To the Editor.
—We are concerned that the otherwise valuable study by Dr Mustin and colleagues1 has been misinterpreted by the media and will be similarly misunderstood by others unaware of the limitations of secondary data analysis.Mustin et al1 concluded that while most US infants have adequate well-child visits, many remain inadequately immunized. The authors defined adequate well-child visits as one prior to 3 months of age plus one between the ages of 3 and 6 months. They defined adequate immunization as receipt of three diphtheria and tetanus toxoids and pertussis (DTP) vaccines and two oral poliomyelitis vaccines (OPVs) by 6 months of age. Because three visits at least 4 weeks apart are required to administer these immunizations, by the authors' definitions many infants will be classified as having received adequate well-child visits, but will be inadequately immunized at 6 months of age. Furthermore, some of the