Months undervaccinated is based on cumulative days undervaccinated during
the first 24 months of life for 1 or more doses of a recommended vaccine.
Delay begins after the end of a recommendation period and continues until
the child is vaccinated or reaches 24 months of age. Vaccines delayed out
of 6 vaccines: diphtheria and tetanus toxoids and acellular pertussis; poliovirus;
measles, mumps, and rubella; Haemophilus influenzae type
b; hepatitis B; and varicella. A child is up-to-date if he/she received 4
of more doses of diphtheria and tetanus toxoids and acellular pertussis vaccine;
3 or more doses of poliovirus vaccine; 1 or more doses of measles, mumps,
and rubella vaccine; 3 or more doses of hepatitis B vaccine; 3 or more doses
of hepatitis B vaccine; and 1 or more doses of varicella vaccine. Percentage
of children based on weighted data. Asterisk indicates that data are from
the 74% (SE, 0.6%) of children who were up-to-date at the time of the household
interview (age, 24-35 months). Dagger indicates that data are from the 67%
(SE, 0.2%) of children who were up-to-date at age 24 months.
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Luman ET, Barker LE, Shaw KM, McCauley MM, Buehler JW, Pickering LK. Timeliness of Childhood Vaccinations in the United States: Days Undervaccinated and Number of Vaccines Delayed. JAMA. 2005;293(10):1204–1211. doi:10.1001/jama.293.10.1204
Author Affiliations: National Immunization
Program, Centers for Disease Control and Prevention, Atlanta, Ga (Drs Luman,
Barker, and Pickering and Mss Shaw and McCauley); and Rollins School of Public
Health, Emory University, Atlanta, Ga (Drs Luman and Buehler).
Context Only 18% of children in the United States receive all vaccinations at
the recommended times or acceptably early.
Objective To determine the extent of delay of vaccination during the first 24
months of life.
Design, Setting, and Participants The 2003 National Immunization Survey was conducted by random-digit
dialing of households and mailings to vaccination providers to estimate vaccination
coverage rates for US children aged 19 to 35 months. Data for this study were
limited to 14 810 children aged 24 to 35 months.
Main Outcome Measures Cumulative days undervaccinated during the first 24 months of life for
each of 6 vaccines (diphtheria and tetanus toxoids and acellular pertussis;
poliovirus; measles, mumps, and rubella; Haemophilus influenzae type b; hepatitis B; and varicella) and all vaccines combined, number
of late vaccines, and risk factors for severe delay of vaccination.
Results Children were undervaccinated a mean of 172 days (median, 126 days)
for all vaccines combined during their first 24 months of life. Approximately
34% were undervaccinated for less than 1 month and 29% for 1 to 6 months,
while 37% were undervaccinated for more than 6 months. Vaccine-specific undervaccination
of more than 6 months ranged from 9% for poliovirus vaccine to 21% for Haemophilus influenzae type b vaccine. An estimated 25%
of children had delays in receipt of 4 or more of the 6 vaccines. Approximately
21% of children were severely delayed (undervaccinated for more than 6 months
and for ≥4 vaccines). Factors associated with severe delay included having
a mother who was unmarried or who did not have a college degree, living in
a household with 2 or more children, being non-Hispanic black, having 2 or
more vaccination providers, and using public vaccination provider(s).
Conclusions More than 1 in 3 children were undervaccinated for more than 6 months
during their first 24 months of life and 1 in 4 children were delayed for
at least 4 vaccines. Standard measures of vaccination coverage mask substantial
shortfalls in ensuring that recommendations are followed regarding age at
vaccination throughout the first 24 months of life.
The standard measure of vaccination coverage is the percentage of children
who have accumulated the required number of vaccine doses, without regard
to timing of the vaccinations. However, to maximize protection from vaccine-preventable
diseases, the recommended childhood immunization schedule specifies ages at
which each of the approximately 20 vaccinations should be administered during
the first 18 months of life.1
Remaining appropriately vaccinated at all times decreases a child’s
risk of contracting vaccine-preventable diseases and prevents disease outbreaks.2,3 Previous studies that examined timeliness
of vaccinations have been limited in scope.4-20 A
more extensive national study published in 2002 found that 18% of children
received all vaccinations at the recommended ages or acceptably early (ie,
within minimum age allowances) and only 9% at the recommended ages.21 However, this study did not examine the degree of
delay, such as duration of time that children were undervaccinated or the
number of vaccines delayed.
