School officials in three Northern Virginia localities said yesterday they had decided not to enforce new state health regulations that require entering students to have a series of immunizations.

Officials in Arlington, Fairfax and Loudoun counties said they feared the regulations would force numerous arrived Vietnamese refugees, to miss too much of the school year. Classes began yesterday in Arlington and Fairfax and a week ago yesterday in Loudoun.

"A lot of people were not aware of the law. It sneaked up on people all over the state," said George Hamel, a spokesman for the Fairfax County public schools.

Alexandria schools did require proof of full immunization yesterday before admitting students. Donald Dearborn, an assistant superintendent for the city's schools said: "to use an old Oriental phrase, 'No tickee, no entrance.'"

The new regulations were drafted partly in response to a high rate of measles suffered by Virginia students last year. Robert Jackson, the assistant state health commissioner said yesterday that "almost 20 percent of the [measles] cases reported to the U.S. Center for Disease Control in Atlanta last year were from Virginia." About 70 percent of those cases, said Jackson, occurred among elementary school students.

Arlington School Superintendent Larry Cuban directed elementary school teachers in the county to admit students to the first day of classes yesterday if the student had proof of a physical and at least one immunization.

"I have received nothing in writing . . . [and] until I find out exactly what the state health officer [requires], we are doing precisely what we've been doing for the last few years," Cuban said.

Jackson said in Richmond that there is no room for interpretation in a new state law that mandates that all students be "successfully immunized" against such communicable diseases as polio, rubella and measles before entering public schools.

"The fundamental, statewide policy is that children should be immunized before getting to school," said Jackson. Jackson said, however, he agreed there had to be some "flexibility," particularly in a situation such as Arlington's where a large number of new students are expected from Vietnamese "boat people" families.

"As long as school systems are working toward the objective (of the new rules), we're not about to jump in and start citing people and taking people to court," said Jackson. "On the other hand, that is always in the background as a possibility."

"New laws are like new cars," said Jackson. "They require a breaking in period."

In Arlington, Dr. Martin Wasserman, chief of the county's Department of Human Resources, said yesterday there was no disagreement over the intention of the law.

"We would agree that a successful immunization requires more than the first shot," Wasserman said. "The problem is the child is then in limbo. I don't think all the ramifications particularly as they affect the Indochinese, were thought out at the time legislation was drafted."

Wasserman said he advised superintendent Cuban to continue last year's admission policy.

The new health regulations also require students to present proof of a recent physical exam before entering school. In the past, students were allowed 10 days after the start of classes to obtain a physical. All school jurisdictions said yesterday they were complying with that regulation, despite some problems caused by it.

"It's unreal here, our place is in complete chaos," said an appointment secretary at the Rader Clinic at Fort Myer Army post in Arlington. "In the past week or so we've had a thousand phone calls from people who are new in the area who have children that haven't had a physical."

The Rader Clinic, which serves military personnel and their dependents throughout the metropolitan area, has been conducting an average of 84 physical exams a day, said Maj. Wayne Vinson, the clinic's administrator. Vinson said the clinic will continue to give physicals through next week.

In Alexandria, the health department will conduct a special immunization clinic for new students today from 1 p.m. to 4:30 plm. at the city's health clinic at 517 North St. Asaph Street.

Virginia school officials in other localities could give no statistics yesterday on how many students were refused admission to class because of the new health regulations.