PEDIATRICS Vol. 110 No. 5 November 2002, pp. 935-939
Timeliness of Childhood Immunizations
From the National Immunization Program, Centers for Disease Control and Prevention, Atlanta, Georgia
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Objective.To examine the timeliness of vaccine administration among infants and young children in the United States.
Methods. We analyzed age at receipt of vaccines among 16 211 children aged 24 to 35 months in the 2000 National Immunization Survey and examined receipt at the recommended time of each dose and selected vaccination series, as well as receipt at 4 additional time frames: acceptably early, late, never by 24 months, and too early to be considered valid. We also examined the relationship between timeliness of vaccinations and characteristics of the child, mother, and immunization provider, using multivariate logistic regression.
Results. Only 9% of children received all recommended vaccines at the recommended ages. The rates varied significantly by antigen, ranging from 24% for all Haemophilus influenzae type b doses to 75% for all hepatitis B doses as recommended. Overall, 55% of children did not receive all recommended doses by 24 months of age, and 8% of children received at least 1 vaccination dose too early to be considered valid. Factors associated with not receiving all vaccines as recommended were having more children in the household, mothers younger than 30 years, use of public providers, and multiple vaccination providers.
Conclusions. By 24 months of age, 9 of 10 children received at least 1 vaccine outside the recommended age ranges. High vaccination status of children at 24 months of age does not reflect the reality that many vaccinations are not given at the appropriate ages. Timeliness of vaccination is critical to prevent disease outbreaks, protect children through their first 2 years of life, and minimize the need to repeat doses.
Key Words: childhood immunizations • vaccination • vaccination coverage • timeliness • age-appropriate
Abbreviations: NIS, National Immunization Survey • ACIP, Advisory Committee on Immunization Practices • DTaP/DTP, diphtheria and tetanus toxoids and acellular or whole-cell pertussis • MMR, measles-mumps-rubella • Hib, Haemophilus influenzae type b • MSA, metropolitan statistical area
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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In the United States, the national childhood immunization schedule calls for children to receive approximately 15 vaccinations by 19 months of age and specifies ages for administration of each vaccine dose.1 These ages were selected to maximize protection as early as possible while minimizing potential risks and have been validated in controlled clinical trials.2
Previously published studies of national immunization rates have focused on whether children accumulate the recommended number of vaccinations by a certain age, usually by 19 to 35 months.3 In 2001, the percentage of children who received all recommended doses by selected milestone ages (3, 5, 7, 13, 19, and 24 months) was reported.4 Although this study provided more information than previously available regarding timing of vaccinations, it did not reveal whether doses were administered during the recommended age ranges. Although some studies have examined timeliness of vaccinations, their scope has been limited to specific states,5,6 counties,7,8 cities,9–12 or health care providers.13,14 This study is the first to examine the timeliness of vaccine administration among a nationally representative sample of children in the United States.
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METHODS |
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National Immunization Survey
The National Immunization Survey (NIS) has been conducted by the Centers for Disease Control and Prevention since 1994 to estimate vaccination coverage rates for the US noninstitutionalized population of children between the ages of 19 and 35 months. The NIS is a random-digit-dialing telephone survey of households with age-eligible children followed by a mail survey of the children’s vaccination providers to validate immunization information.
The current study was based on data collected in January to December 2000, during which 35 960 households with age-eligible children were identified, and complete interviews were conducted for 93%. Analyses of NIS data are restricted to children whose immunization history is verified by their health care providers (67%). We further restricted our analysis to the 71% of the children sampled who were at least 2 years of age to assess vaccinations obtained during the first 2 years of life. In total, we analyzed data for 16 211 children aged 24 to 35 months. The weighting estimation methodology of the NIS adjusts vaccination coverage estimates for household nonresponse, households with multiple telephone lines or without telephones, and vaccination history nonresponse.15 Details of the NIS methods, including Institutional Review Board approval for analysis of NIS data, appear elsewhere.16,17
Definitions
Vaccination doses recommended by the Advisory Committee on Immunization Practices (ACIP), the American Academy of Pediatrics, and the American Academy of Family Physicians during the period that children in this survey were younger than 2 years (1996–2000) include 4 doses of diphtheria and tetanus toxoids and acellular or whole-cell pertussis vaccine (DTaP/DTP), 3 doses of poliovirus vaccine, 1 dose of measles-mumps-rubella vaccine (MMR), 3 or 4 doses of Haemophilus influenzae type b vaccine (Hib), 3 doses of hepatitis B vaccine, and 1 dose of varicella vaccine.18
We analyzed receipt of these vaccine doses according to the schedule approved by the ACIP, which includes recommended ages for routine administration and minimum ages at which doses are considered valid (Table 1). The ACIP acknowledges that vaccines administered before the recommended ages but after minimum acceptable ages may not be optimal but will likely lead to adequate protection. To assist providers in determining whether doses administered early should be readministered, the ACIP defined a 4-day grace period before the minimum age. Vaccinations administered during this grace period are also considered acceptable; those administered before this grace period should be repeated.2
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In this analysis, age recommendations given in months and weeks were converted to days because the number of days in a month varies. We considered each recommended age range to begin at the fewest number of days and to end at the greatest number of days that could compose the given number of months. For example, the recommended age of 2 months equated to an age range of 59 through 91 days. This lenient interpretation of the recommendations may slightly overestimate the number of children vaccinated at the recommended ages.