In this study, we examine timeliness of receipt of vaccination among
a nationally representative sample of children in the United States for each
recommended vaccine and for all vaccines combined. We evaluate the cumulative
number of days children spent undervaccinated during the first 24 months of
life, the number of late vaccinations, risk factors for severe delay, and
the relationship between delay and traditional estimates of vaccination coverage.
We analyzed data from the 2003 National Immunization Survey (NIS), which
was conducted by the Centers for Disease Control and Prevention to estimate
vaccination coverage rates for US children aged 19 to 35 months. The NIS uses
random-digit dialing to survey households with age-eligible children, followed
by a survey mailed to the children’s vaccination providers to validate
vaccination information.22 The caregiver provided
verbal consent for participation. The NIS was approved by the Centers for
Disease Control and Prevention’s institutional review board and the
current study was approved by the National Immunization Program, Centers for
Disease Control and Prevention.
This analysis is based on children with a completed interview (89% of
households identified with an age-eligible child) and adequate vaccination
history information from the vaccination provider(s) (69% of children with
a completed interview). We further restricted our analysis to children who
were at least 2 years old at the time of the interview with the caregiver
(70%) to assess vaccinations received during the first 24 months of life.
In total, we analyzed data for 14 810 children aged 24 to 35 months.
Vaccination doses routinely recommended in early childhood by the Advisory
Committee on Immunization Practices, the American Academy of Pediatrics, and
the American Academy of Family Physicians include 4 doses of diphtheria and
tetanus toxoids and acellular pertussis vaccine (DTaP); 3 doses of poliovirus
vaccine; 1 dose of measles, mumps, and rubella vaccine (MMR); 3 or 4 doses
(depending on vaccine manufacturer) of Haemophilus influenzae type b vaccine (Hib); 3 doses of hepatitis B vaccine; and 1 dose of
varicella vaccine.1 Pneumococcal conjugate
vaccine and influenza vaccine also are recommended routinely during early
childhood1; however, because recommendations
for their use are relatively new23,24 and
coverage has not yet reached levels of other recommended childhood vaccines,25 they are not included in this analysis. For simplicity,
single-disease (poliovirus, Hib, hepatitis B, and varicella) and standard
combined-disease formulations (DTaP and MMR) are referred to as “vaccines”
throughout this report.
We analyzed receipt of vaccines according to the harmonized schedule
approved by the Advisory Committee on Immunization Practices, the American
Academy of Pediatrics, and the American Academy of Family Physicians, which
includes recommended ages and age ranges for routine administration, minimum
ages at which doses are considered valid, and minimum intervals between doses
within a series (Table 1).1 We
excluded vaccine doses that were administered invalidly early, which was defined
by the Advisory Committee on Immunization Practices as a dose administered
prior to 4 days before the minimum acceptable age or interval.
For each day during the first 24 months of life for each child in the
survey, we determined vaccination status for each recommended vaccine. Vaccination
dates and birth date were used to determine age at vaccination. Age recommendations
given in months and weeks were converted to days. We considered each recommended
age to end at the greatest number of days that could compose the given number
of months (Table 1). For example, the
recommended age of 2 months ends when the child turns 3 months old at 90 to
92 days. For this analysis, a child was considered undervaccinated at 93 days
if he/she had not received 1 or more of the doses recommended at 2 months
Days during which the child was undervaccinated for each specific dose
accumulated until the child was vaccinated or reached 24 months of age. Days
of undervaccination for multidose vaccines were calculated by summing all
days during which the child was undervaccinated for 1 or more doses of the
respective vaccine. For example, suppose a child started the DTaP series late
at age 165 days (5½ months), received the second dose at age 210 days
(7 months), the third dose at age 270 days (9 months), and the fourth dose
on schedule at 550 days (18 months). The child is considered undervaccinated
for dose 1 on days 93 to 164, for dose 2 on days 154 to 209, and for dose
3 on days 215 to 269. Days in overlapping periods of undervaccination (days
154-164) are only counted once, so the child would be considered undervaccinated
for DTaP for a total of 172 days (days 93-209 and days 215-269).
