Immunization rates are at all-time highs, and once-dreaded childhood diseases like polio and diphtheria are at or near record lows. But growing numbers of parents and a small group of scientists question the safety of some vaccines, claiming they can cause severe adverse reactions. They also contend that vaccines shouldn't be mandatory for illnesses like chickenpox and hepatitis B -- which are mild or rare in children -- and that tests on the vaccines have been inadequate. In addition, some scientists say that producing genetically engineered vaccines without knowing the long-term side effects is foolhardy. But drugmakers and health officials say there is no proof of a causal relationship between vaccinations and severe adverse reactions and that maintaining public health demands widespread mandatory immunization.