Immunization Levels and Risk Factors for Low Immunization Coverage Among Private Practices

DISCUSSION

This assessment of immunization coverage among 45 randomly selected private medical practices in California found overall coverage levels to be strikingly low. The median coverage level for the basic series was 54% at 24 months of age, well below the Healthy People 2000 goal of 90% coverage among all 2-year-old children. Substantial evidence shows that children who are immunized in the private sector often have lower immunization coverage levels than their counterparts who are immunized in the public sector.^6,,13,18,19 Despite these compelling results, there is a dearth of research on immunization coverage in the private setting, where two thirds of California's children and more than half of the nation's children are vaccinated. A great deal of research has suggested that provider-associated characteristics and behaviors are some of the most important risk factors associated with underimmunization. However, further description of this relationship among private medical practices has also been lacking. We must understand specific risk factors in this setting, so that we can efficiently focus interventions to improve immunization coverage.

This study was undertaken to further elucidate risk factors for low immunization coverage in private medical practices in California. We found that having <50% active pediatric patients in a practice was the most important independent predictor of underimmunization. After controlling for number of visits made to the provider and various other factors, children in practices with fewer than 50% active pediatric patients had nearly 6 times the odds of missing immunizations, compared with children in practices with greater than 50% active pediatric patients. It may be that practices with a minority of active patients function largely as urgent care providers and, thus, are not accessed by families for well-child care, including immunizations. It is unknown whether the children in these practices are medically homeless, meaning that they lack a primary care provider who offers immunizations and other preventive care. However, the percentage of active children in a practice may be a marker for an important risk factor for underimmunization that has not been previously reported. Although this theory is speculative, we believe that additional research in this area is warranted, because it may allow public health officials to target interventions toward medical offices in which children are at tremendous risk of missing immunizations.

In this study, the second most important predictor of vaccination status was the number of visits made by a child to the practice. Children who made fewer than 8 visits were at nearly 5 times the risk of being underimmunized, compared with children with 14 or more visits. This result is not surprising because increased contacts with providers allow for increased opportunities to immunize. This finding, combined with similar results of previously reported studies,^6,,13suggests that children who make few office visits are a critically important target population for immunization interventions. Although the parents of these children may contribute to the problem by failing to make and keep appointments, many children actually make enough visits to receive all of their vaccinations (a minimum of 5) but still are not UTD because providers miss opportunities to immunize.¹ Conventional wisdom holds that missed opportunities for vaccination are 1 of the single most important causes of low vaccination levels.¹ Missed opportunities are caused by less than optimal provider immunization practices, such as deferring immunizations for invalid reasons and failing to administer simultaneous vaccinations. An intervention study that reduced missed opportunities found that the greatest improvement in immunization status was seen in children with the least office visits.²⁰ Our findings support an emphasis on this subgroup of children who make few visits to their providers and would benefit most from efforts to eliminate missed opportunities. Providers may be able to encourage these high-risk children to return for well-care visits by instituting reminder and recall systems.^21–23

Another significant risk factor for being incompletely immunized was the county in which the practice was located. Children in practices located in San Joaquin and San Bernardino counties had a greater chance of being underimmunized. Although the exact reasons for this are unknown, it is noteworthy that these counties have a higher percentage of persons living in poverty and a lower median family income than Contra Costa, the comparison county. Additional work should be performed to determine whether there are specific, modifiable factors associated with underimmunization in these areas.

Surprisingly, the percentage of Medicaid patients in a practice was not a clear predictor of vaccination status. Children in practices with a majority of Medicaid patients had no greater risk of underimmunization than did children in practices with <10% Medicaid patients. However, children in practices with an intermediate number of Medicaid patients (10%–50%) had twice the odds of underimmunization. Thus, the predicted result, increased odds of underimmunization associated with increased percentage of Medicaid population, was not found. These results may suggest that practices with a more homogeneous patient population (ie, largely Medicaid or largely private insurance) may do a better job of providing complete immunization coverage to their patients. Perhaps these practices are better able to adapt their immunization behaviors to suit their patient population. It is also possible that some of the physicians' estimates of percentage of Medicaid patients were inaccurate, resulting in misclassification of the practices.

