Thousands of students in Allegheny County are at risk of being excluded from school over the next six weeks if they don't provide proof they have met new immunization requirements.

In some districts, the deadline is the first day of school, including Allegheny Valley, where school starts today. As of Friday afternoon, 45 students had not provided documentation. That number fell from 75 earlier in the week.

In other districts, including Pittsburgh Public Schools, the deadline is Oct. 1. The city district most recently estimated that 6,000 students still need to present proof of vaccination or a waiver for medical or religious reasons.

But that number is changing as new records are entered into the district's data-keeping system. Janet Yuhasz, district health services coordinator, thinks the actual number is lower.

The Allegheny County Health Department estimates that 7,000 students countywide have not met immunization requirements, but in light of the city district's most recent count, the actual number may be higher, spokesman Guillermo Cole said.

There has been no overall count since April, when the number covering most county schools was about 36,000 of 181,000 students.

Many districts this summer sent e-mails and letters and made phone calls to encourage parents to take their children to a doctor or to free vaccination clinics offered by the health department. The flurry of activity is a result of a health department regulation adopted in 2007 requiring a second dose of the chickenpox vaccine, or proof a child has had the disease, for all students in kindergarten through grade 12.

The new requirements also call for students in grade seven and higher to get a meningitis vaccine and a combination vaccine called Tdap that prevents tetanus, diphtheria and pertussis, or whooping cough.

Medical and religious exemptions are permitted.

The requirement applies to all students in public schools, including cyber and other charter schools, and nonpublic schools as well as home-schooled students.

Many of those missing some of the shots are short the ones required for seventh grade or later.

Students who do not have all of the required immunizations are to be excluded from school if they miss the deadline.

The requirement initially was to take effect on the first day of school last year, but the health department provided an eight-month extension to around May 1.

It was again extended to the beginning of this year. And now it has been extended to Oct. 1, although districts are free to set an earlier deadline.

Mr. Cole said that is the final extension.

A small number of schools held to the May 1 deadline, including Mt. Lebanon School District.

"We did our big push last year," said Mt. Lebanon spokeswoman Cissy Bowman. "We're in good shape."

Allegheny Valley spokeswoman Janice Zastawniak said the district stuck with the deadline of the first day of school because "we already had that information out to parents and we didn't want to confuse the issue."

Ms. Yuhasz said allowing school to get started for about a month gives time to make sure all of the records are properly recorded and no child is erroneously denied entry.

She said the information on shots comes from a variety of sources, including the Pennsylvania Statewide Immunization Information System, the county health department, doctors and parents.

"It takes time to do it. We have a lot of hands on deck," she said. "The numbers get better every day. We're entering hundreds upon hundreds."

She said the school year ended with about 11,300 city school students still needing proof of immunization or exemption.

Robert Paserba, superintendent of schools for the Diocese of Pittsburgh, said he expects Catholic school students will have their shots up-to-date on the first day of school.

He said parents "know it's part of the requirements, like paying tuition."

If necessary, he said each school will work with families individually for a quick turnaround.

School vaccinations still can be received free of charge from the county health department at its regular walk-in clinic at 3441 Forbes Ave., Oakland, from 9 a.m. to 4 p.m. Monday, Tuesday, Thursday and Friday and 1 to 8 p.m. Wednesday.