When a vaccine dose is given so late that the next dose in the series
cannot be given during the routinely recommended time frame, a catch-up schedule
is used that specifies minimum intervals between catch-up doses. For our primary
analysis described above, all days during which a child was not vaccinated
as routinely recommended were counted as undervaccinated. However, we also
explored the impact of not counting days during these required intervals.
While children are still underprotected during the required catch-up intervals,
this more lenient outcome measure accounts for the fact that the subsequent
dose is not yet due. For DTaP, a 28-day interval is required between doses
1 and 2 and doses 2 and 3. Thus, in the above example, the child would be
considered undervaccinated by this more lenient definition on days 93 to 164
for dose 1, days 193 to 209 for dose 2, and days 239 to 269 for dose 3, for
a total of 120 days.
Days undervaccinated for the complete vaccine series were calculated
by summing all days during which the child was undervaccinated for 1 or more
doses of any vaccine(s). Because periods of undervaccination for different
vaccines and doses may overlap, the total number of days undervaccinated may
be less than the sum of the vaccine- and dose-specific days.
For each vaccine and for all vaccines combined, we report the mean number
of days that children were undervaccinated during the first 24 months of life.
We report the percentage of children who were delayed 0 days, 1 to 7 days,
8 to 31 days, 1 to 2 months, 3 to 6 months, 7 to 12 months, or more than 12
months. We also examined the number of vaccines for which children were delayed.
We further examined children who were severely delayed in their vaccinations,
which we defined as being undervaccinated for more than 6 months and delayed
for 4 or more of the 6 vaccines.
To illustrate differences between these measures of timeliness and standard
measures of vaccination status, we examined the relationship between delay
and up-to-date status at the time of interview as well as up-to-date status
at 24 months of age. For a child to be considered up-to-date, he/she must
have received at least 4 doses of DTaP vaccine, 3 doses of poliovirus vaccine,
1 dose of MMR vaccine, 3 doses of Hib vaccine, 3 doses of hepatitis B vaccine,
and 1 dose of varicella vaccine.
For vaccine doses due at 2 months (DTaP, hepatitis B, Hib, and poliovirus),
the total possible delay is 21 months (ie, 24 months minus 3 months, at which
time the child becomes undervaccinated if missing the first dose). For the
single dose of MMR, the total possible delay is 8 months because undervaccination
begins at 16 months. Similarly, for doses not due until 18 months (the third
dose of hepatitis B vaccine, the fourth dose of DTaP vaccine, the third dose
of poliovirus vaccine, and the single dose of varicella vaccine), the total
possible delay is only 5 months. Thus, these later doses have limited potential
impact on the number of days undervaccinated during the first 24 months of
Several complexities of the vaccination recommendations were incorporated
in this analysis. First, children who receive the first dose of Hib vaccine
after 6 months of age need fewer Hib doses; thus fewer doses were required
for these children to be considered completely vaccinated.26 Second,
necessity of the 6-month dose of Hib vaccine depends on the manufacturer of
the doses given at ages 2 and 4 months.26 Because
the NIS does not collect manufacturer information, lenient assumptions were
made regarding the need for the 6-month dose, as previously described.21 In brief, children who received 4 doses were assumed
to be following the 4-dose schedule (ie, to need a dose at 6 months), while
children who received fewer than 3 doses were assumed to be following the
3-dose schedule. For children who received 3 doses, need for the 6-month dose
was assessed based on the timing of the third dose. Finally, because varicella
vaccination is not required for children with natural immunity,27 we
considered varicella vaccination to be satisfied by either administration
of the vaccine or reported chickenpox disease.
