And with schools expected to remain open unless the virus becomes more severe, there’s little standing in the way of H1N1′s spread.

At the same time, schools are likely to serve as centers for mass immunizations, which could sharply reduce H1N1′s reach, according to the Centers for Disease Control and Prevention and state and local authorities. So far, the swine flu does not appear to be more dangerous than the typical seasonal flu. But medical authorities are concerned that it could infect many more people — thereby increasing the potential number of deaths — because so few people have immunity against it.

The mass immunization program, likely to be the largest of its kind since the polio vaccine was given to about 100 million Americans in the 1960s, will play out with some differences between states and local jurisdictions. For instance, still waiting to be resolved are questions about who gets the vaccine, whether schools are used as vaccination sites, whether parents are present when children are vaccinated and whether the vaccine is administered by injection or nasal spray.

Health officials in Virginia, Maryland and the District said that at least some school campuses will be used as vaccination sites. Schools reopen today in the District and in parts of suburban Maryland.

“There’s considerable interest out there from the local health departments and school districts to do it in the schools,” said Jim Farrell, director of the immunization division of the Virginia Department of Health.

Elsewhere, officials suspect that schools will be used less.

“Our school health system . . . is not very well-funded,” said David Fleming, public health director and health officer for Seattle and King County, Wash. “We don’t have the staff in the schools to do it. There’s also the cumbersome process of getting parental permission. So doing it during school hours may not make a lot of sense.”