A substantial decrease in UTD status for the basic series occurred between 12 and 24 months of age. Both study-wide mean and practice median coverage dropped from nearly 75% UTD at 12 months of age to <60% UTD at 24 months of age. In nearly all cases, the individual coverage levels of practices for the basic series were also substantially lower at 24 months of age versus 12 months of age. Similar drops in immunization coverage between 12 and 24 months of age have been previously reported by numerous researchers.^6,,13,24,25

In contrast, coverage levels for the basic series plus 3 HBV were essentially the same for 12- and 24-month-old children, with study-wide means and practice medians at ∼50%. This finding suggests that many practices still had not begun routine administration of HBV in the first year of life, despite the 1991 recommendation of the CDC for universal immunization of infants against hepatitis B.²⁶

The only office immunization policy that was significant in the multivariate model was administration of DTP4 at a separate visit from the Hib booster. This study found that children in practices that more frequently administer these vaccinations separately have twice the odds of missing immunizations, compared with children in practices that usually administer DTP4 and the Hib booster simultaneously. This finding is particularly important in view of the stagnant, low coverage levels for DTP4. In 1997, national coverage levels for DTP4 did not improve over 1996 levels of 81%.⁴ Some providers believe that giving DTP4 and Hib booster at separate visits will encourage parents to bring their children back for subsequent well-care appointments. However, a recent study²⁷ found that 74% of parents were motivated to bring their children to well-child visits regardless of the need for immunizations and only 18% of parents needed a vaccine incentive to motivate them.

It is likely that other immunization policies are also important risk factors for underimmunization. This study was limited in its ability to show such effects for various reasons. First, the practice of splitting DTP4 and the Hib booster was the only immunization procedure that was determined from the medical record data. All other policies that were tested for significance were based on physician's questionnaire responses, the accuracy of which is unknown. Second, some reported policies were not used in the analysis because of discrepant answers from physician and nonphysician questionnaire respondents in the same practice. The third problem with detecting significant associations between office immunization policies and coverage levels resulted from the fact that only 1 physician from each practice completed the questionnaire. In offices with more than 1 physician, the surveyed physician's responses may not have been representative of all immunization providers in that office.

Another potential weaknesses of this study is that the sample of practices may be biased. Although sites were randomly selected from each county, the relatively low number of practices that could be located and the relatively low participation rates among located practices may have yielded a biased sample. For example, the large number of practices that could not be located may have been attributable in part to recent restructuring of the private health care system in response to the increase in managed care plans. Furthermore, providers who thought their immunization levels were especially low or high may have chosen not to participate in the study for reasons such as fear or lack of perceived need, respectively.

The percentages of fully immunized 2-year-old children found in the practices included in this study were lower, overall, than the coverage levels estimated by the NIS among children 19 to 35 months of age during the same period.²⁸ This difference may in part reflect an older median age of 27 months of age among children included in the NIS survey. However, the primary reason for this discrepancy is most likely attributable to methodological differences between the 2 studies. The NIS is a population-based study that relies heavily on information from households that are reachable by phone. This study is practice-based and, therefore, captures only the data contained within the medical records from the study sites. Because patients are geographically mobile and/or have variable insurance status, children may leave a practice or receive immunizations elsewhere, without notifying the provider. Thus, as investigators of an office-based study, we faced some of the same problems that plague vaccine providers: determining which children are active patients of the practice and whether they have been vaccinated elsewhere. In the future, these problems may be solved by a well-functioning immunization registry. Until then, it is important to acknowledge that patients move in and out of practices. Although little is known about the number of providers visited by preschool-aged children in California, 1 study in Baltimore found that the vast majority (81%) of children saw the same provider for all primary care during the first 2 years of life.¹² Nonetheless, in an effort to account for the nomadic behavior of some patients, this study included only active children in the calculation of a practice's vaccination rate. However, parents were not contacted to confirm the status of their children. It is likely that some patients who had left the practice were included in the study and some children who had received immunizations elsewhere were incorrectly classified as NUTD.

Misclassification, either of a child's active status or a child's immunization status, is one of the inherent difficulties of practice-based studies. Darden et al²⁹ attempted to quantify the impact of these misclassifications on their office-based study. They found that when parent contact information was used to exclude inactive patients, vaccination coverage estimates increased by 4.5%. In addition, they found that supplemental information supplied by parents increased vaccination coverage estimates by 3.6% over those estimates based solely on medical records. The findings of Darden et al²⁹ are reassuring because they suggest that misclassification may be a relatively small problem. Nonetheless, we would be remiss in failing to mention the possibility of confounding by such misclassifications. For example, this study found that smaller proportion of actives in a practice and fewer number of visits per child were the strongest predictors of underimmunization. However, practices with a large proportion of inactive pediatric patients may also have had more misclassifications of inactive children as active. Similarly, children with fewer visits may have had more misclassifications as NUTD, perhaps because they were immunized elsewhere. In both cases, these potential misclassifications may have confounded the relationship between the predictors and immunization coverage levels.

Despite the inherent difficulties, practice-based studies are a valuable research tool that allow a view of immunization coverage different from that provided by the NIS. Aside from the obvious benefit of clarifying the relationship among provider behaviors and immunization coverage, practice-based studies may also provide unique insights into vaccination barriers. For example, the practices in this study that had fewer than 50% active pediatric patients may have contained a subpopulation of patients that was highly nomadic or medically homeless. Similarly, the children that made few visits to their providers may also represent a subset of the preschool-aged population. Such patients may be more difficult to reach by phone and, therefore, may not be captured in population-based studies, such as the NIS.