Polio Extraimmunization in Children Younger Than 2 Years After Changes in Immunization Recommendations
* Center for Health Studies, Group Health Cooperative, Seattle, Washington
University of Chicago, Pritzker School of Medicine, Chicago, Illinois
University of Washington, Departments of Pediatrics and Epidemiology, Seattle, Washington
|| Center for Health Research, Northwest Kaiser Permanente, Portland, Oregon
¶ Division of Research, Kaiser Permanente of Northern California, Oakland, California
# UCLA Center for Vaccine Research, Harbor-UCLA Medical Center, Torrance, California
** Kaiser-UCLA Vaccine Research Group, Southern California Kaiser, Permanente, Panorama City, California
National Immunization Program, Vaccine Safety and Development Activity, Centers for Disease Control and Prevention, Atlanta, Georgia
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ABSTRACT |
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Objective. To investigate trends over time in polio extraimmunization among children in 4 large health maintenance organizations and to study the association with recent changes in polio immunization policy.
Methods. Using 176 169 children who were born after 1994 and enrolled for their first 2 years of life, we assessed rates and trends of polio extraimmunization in the Vaccine Safety Datalink project. We used logistic regression to test the association of extraimmunization with different polio immunization schedules and with sociodemographic characteristics and used Poisson regression to test changes in rates over time.
Results. Overall, 10.5% were extraimmunized for poliovirus; children on the all inactivated polio virus or sequential schedule were one half as likely as those on the all oral polio virus schedule to be extraimmunized by 2 years of age. There was a significant decrease in extraimmunization over time, with <5% of children born at the end of 1997 being extraimmunized, compared with >15% at the beginning of 1994.
Conclusions. Poliovirus extraimmunization rates have fallen dramatically in association with the change-over to the all inactivated polio virus schedule.
Key Words: extraimmunization • polio vaccine
Abbreviations: OPV, oral polio virus • IPV, inactivated polio virus • DTP, diphtheria and tetanus toxoids and whole-cell pertussis • DTaP, diphtheria and tetanus toxoids and acellular pertussis • Hib, Haemophilus influenzae type b • HBV, hepatitis B virus • MMR, measles-mumps-rubella • HMO, health maintenance organization • GHC, Group Health Cooperative of Puget Sound • SCK, Southern California Kaiser Permanente Health Care Program • NCK, Northern California Kaiser Permanente Health Care Program • NWK, Kaiser Permanente Northwest
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INTRODUCTION |
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Childhood immunizations provide an effective means of reducing morbidity and mortality from infectious disease. Efforts to ensure timely and thorough administration of immunizations have significantly increased immunization coverage rates among US children. As a result of these efforts, current coverage rates have reached an all-time high, with a corresponding decline in the incidence of vaccine-preventable diseases.1,2
Although thorough immunization coverage remains a top priority of US vaccination policy, recent attention has turned to understanding better and ameliorating the problem of extraimmunization. Extraimmunization wastes resources, causes unnecessary pain, and increases children’s risk for adverse events after vaccination. Extraimmunization may result from either superfluous vaccination (vaccination after successful completion of a series) or early vaccination, which often necessitates correction with additional vaccination to ensure adequate immune response or to meet school requirements. Early vaccinations are those given before a minimum age has been achieved or a minimum between-dose time interval has elapsed.
A recent study of children aged 19 to 35 months showed that extraimmunization is a frequent and costly problem.3 This study found that 21% of children who were born between February 1994 and May 1996 were extraimmunized for 1 or more of the following antigens: poliovirus (oral [OPV] or inactivated [IPV]), diphtheria and tetanus toxoids and whole-cell or acellular pertussis (DTP or DTaP), Haemophilus influenzae type b (Hib), hepatitis B virus (HBV), or measles-mumps-rubella (MMR). Poliovirus was the antigen most commonly given in excess, with 14.1% of children extraimmunized for this series. For each of the other vaccines, fewer than 5% of children were extraimmunized. Another, smaller study estimated that 18% of children were extraimmunized for their age, and 9% were extraimmunized for polio.4
These estimates of polio extraimmunization rates, although valuable, were based on data that preceded or coincided with major changes in polio immunization recommendations that occurred between 1997 and 20005–9 (Table 1). Recent data indicate that extraimmunization rates have begun to decline since these policy changes were put in effect, with 10.6% of children in a 2000 survey extraimmunized, compared with 21.1% of children surveyed in 1997.10 Because estimates of polio extraimmunization rates that reflect current vaccination practices are lacking, we evaluated the impact of changing recommendations on polio extraimmunization in children younger than 2 years.
