There was a substantial reduction in varicella associated morbidity, mortality, and health care costs since the implementation of a universal 1-dose childhood varicella vaccination program in the United States in 1995.1 However, approximately 15% of vaccine recipients do not achieve protective levels of antibodies after a single dose.2 Additionally, some study suggested that vaccine-induced immunity may wane with time.3 In 2006, the Advisory Committee on Immunization Practices (ACIP) approved the new recommendation regarding the use of varicella-containing vaccines for prevention of varicella.4 The new recommendation included a 3-pronged approach: (1) implementation of a routine 2-dose varicella vaccination program for children, with the first dose administered at 12 to 15 months and the second dose at 4 to 6 years; (2) a second dose catch-up varicella vaccination for children, adolescents, and adults who previously had received 1 dose; and (3) routine vaccination of all healthy persons aged ≥13 years without evidence of immunity.
Although data from a large randomized trial suggested that 2-dose regimen was significantly more effective than a single injection in protection against varicella,5 a recent study of outbreak in elementary school children with low 2-dose coverage (40%) suggested that the vaccine effectiveness of 1 and 2 doses were similar.6 Although more studies in large populations are needed to evaluate the effectiveness of the second-dose vaccine, at present, it is important to understand and continuously monitor the second-dose coverage among eligible age groups to guide future policy and intervention design. At present, few data are available for second-dose varicella vaccination coverage among children 4 to 6 years of age, and little was known about the coverage by gender, race/ethnicity, and socioeconomic status (SES). Knowledge of the coverage by these characteristics may provide important insights for developing strategies tailored to subgroups with lower coverage rate.
To evaluate the patterns of uptake of second-dose vaccination in an insured population, we assessed the second-dose coverage for varicella-containing vaccine among children and adolescents in Kaiser Permanente Southern California (KPSC) health plan between September 2006 and September 2009.
MATERIALS AND METHODS
KPSC is the largest managed care organization in Southern California, serving approximately 3.3 million members who are broadly representative of the diverse racial/ethnic and socioeconomic background of the source population in this geographic region. In September 2006, KPSC implemented the new ACIP recommendation regarding the 2-dose varicella vaccination. Varicella vaccines are offered to eligible members with no additional out of pocket cost. With a design similar to a repeated survey, we conducted cross-sectional assessments of the second-dose varicella vaccine coverage on every 30th of September in 2006–2009. We included children and adolescents enrolled in the KPSC health plan who met the following inclusion criteria: (1) 4 to 17 years old on the assessment date, (2) without a recorded history of diagnosed varicella, and (3) with at least 1 dose of single-antigen varicella vaccine or MMRV (measles, mumps, rubella, and varicella) administered after 12 months of age (the first dose must not have been given earlier than 4 days before the first birthday). Immunization records were obtained from Kaiser Immunization Tracking System (KITS), which has been in place in KPSC since 1980. KITS contains member's unique medical record number and vaccination history. Varicella vaccines delivered outside the health plan are entered into the KITS with appropriate documentation to substantiate its delivery.
Calculation of Second-dose Coverage.
The second-dose vaccination coverage was defined as the proportion of those who had received the second-dose varicella vaccine among the children and adolescents who previously had received 1 dose. According to the ACIP recommendation, a minimum of a 28-day interval between 2 doses of vaccine was required for the second dose to be considered as a valid dose.4 The overall second-dose coverage was calculated separately for each of the 4 assessment dates, as well as by age, sex, race/ethnicity, age at the first dose (12–15 month versus later than 15 months), and SES indicators. Age-specific coverage also was estimated among strata defined by sex and race/ethnicity. SES indicators included Medicaid enrollment status and neighborhood education and median annual household income levels. To obtain neighborhood income and education levels, the members' addresses were mapped to US Census block data. If a subject enrolled in the California's Medicaid health care program any time within 12 months before the assessment date, he/she was considered a Medicaid enrollee.
χ2 test for trend was performed to test if there was a significant increase in second-dose coverage over time. χ2 test was conducted to examine whether second-dose coverage was significantly different across subgroups. Multivariable logistic regression models were used to estimate independent associations of demographic and SES characteristics with the uptake of the second-dose vaccine in each year during 2007 to 2009. Pearson correlation coefficients were first calculated to identify associations (potential collinearity) between the SES indicators. No strong collinearity was found, although neighborhood education and median household income levels were positively correlated (ρ = 0.65). On the basis of the results of model fitting tests and prior knowledge, all demographic and SES indicators were included in the final model. All analyses were conducted by using SAS statistical software, version 9 (SAS Institute, Inc., Cary, NC).
