Infection and immunization.
Herd infections, especially in children, are strongly influenced epidemiologically by social and demographic factors which have contributed favorably to a general decline in incidence and mortality during the past 50 years or more. Intervention procedures such as immunization cannot be evaluated or planned realistically except against these background factors. Assessed in this way, immunization against diphtheria and poliomyelitis was unequivocally effective in reducing incidence and morbidity of these diseases. By comparison, pertussis vaccine has a very limited protective effect, the value of which, as morbidity decreases, may be offset by the intrinsic toxicity of the vaccine and by the possibility of infrequent but severe brain damage in some children.