Journal Issue: Children and Divorce Volume 4 Number 1 Spring/Summer 1994
Because there are different vaccines with different dosage schedules, there are many possible ways of assessing the immunization status of young children. Objectives are frequently stated in terms of the proportion of children of a specified age who are fully immunized at a point in time.
The United States had established for 1990 an objective that at least 90% of all children should have fully completed their basic immunization series (four doses of DTP, three doses of OPV, and one dose of MMR) by age two.8,9 This objective, however, was not met. Based on the 1991 NHIS, only 37% of two-year-olds were fully immunized for these seven diseases.9
The United States, however, recently relaxed its criteria for DTP immunization so that children with at least three doses by age two are counted as immunized. Under these less stringent criteria, 49% of two-year-olds were adequately immunized in 1991.10 Using a comprehensive criterion for measuring immunization status yields lower measures of immunization than are recorded for disease-specific immunization levels. For two-year-olds, disease-specific immunization levels were 52% for OPV, 67% for DTP (at least three doses), and 80% for MMR in 1991.
In contrast to the relatively low levels of full immunization recorded for young children, recently released data suggest that about 97% of children in the United States are adequately immunized before or shortly after starting school.9,11
Figure 2 presents data on disease-specific immunization rates for two-year-olds for 1985, 1991, and 1992. The immunization rates presented are not strictly comparable over time because the data from 1985 are from the USIS and data for 1991 and 1992 are from the NHIS. There have been changes over time in the way refusals, unknowns, and "don't know" responses were handled. In addition, between 1991 and 1992, there was a change in how responses based on parental recall were handled.
In 1991, parents were required to specify the exact ages at which vaccinations were administered for the full number of doses to be credited. Beginning in 1992, a parental response that a child had received all doses of a particular vaccine was accepted even if the time of immunization was not specified. The CDC expects this change in methodology to enhance the accuracy of the data. It also has the effect of increasing reported vaccination levels, perhaps substantially.12
Immunization levels for measles increased considerably between 1985 and 1991, apparently in response to an intensive national inoculation campaign that followed a large outbreak of the disease between 1989 and 1991.13 As a result, in part, of increased levels of vaccination, only 175 cases of the disease were reported in the first half of 1993, down from nearly 14,000 cases reported for the same period in 1990, the height of the epidemic. In contrast, the percentage of two-year-olds immunized against polio, diphtheria, tetanus, and pertussis (whooping cough) was essentially unchanged between 1985 and 1991. Rates of immunization for these diseases registered substantial increases between 1991 and 1992, but it is too soon to tell whether these increases reflect intensified efforts to immunize preschool children or changes in the way immunization status is measured.
There is some question as to how accurately national survey data that are based largely on adult recall of a series of pediatric health system encounters reflect true immunization levels among two-year-old children. In the 1991 NHIS, only 49% of white respondents and 41% of respondents from other racial groups consulted immunization ("shot") records for all the vaccination questions or reported no vaccinations.9
As illustrated in Figure 3, reported immunization rates are substantially higher for children for whom shot records are available at the time of the survey interview. Whether immunization levels are systematically underestimated because shot records are not consulted at the time of interview for more than half of children in national surveys is unknown. Because an important function of the shot record is to facilitate following the complex maze of recommended child immunizations, it is probable that children for whom shot records can be readily consulted in response to a survey interview are more likely to have received immunizations on recommended schedules than are children for whom documentation is not available. To improve the measurement of immunization coverage through the NHIS, beginning in 1994, a subsample of responses from parents without records will be verified with providers to adjust survey results to better reflect true immunization levels.5
Preschool-age white children are more likely to be fully immunized than black children of the same age.10 As illustrated in Figure 4, the primary difference in the immunization status of white and black children, particularly with regard to DTP and OPV, is that white children are more likely to be fully immunized by age two while a larger proportion of black children are only partly immunized by age two. Yet, because there are many more white children than black children in the general population, almost 75% of all undervaccinated two-year-olds are white.12 Note also that, because almost 90% of children are at least partly immunized at age two, most children will require only an abbreviated series of inoculations to be fully immunized at school entry. This may account, in part, for the high level of immunization coverage among children at school entry.