For each dose, we report the percentage of children who received the dose: 1) within the recommended age range (recommended), 2) within 4 days before the minimum acceptable age until the routinely recommended age (acceptably early), 3) after the recommended age range but before 24 months (late), 4) never received before 24 months old (never), and 5) too early to be considered valid (ie, >4 days before the minimum acceptable age; invalid). We also report the percentage of children who received at least 1 vaccine dose within these time frames, as well as the percentage of children who received all doses in selected series as recommended. We did not evaluate intervals between doses to exclude doses administered too close together.
Hib vaccine is unique because a complete series may be composed of either 3 or 4 doses. For most Hib vaccines, a 4-dose schedule is recommended (at 2, 4, 6, and 12–15 months); however, the 6-month dose is not needed if Haemophilus influenzae type b conjugate vaccine (meningococcal protein conjugate) (PedvaxHIB or ComVax [Merck, Whitehouse Station, NJ]) is used for the 2-month and 4-month doses.18 Vaccine manufacturer information is not collected in the NIS, so some lenient assumptions were made for this study. Children who received 4 or more doses of Hib were assumed to be on the 4-dose schedule (72% of the children). Children who received 0, 1, or 2 doses were assumed to be on the 3-dose schedule (ie, assumed not to need the 6-month dose; 8%). For children who received 3 doses of Hib, determination of the need for the 6-month dose was based on the age at dose 3. Children who received dose 3 between 12 and 15 months of age (10%) were assumed to be on the 3-dose schedule. Children who received dose 3 during or before 6 months of age (6%) were assumed to be on the 4-dose schedule. For those who received dose 3 between 7 and 11 months of age, simultaneous receipt with DTaP/DTP was considered. Usually, providers administer all vaccine doses due at a given age during the same office visit,19 and for the first 3 doses, the DTaP/DTP and 4-dose Hib recommended schedules are identical. Therefore, when only 3 doses of Hib were administered and the third doses of Hib and DTaP/DTP were administered simultaneously between 7 and 11 months (3%), we assumed that the child was following the 4-dose Hib schedule. When dose 3 of Hib was administered between 7 and 11 months but not with dose 3 of DTaP/DTP (<1%), we assumed that the child was following the 3-dose Hib schedule.
To describe further the factors associated with timeliness of vaccine administration, we evaluated various characteristics of the child, mother, and immunization providers. In the household portion of the survey, parents or caregivers reported race/ethnicity of the child, number of children in the household, and education level of the mother. Poverty status was determined on the basis of household size, composition, and income reported by the survey respondent, as defined by the United States Census Bureau.20 Urbanicity (metropolitan statistical area [MSA] central city, MSA noncentral city, non-MSA) was determined by respondents’ telephone area code/exchange.21 Provider facility type (public, private, hospital, or other) was reported by the providers. The number of providers was based on the number of usable vaccination records submitted for the child.
Statistical Analysis
Percentage estimates and their 95% confidence intervals were calculated using SUDAAN version 7.5.3, a statistical package used to analyze complex survey data.22 Multivariate logistic regression was used to test simultaneously the significance of maternal, child, and provider characteristics associated with not receiving all vaccines as recommended.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Table 2 shows the percentage of children who received each vaccine dose within the 5 time frames analyzed. Most children received each individual vaccine dose during the recommended ages, with considerable variability by dose (93% for the first dose of hepatitis B to 45% for the final dose of Hib vaccine). However, many vaccinations were administered at ages that are considered acceptable but younger than recommended. Overall, 38% of children received at least 1 vaccine dose at an acceptably early age before the recommended age, ranging from approximately 1% of children receiving the second and third doses of hepatitis B, the last dose of Hib, and the single doses of varicella and MMR vaccines acceptably early to approximately 16% receiving the first doses of DTaP, Hib, and poliovirus vaccines acceptably early. Approximately 58% of children received a vaccination later than recommended; this ranged from 2% for the third dose of hepatitis B to 27% for the third dose of DTaP/DTP. Most children (55%) did not receive all recommended vaccinations by 24 months; this ranged from 3% for the first doses of hepatitis B, Hib, and poliovirus to 36% for varicella. Overall, 8% of children received at least 1 dose too early to be considered valid, with nearly 5% receiving the third dose of hepatitis B before the valid age. Invalid doses probably are underestimated, because we did not exclude doses considered invalid as a result of improper spacing.
