As schools and colleges prepare to welcome students back into session this month, they face an unpredictable yet unavoidable disruption -- pandemic flu.

The H1N1 virus has been circulating all summer, shutting down overnight camps and keeping doctors' offices busy. Now health officials are bracing for an even bigger spread of the novel virus as students of all ages congregate in the close quarters of buses, classrooms, dorms and cafeterias.

Complicating matters is the lag between the start of school and the anticipated October delivery date for the first, limited batch of vaccine.

"We're concerned about a second wave in September," Dr. Jeffrey Engel, state health director, said about the flu. "We know it's here now, and active. We're thinking all it's going to require [to spread] is crowding."

State health leaders -- along with officials from county health departments, public schools and area universities -- are drafting plans for handling large-scale flu outbreaks within their ranks.

Much depends on additional guidance from the federal Centers for Disease Control and Prevention, which leads the public health response.

Last week, the CDC issued a list of who should receive the first inoculations in October, when between 120million and 160 million doses of vaccine are expected to be delivered. Pregnant women, people who tend babies under 6 months old, and health-care workers who treat patients will stand first in line for the initially tight supply.

Among those also considered high priority are children and young adults up to the age of 24. As a result, flu shot clinics may be planned at schools and colleges as the vaccine becomes available.

And the clinics would likely run twice, because the new vaccine calls for two shots -- an initial inoculation and a booster about 21 days later to ensure protection against the new virus. Those shots are in addition to the regular seasonal flu jab, which must be given separately.

Wake County health officials are discussing how to run mass vaccination clinics for the first time at schools, and are talking with area pediatricians about the best approach for reaching the estimated 140,000 public school children in the county and thousands of others who are homeschooled or attend private academies

Dr. Peter J. Morris, medical director for Wake County Human Services, said one idea is to divide the duties, with mass clinics targeting children at schools, and pediatricians concentrating on preschool children and youngsters who have complicating illnesses.

"We're all brainstorming," Morris said, noting that the scale of the H1N1 vaccination drive is likely to dwarf the typical demand for seasonal flu shots. About 40 percent to 50 percent of children usually get vaccinated.

No one knows, however, how much pandemic flu vaccine will be shipped and when.

"The details are just not clear at this point," said Dr. Mary Covington, assistant vice chancellor for Campus Health Services at UNC-Chapel Hill.

She and others said the unpredictable nature of the expected outbreak makes planning a challenge, but at least one factor appears unchanged. So far, the virus has not mutated into a more lethal strain. Most people who have been infected with the bug have had mild to moderate symptoms.

In addition, she said, it continues to respond well to anti-viral drugs that ease symptoms, and such drugs are especially helpful for people who are at high risk of complications. Counties all have stockpiles of the anti-virals in case retail supplies grow scarce.

As a result, worst-case scenarios of massive school closings and business shut-downs are not at the top of response plans.