Montgomery County school officials estimate that as many as 3,500 public secondary school students have not yet been immunized against measles despite the fact that today is the deadline to get the state-required vaccination.

In Prince George's County, about 700 students had not yet received the vaccination as of yesterday, according to Louis Blauvelt, supervisor of health services for the county schools.

Youngsters who have not been immunized will be sent home from school today, according to school officials in both counties.

"We expect there will be very few sent home because the students have been told if they haven't complied (with the immunization regulation), they should just now show up," said Dr. Kenneth Muir, Montgomery schools spokesman.

Bad weather resulted in a light turnout yesterday at an immunization clinic set up in Walter Johnson High School in North Bethesda, according to Frank Berfield of the county health department.

Berfield said a final clinic will be held Friday from 9 a.m. to 5 p.m. at Woodside Elementary School, at Ballard Street and Georgia Avenue in Silver Spring, for those public and private school students who have missed the deadline.

Students in Prince George's have to make an appointment with the health department in Cheverly to get their vaccination now that the deadline has passed, Blauvelt said.

Youths under 18 must be accompanied at the clinics by their parents who are required to sign a consent form, health department officials said.

This is the first time the state has required that students in grades seven through 12 be immunized against measles, according to health officials. Children in kindergarten through sixth grade are required to be immunized against polio, rubella, diphtheria, pertussis (whooping cough) and tetanus as well as measles.

Maryland state law requires that youngsters be immunized with a live measles vaccine once they are a year old. Youngsters who may have been vaccinated with a killed measles vaccine (no longer in use in Maryland) should be revaccinated with the live vaccine, according to Dr. Eugene Rosenberger, chief of epidemiology for the Montgomery health department.

In Virginia, some preschool and kindergarten students have already been suspended for lack of immunizations. Students in grades one through 12 in Fairfax have until Dec. 9 to show proof of immunization against communicable diseases or they will face suspension. Loudoun County students must show proof of immunization by Jan. 3. Prince William and Arlington County students were required to show proof on the first day of school in September.