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METHODS |
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Study Population and Immunization Data
This study used data from the Vaccine Safety Datalink project supported by the National Immunization Program at the Centers for Disease Control and Prevention. The data include demographic information and vaccination histories of children enrolled at 4 health maintenance organizations (HMOs): the Group Health Cooperative of Puget Sound (GHC; Seattle, WA), Kaiser Permanente Northwest (NWK; Portland, OR), Kaiser Permanente Medical Program of Northern California (NCK; Oakland, CA), and Southern California Kaiser Permanente Health Care Program (SCK; Los Angeles, CA).
Vaccine Safety Datalink data have been used in a variety of childhood immunization studies.11 Each child possesses a unique identifier, and demographic data include birth date, gender, Medicaid status, and, with the exception of GHC, race. Vaccination histories were derived from computerized immunization tracking systems maintained by each HMO and included data concerning vaccination date, type of vaccine administered, and clinic at which the vaccine was administered.
Our study analyzed 176 169 children who were born between January 1, 1994, and December 31, 1997 (for NCK, SCK, and NWK), or December 31, 1996 (for GHC), and were continuously enrolled for the first 2 years of life. The different endpoints for the time periods were used because of differences in data availability at the time of our analysis. The enrollment requirement was made to ensure completeness of immunization histories. The time period corresponds to the cohort of children who reached 2 years of age between January 1, 1996, and December 31, 1999. Using the same time period in the calculation of extraimmunization rates over time, we studied all children who were aged 1 to 2 and enrolled between January 1, 1996, and December 31, 1999 (for NCK, SCK, and NWK), or December 31, 1998 (for GHC), and continuously enrolled from birth (N = 253 427).
Immunization Definitions
Because no more than 3 polio immunizations before 2 years of age were recommended during the study period, we considered children who received 4 or more polio immunizations before 2 years of age to be extraimmunized for polio. Children with at least 3 polio immunizations were categorized according to the type of vaccine administered for their first 3 immunizations: all-OPV, all-IPV, IPV-IPV-OPV sequential schedule, or a nonstandard mix of IPV and OPV.
To compare rates of polio extraimmunization to previous studies, we calculated the percentage of children in our sample who were extraimmunized by 2 years of age. However, this type of analysis did not allow us to determine when the extraimmunizations occurred. Therefore, we also assessed the rate of extraimmunization occurrences among 1- to 2-year-olds as a function of time, to compare these rates before and after the introduction of recommendation changes. To evaluate extraimmunization over time, for each month, we divided the number of children who were between age 1 and 2 years and were extraimmunized by the total number of children who were between 1 and 2 years at some time in that month. Children were required to be enrolled continuously from birth through the end of the given month. The 1- to 2-year-old age range was chosen because no children in our sample who were younger than 1 year were extraimmunized.
To evaluate whether any trends seen among polio immunizations were also seen in other vaccine series, we calculated extraimmunization rates for DTP/DTaP. Children who received 5 or more DTaP vaccinations before the age of 2 years were considered to be extraimmunized for diphtheria, tetanus, and pertussis. To have greater confidence that each automated record represented a distinct vaccination, we considered vaccinations of the same type given within 1 week of each other to be duplicate record entries. These duplicate records (0.2% of polio and 0.5% of DTP/DTaP immunizations) were eliminated from our analyses.
Children who received fewer than 3 polio immunizations before 2 years of age were considered to be underimmunized. Children who received 3 immunizations but had 1 or more immunizations given prematurely, without receiving a make-up dose, were also considered to be underimmunized. Minimum age for vaccination and minimum between-dose intervals were determined according to the prevailing recommendations at the time each vaccination was administered.