To identify factors associated with severe delay of vaccination, characteristics
of the child, mother, and immunization providers were evaluated: caregiver-reported
race/ethnicity of the child (based on standard options); number of children
in the household; mother’s marital status, age, and education level;
family poverty status28; residence in a metropolitan
statistical area and census region derived from telephone numbers; and number
of responding providers and their facility types (public, private, or other/mixed).
Estimates of percentages, means, odds ratios, and SEs were calculated
using SUDAAN statistical software (version 8.0, Research Triangle Institute,
Research Triangle Park, NC).29 Bivariate analysis
and factor-level χ2 tests were conducted to assess risk factors
for severe delay of vaccinations in early childhood. The level of significance
was set a priori at .05. To assess simultaneous contributions of each factor
on risk of severe delay, we constructed multivariate logistic regression models
with children who were least likely to be severely delayed in the bivariate
analysis serving as the reference group for each factor. We conducted standard
backward elimination, removing variables in the model until all remaining
factors had P values of less than .10, to arrive
at a final main-effects model describing factors associated with severe delay.
All analyses were weighted to account for the complex sampling design of the
Children were undervaccinated a mean of 172 days (median, 126 days)
cumulatively during the first 24 months of life (Table 2). Approximately 74% of children were delayed for 1 or more
vaccinations during the first 24 months of life; among them, the mean days
undervaccinated was 232. Relatively few children had short periods of delay
(3% for 1-7 days, 6% for 8-31 days, and 12% for 1-2 months) while more children
were delayed for longer periods (17% for 3-6 months and 22% for 7-12 months).
Approximately 15% of all children were undervaccinated for more than 12 of
their first 24 months of life. Among children with any delay, 52% were undervaccinated
for more than 6 months.
Children were less likely to be delayed for each individual vaccine
than for the series as a whole. The percentage of children with any delay
ranged from 54% for the recommended doses of Hib vaccine and 48% for DTaP
vaccine to 32% for poliovirus vaccine and less than 30% for hepatitis B, MMR,
and varicella vaccines (Table 2). However,
considerable numbers of children remained undervaccinated with each vaccine
for a substantial portion of their first 24 months of life. Approximately
10% to 20% of children were undervaccinated for more than 6 months for DTaP,
hepatitis B, Hib, and poliovirus vaccines. For MMR and varicella vaccines,
the total possible delay is considerably shorter. Nevertheless, 11% of children
were undervaccinated for 7 or more of the possible 8 months of delay for MMR
vaccine and 20% were undervaccinated for 3 or more of the possible 5 months
of delay for varicella vaccine.
The number of vaccines for which children had delays is presented in Table 3. Overall, 47% of children had no delayed
vaccinations or were delayed for only 1 of the 6 vaccines. Approximately 28%
of children had delays for 2 or 3 vaccines, and 25% had delays in receipt
of 4 or more of the 6 vaccines.
Approximately 21% of children were undervaccinated for more than 6 months
and for 4 or more antigens (Table 3)
and were considered severely delayed in their early childhood vaccinations.
In multivariate analysis, factors associated with severe delay of vaccinations
included having a mother who was unmarried or who did not have a college degree,
living in a household with 2 or more children, being non-Hispanic black, having
2 or more vaccination providers, and using public vaccination providers (Table 4).
When the required interval between catch-up doses was not counted as
days undervaccinated, results changed only slightly. For the complete vaccination
series, days undervaccinated declined from a mean of 172 days to 163 days.
