HEW Secretary Joseph A. Califano Jr., claiming his nationwide immunization campaign produced a sharp drop in childhood diseases in its first year, said yesterday he is intensifying the effort to reach poor and preschool youngsters.

The head of the Health, Education and Welfare Department said the most dramatic evidence of the program's success was a 65 percent decline in the number of measles cases reported, from 16,701 in the first 12 weeks of last year to 6,064 this year.

Cases of German Measles (rubella) and mumps are down 57 percent and 29 percent respectively, Califano said.

Many states with mandatory immunization laws on the books ordered the statutes enforced last year and refused to admit children to schools without proof of immunization.

Califano showed films of several television spots using "Star Wars" robots C3P0 and R2-D2 and television personality Captain Kangaroo to dramatize the dangers of the common childhood diseases and said he has asked broadcasters in the major metropolitan markets to show the public service spots.

Ads also have been prepared for use by newspapers and magazines.

A year ago, Califano set a goal to raise the level of children immunized against the seven preventable childhood diseases to 90 percent by next fall.

He said yesterday he doesn't know how close the department is to that goal, because a national survey would cost about a half million dollars and he would rather spend the money on the program.

In addition to measles, mumps and rubella, the preventable diseases are polio, diptheria, whooping cough and tetanus.