Finally, a number of advisory committees have suggested that the administration of extra antigens is acceptable when a combination vaccine would reduce the required number of injections (or if it is the only means of delivering the indicated antigen).12 Therefore, when evaluating combination vaccines (eg, DTP-HIB), an extraimmunization was counted as such only when the extraimmunization was present for both antigens.
Statistical Analysis
Statistical analyses were performed using SAS Software (SAS Inc, Cary, NC) and Stata Statistical Software (Stata Corp, College Station, TX). We used logistic regression to test whether polio schedule type (all-OPV, all-IPV, IPV-IPV-OPV sequential schedule, or other mix of IPV and OPV) was associated with the probability of a child’s being extraimmunized by 2 years of age. Because we hypothesized that children who were up-to-date at an earlier age were at greater risk to be extraimmunized, our model adjusted for the number of polio immunizations received before 7 months of age. Sociodemographic factors of interest included birth cohort, gender, Medicaid status, race (white, black, or other), and continuity of care. Models of immunization error rates over time were performed using Poisson estimation. The regression model tested for the significance of a time trend variable.
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RESULTS |
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Most children were enrolled at either NCK (43.7%) or SCK (46.9%; Table 2). Among children with available data, 61.8% were white and 9.1% were black, 3.2% were on Medicaid, and 94.2% received all of their vaccinations at the same clinic. Overall, 5.2% of children were never immunized, 7.9% of children were underimmunized, 76.5% were properly immunized, and 10.5% were extraimmunized for poliovirus. Of children who were properly immunized, 84.5% received the all-OPV schedule, 2.0% received the all-IPV schedule, 12.1% received the IPV-IPV-OPV sequential schedule, and 1.4% received a nonstandard mix of IPV and OPV. Of children who were extraimmunized, 91.2% received the all-OPV schedule, 0.9% received the all-IPV schedule, 4.6% received the IPV-IPV-OPV sequential schedule, and 3.3% received a nonstandard mix of IPV and OPV.
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The results from the regression analysis (Table 3) indicate a significant downward trend in extraimmunization rates over time. Children on the all-IPV or IPV-IPV-OPV sequential schedule were approximately two thirds to three fourths as likely as children on the all-OPV schedule to be extraimmunized by 2 years of age. Children who received a nonstandard mix of IPV and OPV were 3 times as likely to be extraimmunized. Children who received a third polio vaccination before age 7 months were 2.3 times as likely to be extraimmunized for polio. Whites and blacks were less likely to be extraimmunized than children of other races. Continuity of care played a significant role; children who received 1 or more vaccinations at different clinics were 2.6 times as likely to be extraimmunized.
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Figure 1 shows the downward trend across birth cohorts in the percentage of children extraimmunized. More than 15% of children who were born at the beginning of 1994 were extraimmunized, compared with fewer than 5% of children who were born at the end of 1997. The downward trend was fairly consistent with the exception of the first half of 1997, when the rates increased before declining again. In contrast, the percentage of children who were extraimmunized for DTP/DTaP was relatively constant (between 1% and 2%), with a slight increase occurring in 1997.
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Figure 2 shows a similar trend in the number of extraimmunizations per enrollee between the ages of 1 and 2 years. Between 1996 and 1999, this rate fell from >1% of enrollees extraimmunized in a given month to <0.2%. The rate trends downward in 1996, plateaus in 1997 and early 1998, then declines precipitously until the end of 1999. Poisson analysis confirmed that the decrease in extraimmunization over time was statistically significant (P < .001) and that this trend was significantly greater after 1998 than before 1998 (P < .001).
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There was a rapid decline in all-OPV use after the recommendation changes (Fig 3). Use of OPV began to decline among children who were born in the latter half of 1996. By mid-1997, children were more likely to be placed on an alternative schedule. Of note is that the decline in extraimmunization preceded the decline in OPV use, suggesting that not all of the decrease in extraimmunization was attributable to IPV use.