For individual vaccines, this reduction ranged from 0 to 10 days. The percentage
of children undervaccinated for more than 6 months remained approximately
the same (36% vs 37%), and the percentage of children severely delayed remained
Children who were completely vaccinated at the time of the household
interview or at 24 months had fewer days undervaccinated and fewer vaccines
delayed than did those who were not completely vaccinated. However, even among
the 74% of children who were up-to-date at the time of the interview, 23%
were undervaccinated for more than 6 months during the first 24 months of
life, with fewer than half undervaccinated for less than 1 month (Figure). In addition, 42% were delayed for multiple
vaccines. Similarly, among the 67% of children who were up-to-date at 24 months
of age, 18% were undervaccinated for more than 6 months and 37% were delayed
for multiple vaccines. Traditionally estimated vaccination coverage decreased
with longer period undervaccinated; vaccination coverage at the time of interview
was 100% among children undervaccinated for less than 1 month, 82% (SE, 0.9%)
among children undervaccinated for 1 to 6 months, and 46% (SE, 1.1%) among
children undervaccinated for more than 6 months. Twenty-four month coverage
was 100% among children undervaccinated for less than 1 month, 74% (SE, 1.1%)
among children undervaccinated for 1 to 6 months, and 32% (SE, 1.0%) among
children undervaccinated for more than 6 months.
Vaccination coverage rates have reached record-high levels in the United
States, with estimated coverage higher than 85% among children aged 19 to
35 months for DTaP and varicella vaccines, higher than 90% for poliovirus,
MMR, hepatitis B, and Hib vaccines, and 73% for the combined series of all
of these vaccines.25 These results highlight
substantial successes of national, state, and local vaccination programs in
increasing the proportion of children who have accumulated the required number
of vaccinations by 19 to 35 months of age. However, reliance on this indicator
alone masks substantial delays during the first 2 years of life prior to becoming
Few children receive all vaccinations on time.17,18,21,30 Moreover,
we found that the delays are not trivial: only 1 in 3 children were age-appropriately
vaccinated or undervaccinated for less than 1 month cumulatively during the
first 24 months of life; 37% of children were undervaccinated for more than
6 months; and more than half of children were delayed with multiple vaccines.
Among children with any delay, most were undervaccinated for more than 6 months.
Even among children who would be considered up-to-date by traditional coverage
estimates, 23% were undervaccinated for more than 6 of the first 24 months
of life and 42% were delayed for multiple vaccines. These results confirm
that opportunity exists for improvement in vaccine administration in the United
States to ensure that all children remain fully vaccinated and optimally protected
from vaccine-preventable diseases throughout early childhood—the time
when children are most at risk for illness and severe complications from many
Most troubling are children who are undervaccinated for more than 6
months during the first 24 months of life and those who are delayed for 4
or more vaccines, and especially the 21% of children who are severely delayed.
These delays signify problems with access to care and vaccine delivery and
highlight risk of disease more than do delays of a few days or delays in receipt
of a single vaccine. Furthermore, most children who were undervaccinated for
more than 6 months did not catch up by the time the caregiver was interviewed
for the NIS when the child was aged 24 to 35 months.
Risk of disease due to delay in vaccine administration varies, and depends
on the vaccine, disease circulation, transmissibility, likelihood of importation,
and severity of outcome. While vaccine-preventable disease incidence is generally
low in the United States,31 vaccination timeliness
is particularly important for diseases that have the potential to cause large
outbreaks, such as measles, and for diseases that currently are circulating,
such as pertussis.
We found that 11% of children were undervaccinated for measles for more
than 6 of a possible 8 months during the first 24 months of life and another
3% were undervaccinated for 3 to 6 months. Because only a single dose of measles
vaccine is recommended in early childhood, these delays represent a period
of complete lack of protection against measles. Elimination efforts have greatly
reduced the incidence of measles in the United States to only 56 reported
cases in 2003.31 However, risk of importation
remains high due to global measles circulation, high rates of transmission,
and the large volume of travelers to and from the United States.32 With
each imported measles case, population immunity is tested and an outbreak
or epidemic could result if there are enough susceptible hosts due to delays
in vaccination, lack of vaccination, or inadequate immune response to vaccination.
The US measles epidemic in 1989 to 1991 was caused by a failure to provide
timely vaccination,33 and according to the
Centers for Disease Control and Prevention, “only a sustained effort
to provide age-appropriate vaccination will prevent another resurgence of
Pertussis is another example of the importance of timely vaccination.
