and Prevention said WHO's Phase 6 declaration would not change their approach to the disease. They said the CDC had acted for the last month as if a pandemic already existed here.

Still, the official U.S. weekly count for the new swine flu, expected today, will likely be far higher than the previously reported 13,000 infections and 27 deaths. They do not, for example, include Pennsylvania's two deaths, both in the last week, or many of the 15 deaths in New York.

Normally, the seasonal flu has disappeared with warm weather, or seemed to. Public-health officials never looked intensely in summer, when the worst was over, but swine flu is proving resilient.

"What this [WHO] declaration does do is remind the world that flu viruses like H1N1 need to be taken seriously," said Kathleen Sebelius, the U.S. secretary of Health and Human Services, warning that more cases could crop up in the fall.

"We need to start preparing now in order to be ready for a possible H1N1 immunization campaign starting in late September," she said in a statement from Washington.

CDC officials said that they had ordered stockpiles of chemicals needed to manufacture vast amounts of the vaccine. A government decision to move ahead with manufacture could come soon, and decisions about distribution after that.

In Philadelphia, where public school classes continue until June 23, yesterday's new four-page guidance to principals emphasizes spotting sick students and preventing further spread while avoiding the closure of schools.

All school staff are instructed to "actively seek to identify" children with flulike symptoms - fever, cough, sore throat - and take them to the nurse. If the nurse confirms the symptoms, a child should immediately be sent home with a parent or guardian, not on a bus.

"Hand sanitizer and tissues should be readily available in every classroom and office," according to the new policy, and high rates of absenteeism in a school should trigger consideration of canceling major gatherings such as assemblies, sporting events, and field trips.

Parents whose children have no symptoms are being discouraged from keeping their children home, but letters notifying parents of flu in a school are being sent whenever at least five students with flulike symptoms are confirmed by the nurse, officials said. That has already happened in about 10 of the district's 267 schools.

Schools will be closed only if too many staff are absent to ensure safety, they said.

The district expects "not to have to cancel any graduations," said Tomás Hanna, chief of school operations, adding that students with flulike symptoms will be asked not to attend.