Concerned about a nationwide outbreak of measles, officials from the U.S. Center for Disease Control in Atlanta have recently visited Fairfax County schools to help them determine why some children immunized against the disease have contracted measles while others have not.

The worst outbreak of the disease in the Washington area occurred at Henry David Thoreau Junior High School in Vienna, where 28 cases of measles were reported in January, according to school records. Numerous other cases have been reported througout the Washington area.

"We're seeing the measles outbreak all over the country," said David Heymann, an epidemiologist with Center for Disease Control (CDC). "Now we're trying to find out who's getting (the disease) and what recommendations to make."

So far this year, Heymann said, 12,655 cases of red measles have been reported nationwide, compared to 6,945 cases reported in all of 1976. The actual number of cases, however, is probably much higher, he said, since only a "small percentage" of the cases are reported to local health officials.

In Fairfax, CDC officials and the county's Health Department are checking the immunization records of 44 students who contacted the disease in January against a control group of 176 students who also were immunized but did not contact it, Heymann said.

Similar studies are being conducted in several other states, including Ohio, Missouri and Texas. Heymann said the DCD chose sites for the study where "good school and health records are kept."

In each case, Heymann said, local health departments have sent letters to the parents of students in the study asking to determine the date of their child's immunization and the type of vaccine used. CDC officials are then making follow-up telephone calls to the parents to collect the information.

Part of the reason for the current measles outbreak, Heymann said, is that health officials have only recently discovered that the measles vaccine is often not effective when administered before a child is 15 months old.

Until that age, Heymann said, a child is protected from measles by his mother's antibodies is she is immune to the disease. The antibodies also fight off the vaccine, rendering it ineffective, until the age of about 14 months when the antibodies disappear and the infant is on his own, Heymann said.

In addition he said, many of those now contracting the disease are between the ages of 9 and 14 and were immunized shortly after the measles vacccine was first introduced in 1963.

At the time, Heymann said, two types of vaccine were in use - one containing a weaker strain of the actual live virus and another made from a killed virus. The vaccine made from the killed virus had since been taken off market, he said, and those who were vaccinated with it are probably not immune to the disease.