The Washington, D.C., public school system has begun a campaign to have some 12,000 children immunized against a variety of diseases before the beginning of the school year in just a few weeks. Superintendent Paul L. Vance acknowledged that many of the children in question were immunized last year, but he stressed that some must have booster shots to ensure that their vaccinations are up to date. The District of Columbia requires that all children in school be immunized against diphtheria, pertussis, tetanus, measles, mumps, rubella, Haemophilus influenzae type b, hepatitis B, pneumococcal pneumonia, and chickenpox. Last year, the school system was unable to find the immunization records for roughly 41,000 children, a problem that kept some students out of school for several weeks and that was blamed on poor records management for nearly a decade.