A new Virginia law that will prevent students from attending school unless they can show the dates they were immunized against contagious diseases sent hundreds of families scrambling to Northern Virginia health clinics this week to get their children inoculated before schools open.

The new law requires documentation of the dates students received vaccinations against diphtheria, whooping cough, tetanus, polio, rubeola (red measles), rubella (German measles) and mumps, and it requires parents to supply more documentation than the old statute did. Students must have had booster shots against diphtheria, whooping cough, tetanus and polio, usually given by age 4.

By yesterday, almost 3,000 of Prince William's 35,100 students had not turned in their updated immunization records.

In Fairfax, Assistant School Superintendent Beatrice Cameron said, "We expect a good many students won't be able to attend school on Monday."

Schools in Prince William, Fairfax, Loudoun and Fauquier counties open Monday. Those in Arlington and Alexandria open Sept. 6.

The requirement of dated medical records has sent parents looking for their children's old records or stampeding to health clinics.

Waiting rooms at clinics in Fairfax and Loudoun counties are filled to capacity with families waiting for the shots, health department officials say.

The Fairfax County Health Department has given almost 2,000 injections to students the past two weeks.

"We've had an avalanche of people," said Dr. Garth Dettinger, assistant health director for Fairfax County. Dettinger said local clinics have assigned additional staff to give shots and have extended hours.

Normally, the Loudoun County Health Department holds immunization clinics twice a month. This month, the department has been inoculating children every day.

"We got about all we can hold right now," said Dr. Earl Virts, Loudoun County health director. " We've got a lot of older children whose immunizations weren't up to date."

School boards in Prince William and Fauquier counties are hoping that parents will heed the thousands of letters they've mailed urging parents to update medical records to meet state requirements.

Prince William Associate Superintendent Eugene Lee said "one of the major problems we've had from this episode" is with students from other states who do not meet immunization requirements or cannot easily locate immunization records.

At the beginning of summer, the Prince William school system mailed 12,000 letters informing parents that their children's immunization records did not meet state requirements. The Fauquier County school system put 1,000 mailed letters to parents.

Loudoun County Assistant School Superintendent Arthur Welch said many parents are having problems finding their children's immunization records. "This is a whole new ball game now. We have to go back and pick up everyone from grades four to 12," he said.

"I can understand how some parents can be a little oppressed by it . . . especially if you've already done this and you have phobias about needles," said Phyllis Worrell, Fauquier County director of pupil services.

Arlington school officials said they expect at least several hundred students will not meet immunization requirements when school starts after Labor Day.

Alexandria school officials said they are having no problem getting students to meet state requirements.

A student can only be exempt from the state requirements if immunization would pose health problems or conflicts with the student's religion.