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When we analyzed the data by site, we found some differences. All sites showed a generalized downward trend in both percentage of children extraimmunized and the number of extraimmunizations per enrollee. One site had a higher percentage of children extraimmunized (13.7%) than the other 3 (9.9%, 9.0%, and 7.7%). Also, the transient increase in extraimmunization among children who were born in the first half of 1997 was observed at 2 of the 3 sites; 1 site did not include data for children who were born in 1997.
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DISCUSSION |
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A strength of this study was the ability to assess changes in extraimmunization in time periods before and after the new recommendations, using population-based data from 4 large HMOs. Fewer than 5% of children who were born in the last quarter of 1997 were extraimmunized, compared with 11.1% of children in the preceding 15 quarters. These results support previous estimates of poliovirus extraimmunization rates before major changes in vaccination recommendations were instituted. However, our analysis demonstrates that poliovirus extraimmunization rates have fallen notably in association with the changeover to the all-IPV schedule. This study suggests that low rates of poliovirus extraimmunization are achievable with the current all-IPV schedule.
Our analysis also suggests that policy changes may have led to transient rises in polio extraimmunization rates. Extraimmunization rates had begun to decline before the introduction of IPV. These rates leveled off at the time of IPV introduction but resumed the downward trend shortly thereafter. One possibility is that the changes in polio immunization policy caused some short-lived confusion, which in turn increased the erroneous administration of polio vaccine. However, both the anticipation of and reaction to recommendation changes may have increased provider awareness concerning correct polio vaccination practice, leading overall to fewer errors. This might account for the modest decline in extraimmunization preceding IPV introduction and the rapid decline that followed.
The association of low extraimmunization rates with the changeover to IPV may also be attributable in part to a higher threshold for dispensation of injectable vaccines as a result of the pain associated with their administration. Rates of DTP/DTaP extraimmunization were relatively constant during the same time period, suggesting that the decline was specific to changes in polio vaccination practices rather than the result of a generalized downward trend in extraimmunization for all vaccines.
This study had several limitations. Our data were not comprehensive enough to allow stratification by certain, potentially relevant patient characteristics, such as enrollment in Women, Infants and Children, Head Start, and so forth. Because our sample was confined to HMO enrollees, our findings may not be representative of the population of US children. In addition, the continuous enrollment criterion excluded children who switched health plans during their first 2 years of life. Such children are probably predisposed to extraimmunization because they receive their immunizations from different providers, who may or may not have complete immunization histories. Therefore, our data probably represent a low-end estimate of the true national rate of extraimmunization. Finally, it is likely that unmeasured factors also contributed to the observed decline in extraimmunization. Although it may be difficult to perform additional studies of this topic in the private sector or among patients who switch plans repeatedly, we believe that such studies are needed.
More recent rates of polio extraimmunization are lower than estimates in the period preceding IPV introduction and are similar to extraimmunization rates for other vaccine series. Despite this decline, polio represents only 1 vaccine in an extensive vaccination regimen. When all vaccinations are considered, a high percentage of children may be extraimmunized for any 1 vaccine. As new vaccines continue to be added to an already complex vaccination schedule and as new combinations of vaccines become available, the problem of extraimmunization will remain. It thus will be important to monitor rates of extraimmunization in the future, as is currently done for underimmunization. As families move and change providers in our highly mobile society, a single solution to ameliorate this problem is lacking. As a common repository of data, immunization registries hold promise, but their widespread utilization faces many challenges before most physicians embrace them. The ability of registries to reduce extraimmunization remains to be shown. Our results also suggest that use of oral vaccines may be related to higher rates of extraimmunization. More attention to this problem in the context of oral vaccination may be warranted.
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FOOTNOTES |
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Received for publication Jun 3, 2002; Accepted Sep 23, 2002.
Reprint requests to (R.L.D.) University of Washington Department of Pediatrics, Box 358853, 146 North Canal St, Ste 300, Seattle, WA 98103. Email: rdavis{at}u.washington.edu
Dr Mell performed this work while with Center for Health Studies, Group Health Cooperative.
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