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Only 9% of children received all recommended vaccinations at the recommended ages (Table 3). The percentage of children who received all doses as recommended varied substantially by antigen, with highest rates for children who received all hepatitis B doses (75%) and the single MMR dose (73%) as recommended, and lowest rates for children who received all DTaP/DTP doses (27%) and all Hib doses (24%) as recommended. Hib doses received as recommended may be slightly overestimated as a result of assumptions made regarding the 3-dose versus 4-dose schedule. All rates increased when the recommended age and the acceptably early window were considered together, with the overall rate increasing to 18%.
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Timeliness of vaccinations varied by characteristics of the child, mother, and providers. We evaluated the effect of race/ethnicity of the child, number of children in the household, education level of the mother, poverty status, urbanicity, provider facility type, and the number of providers. In multivariate analyses, factors significantly associated with not receiving all vaccinations as recommended were having more children in the household (P < .01), a mother younger than 30 years (P = .03), a public vaccination provider (P < .01), and multiple vaccination providers (P < .01).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Vaccine-preventable disease incidence is generally low in the United States.23 However, studies of measles, Hib, and pertussis reinforce the importance of timeliness of vaccination. Investigation of the measles epidemic of 1989 to 1991 in the United States led the National Vaccine Advisory Committee and the Centers for Disease Control and Prevention to conclude that the primary cause for the epidemic was a failure to provide vaccines on schedule24 and that "only a sustained effort to provide age-appropriate vaccination will prevent another resurgence of measles."25 Although the incidence of Hib is low, illness and death still occur among infants who have not completed the primary series of Hib vaccination. Experts have concluded that eliminating Hib will require improved age-appropriate vaccination of children.26 Pertussis incidence is increasing in infants who are too young to receive 3 doses of DTaP/DTP (ie, younger than 6 months), which is also the age group with the highest burden of disease and the highest rates for complications and death.27 The current study found that 27% of children received their third dose of DTaP/DTP late. Timely vaccinations protect children as early as possible and prevent disease outbreaks. In addition, delay in receiving vaccinations has been shown to have negative implications for the receipt of other preventive health care services.28
Another consequence of not adhering to the recommended childhood immunization schedule is that doses given too early to be considered valid must be readministered. This practice may result in unnecessary risk of adverse reaction and increased costs.29 In this study, we found that 8% of children had invalid doses administered before the minimum age requirements.
Recommendations for the early childhood vaccination schedule are based on ages that have been shown to be safe and effective in controlled clinical trials. The safety and efficacy of early and late vaccination have been evaluated with some antigens such as measles,30–32 hepatitis B,33 and the combination DTaP/Hib vaccine34 but are not known in all cases and may vary by antigen and by dose. Until studies that demonstrate the efficacy of vaccinations given outside the recommended time periods are conducted, providers and parents should adhere to published guidelines.
At every health care visit during early childhood, providers should conduct a thorough assessment of vaccination status, including determining whether the child lacks any doses recommended for his or her age, whether doses already received should be counted as valid according to the recommendations, and when the child should return for the next recommended vaccinations. Providers report difficulty in assessing vaccination status,35 especially when they have limited understanding of the immunization schedule.36 In the future, schedule complexity will increase, as the number of vaccines is expected to triple by 2020.37 In addition, development of new combination vaccines,38 which are beneficial in reducing the number of simultaneous injections, adds to this complexity because physicians must make decisions about which combination vaccines to use in their practice and whether all vaccines in the combination are indicated for the given child. For children who fall behind schedule, the task of catching up as soon as possible is particularly complex. Computer software systems can assist providers in accurately assessing each child’s immunization history and current needs and in making plans for future health care visits.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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By 24 months of age, 9 of 10 children had received at least 1 dose of vaccine outside the recommended age ranges. National vaccination coverage estimates for doses completed by 24 months are at near record-high levels.39 However, children who are completely vaccinated at 2 years of age could have been undervaccinated during much of their first 2 years—the time when children are most at risk of vaccine-preventable diseases. Focusing on vaccination status of children at 2 years or at any single age can fail to detect the many vaccinations not given at the appropriate ages.
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ACKNOWLEDGMENTS |
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We thank Dr Robb Linkins for review of the manuscript.
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FOOTNOTES |
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Received for publication Feb 19, 2002; Accepted May 31, 2002.
Reprint requests to (E.T.L.) Centers for Disease Control and Prevention, 1600 Clifton Rd NE, Mail Stop E-62, Atlanta, GA 30333. E-mail: ecl7{at}cdc.gov
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PEDIATRICS (ISSN 1098-4275). ©2002 by the American Academy of Pediatrics
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