With the opening of school just two weeks away, a group of the nation's leading scientists sounded a frightening alarm this week about the coming second stage of the worldwide swine flu pandemic.

Before the end of this year, the H1N1 virus could kill as many as 90,000 Americans and hospitalize nearly 2 million people, mainly "younger adults and children ... and those with preexisting conditions," says the report by President Obama's Council of Advisers on Science and Technology.

Flu activity could begin within the next two weeks, the panel warned, spreading "exponentially" each day and starting to hit its peak in mid-October - even before significant quantities of new vaccines become available.

If the panel's scenario proves true, the crisis will overwhelm our hospital system. It will also reignite last spring's bitter conflict between the Bloomberg administration and many public school parents over whether schools hit by the flu should be temporarily closed.

Last spring, Mayor Bloomberg insisted on more than one occasion that shutting schools "has absolutely nothing to do with the spread of the disease." The city closed only about 50 schools - and usually only in response to the demands of angry parents.

Former City Health Commissioner Thomas Frieden, now Obama's head of the Centers for Disease Control and Prevention, took the same policy with him to the CDC.

"It is now clear that closing schools is rarely indicated, even when H1N1 is in the school," Frieden said this month.

Health and Human Services Secretary Kathleen Sebelius reiterated the same view on Tuesday, saying in an interview, "Shutting a school down ... doesn't stop the virus from spreading."

But those statement are far different from what the White House panel of independent scientists says.

"The federal government's planned response to a pandemic virus includes community mitigation measures," the report says. It lists among those measures, "social distancing, cancellation of public gatherings, voluntary home quarantine and school closures."

The report points out that during the 1918-19 flu epidemic that killed more than 500,000 Americans, "Cities that responded rapidly by closing schools, churches and theaters; restricting public gatherings, and otherwise discouraging social interaction appear to have reduced transmission and mortality while the measures were in effect."

Those who want to rule out temporary school closings argue that the current virus is mild and doesn't warrant such major disruptions.

But a possible 2 million people being hospitalized hardly sounds like something to sneeze at - especially when most of the people getting sick and dying are children and younger adults instead of senior citizens, the group that normally represents two-thirds of seasonal flu hospitalizations and 90% of flu deaths.

The report notes that in Australia, where it is now winter, more than 11% of 20,000 confirmed cases of H1N1 during the past few months resulted in hospitalizations.

"Even at the current level of virulence," the report says, "the demand on the health care system in some communities is likely to exceed capacity, necessitating measures to slow the spread of the virus."

The panel urges that our leaders learn from the experiences of other countries, such as the United Kingdom, Mexico and Japan, which have used more aggressive policies on school closings so far.

It goes on to say that "there is significant evidence, as well as logic, to support the idea that school closure ... can reduce virus transmission," while it also notes that local communities will have to weigh the "tradeoffs between the medical benefit gained and social disruption caused by school or institutional closure."

The bottom line: School closures and other "social distancing" measures do slow down the spread of the virus. With no substantial supplies of vaccines expected until mid-October, our leaders should not be so quick to rule them out.