This fall, for the first time, new sixth-graders in Virginia will not be allowed into school unless they've been immunized against tetanus, diphtheria and pertussis.

State lawmakers mandated the new vaccine in 2006, following recent fatal outbreaks nationwide of pertussis, also known as whooping cough. The disease, which causes fits of coughing, is very contagious and can be fatal to young children.

Incoming sixth-grade students started taking the new vaccine, called Tdap, during the 2006-07 school year, but state education officials last year gave parents a 90-day grace period to immunize their children.

When the new school year kicks off in a few weeks, however, sixth-grade students will be turned away at the school door unless their parents can show the children have had the Tdap booster.

"It was made very well-known that there would not be a grace period this year, and that message had been stressed all year long," said Tia Campbell, school health specialist for the Virginia Department of Education.

School districts have sent letters to parents during the last school year and the summer reminding them to immunize their children. But it's hard to tell how many parents have taken their new middle school-age children to the doctor for the booster shots.

"We're hoping the parents got the message," said Debbie Sams, nursing supervisor for the Roanoke and Alleghany Health Districts of the Virginia Department of Health.

The health department is hosting clinics in Roanoke, Salem and Covington to offer all vaccines required for school. Parents can also make appointments at the health departments to receive the vaccines.

There will also be a walk-in clinic at the health department in Roanoke on Sept. 4, the first day of school, for city students who show up without vaccines. The school district has not publicized the clinic in order to encourage parents to get their children vaccinated before school starts, said school spokeswoman Tiffany Woods.

All vaccines required for school attendance are free at local health departments.

Health officials still do not have records of immunizations for 77 percent of the city's incoming sixth-graders, 90 percent of Roanoke County's and roughly 44 percent of Salem's, Sams said. That doesn't mean that students haven't had their shots, however, just that they haven't reported it to their school system or the health department.

"We're all getting kind of nervous here," Sams said. "If they all wait until the end of August, they're not going to get an appointment."

Despite the grace period last year, roughly 3,000 sixth-grade students in 60 of the state's school districts were temporarily barred from school because they hadn't received the vaccine. Those included students in Bedford and Henry counties, according to the state Department of Education.

Young children are routinely vaccinated against tetanus, diphtheria and pertussis before they begin kindergarten, according to the U.S. Centers for Disease Control and Prevention. But doctors have not been giving booster shots to older children because some developed a reaction to the vaccine.

As a result, the immunity that young children developed after being vaccinated wore off after a few years, leaving them susceptible to infection. And once infected, it was easy for them to pass pertussis on to infants.

In 1976, there were 1,020 cases of pertussis nationwide, said Curtis Allen, a CDC spokesman. That number jumped to 11,647 in 2003. The following year, the number of people infected more than doubled, reaching 25,827. Of those, 27 died, most of them young children.

The new Tdap vaccine was approved in 2005, the year before Virginia lawmakers made it a requirement for the state's public schools.