We included 389,498 members as of September 30, 2006, 423,017 members as of September 2007, 449,963 members as of September 2008, and 476,941 members as of September 2009 in the prevalence cohorts (Table, Supplemental Digital Content 1, http://links.lww.com/INF/A749). The majority in this study population (approximately 50%) were Hispanic. About 95% of the second doses were given at a KPSC facility.
A rapid uptake of the second-dose varicella vaccine was observed after the implementation of the new recommendation, especially among children aged 5 to 6 years (Fig. 1). The overall second-dose coverage increased from 42% as assessed in 2007 to 75% in 2009 (P for trend <0.0001). The 5 to 6 year olds had the highest rate of second-dose coverage, which reached over 90% as of September 30, 2009 (Table, Supplemental Digital Content 1, http://links.lww.com/INF/A749). White children and adolescents had relatively lower overall and age-specific second-dose coverage compared with the other groups defined by race/ethnicity. The overall coverage varied little among Hispanics, blacks, and Asians. Age-specific rates indicated that the highest rates were observed in 5-year-old Asian children in 2007 (78.9%) and 2008 (92.6%), and 6-year-old Asian children in 2009 (94.9%). Subjects who had delayed first dose (later than 15 months) also had lower second-dose coverage (Table, Supplemental Digital Content 1, http://links.lww.com/INF/A749). No significant difference was found between males and females in all age groups (data not shown). In the multivariable analysis for 2009 data, second-dose uptake was inversely associated with White race/ethnicity (vs. Hispanic, odds ratio [OR]: 0.75, 95% confidence interval [CI]: 0.73–0.76), and neighborhood education level (50%–75% adults with high school degree: OR [95% CI]: 0.89 [0.87–0.91]; >75% adults with high school degree: OR [95% CI]: 0.79 [0.77–0.81]; reference group: ≤50% adults with high school degree). Medicaid enrollees were more likely to seek the second-dose (OR [95% CI]: 1.17 [1.15–1.20]). However, the neighborhood median annual household income was not strongly associated with second-dose uptake ($60,001–$80000 vs. $40,001–$60000: OR [95% CI]: 1.05 [1.03–1.07]; >$80,000 vs. $40,001–$60,000: OR [95% CI]: 1.06 [1.04–1.08]). Similar associations were observed in 2007 and 2008 (data not shown).
After the implementation of 2-dose varicella vaccination program, the second-dose varicella vaccination coverage increased rapidly in KPSC. However, it should be recognized that the estimates in this study may not be directly comparable to the results from other studies because of different methods and designs. The second-dose coverage in this study was calculated among children/adolescents with no recorded history of varicella and with at least 1 dose of varicella vaccine recorded in KITS. The estimates from West Philadelphia in children (2-dose coverage in 2008, 51% in 5 to 9 year olds and 62% in 6 year olds) were limited to those with at least 1 documented dose of any kind of vaccine, and no exclusion was made on the basis of the history of varicella.7 The 2-dose coverage estimated by National Immunization Survey (NIS)8 was limited to adolescents with no recorded history of varicella, but not limited to those who received at least 1 dose of vaccine. If we assume that the 1-dose coverage in our population was similar to the national level (ie, 90%) in 2009, the 2-dose coverage comparable to the NIS estimate would be approximately 82.2% (91.3% × 90%) among 5-year olds, 83.6% (92.9% × 90%) among 6-year olds. The 2-dose coverage in 13 to 15 and 16 to 17 year olds would be 66.4% (73.8% × 90%) and 61.9% (68.8% × 90%), respectively, which were higher than the national (48.6%) and California (56.9%) coverage in 13 to 17 year olds estimated by NIS.
Previous studies have suggested that highly educated parents are less likely to vaccinate their children and rates of compliance are higher in Hispanic and non-Hispanic black low income families.9,10 Together, these findings suggest that SES and race/ethnicity may play a role in childhood vaccine uptake. Results from this study concur with the findings from those studies. In addition, this study was done in an insured population which allows for evaluation of the impact of race/ethnicity and education on vaccination decisions independent of income, because all subjects had relatively comparable access to health care. However, it should be noted that education and income levels were estimated using US Census block data, which may not necessarily represent the actual household education and income levels.
Several factors might have contributed to the rapid uptake of second-dose in KPSC. First, all health plan members have minimum out-of-pocket copay and vaccines routinely recommended are offered to children and adolescents without out-of-pocket cost. Second, the integrated system of the KITS and electronic health record allows the generation of alerts that remind the physician or other providers that a vaccine is due or has been delayed. This electronic health record-enabled technology likely improved the vaccine coverage and ensured up-to-date vaccination. In addition, members also have access to their vaccination records and information about future recommended vaccines via the internet.
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