During 2003, 11 647 cases of pertussis were reported in the United States31; incidence was highest among infants who were younger
than 6 months34 and infants accounted for the
highest proportion of pertussis-related hospitalizations and deaths.35 Among infants who contracted pertussis during the
1990s, at least 44% were undervaccinated for their age. Furthermore, among
the 25 pertussis-related deaths in infants aged 2 to 11 months, 15 had not
received any doses of pertussis vaccine.36 While
these children were too young to have received the complete 4-dose series
of DTaP, data suggest that the risk of pertussis-related hospitalization is
decreased if children have received 1 or 2 doses of vaccine.35 Furthermore,
because siblings are a source of transmission to infants too young to be vaccinated,36,37 timely vaccination of these children
can indirectly protect young infants by decreasing their exposure. We found
that 16% of children were undervaccinated for DTaP for more than 6 of the
first 24 months of life and another 14% were undervaccinated for 3 to 6 months.
In addition to risk of disease and disease outbreaks, delayed vaccinations
have administrative implications. Vaccine providers report difficulty in assessing
vaccination status38 and determining appropriate
catch-up regimens for children who have fallen behind in their vaccinations.39 Also, children who fall behind are less likely to
be fully vaccinated at later times.19 Preventing
children from falling behind and immediate identification of delays present
opportunities to intervene earlier and to ensure full vaccination.
This study is subject to several limitations. First, the NIS is a telephone
survey and relies on identification of vaccination providers by the household
respondent and on complete and accurate reporting of vaccination histories
by these providers. Bias may have occurred due to households without telephones
and household nonresponse. Furthermore, if some vaccination providers were
not identified or if some providers did not report complete histories, we
may have misclassified children as not having been completely vaccinated.
However, the NIS uses a variety of weighting strategies to reduce bias and
to ensure that all children in the United States are represented by children
with adequate provider data.22,40,41 Also,
the NIS lacks additional information that could provide a better understanding
of the reasons for undervaccination and how to ensure that children receive
vaccinations as recommended, such as parental reasons for delay. Information
on all health care visits would enable an assessment of the proportion of
delay attributable to missed opportunities for vaccination. Finally, vaccine
shortages in the United States during 2000 to 2002 may have affected timeliness
of MMR, DTaP, and varicella vaccinations for children in this analysis.42,43 However, the effect of the shortages
on receipt of vaccines has been shown to be small.44
Previously, we21 found that 18% of children
in the 2000 NIS received all vaccines as recommended or acceptably early.
In the current study, we found that 26% did not have any days undervaccinated.
However, these results are not directly comparable due to slight differences
in case definitions. Most notably, in the previous study we considered a child
to be delayed if they were late by any amount, while in the current study
we considered a child to be delayed only after they were late for at least
1 full day (for example, at 93 days rather than 92 days for doses due at age
2 months). When the slightly more stringent definition is applied to the current
study, 9% of children shift from the 0 days undervaccinated category to the
1 to 7 days undervaccinated category, leaving 17% of children without any
days undervaccinated, comparable with previous findings. Results for more
severe levels of delay are not affected.
Results of this study have important implications for clinical practice
and public policy. The major concern for underimmunized children is disease
occurrence, especially in the 21% of children who were severely delayed. Factors
leading to underimmunization that need to be addressed include reducing racial
and ethnic disparities, and assisting women who are unmarried, have less than
a high school education, have multiple children, have multiple vaccine providers,
and use public vaccination clinics. Interventions must be focused on plans
that could best address the needs of these mothers, such as extended office
hours for women who have difficulty taking time away from work, using appropriate
education-level information regarding safety and benefits of vaccination,
and ensuring availability of sibling child care in the workplace.
In addition, systems need to be implemented to identify children who
are behind on immunizations, which will help in 2 areas. The first is to enable
the recall of children soon after they fall behind on their immunizations,
and the second is to ensure that opportunities to immunize children when they
are seen for other medical reasons are not missed. Conversely, immunization
visits provide an opportunity for delivery of other much needed preventive
services recommended by the American Academy of Pediatrics. Immunization coverage
has been shown to be a marker for delivery of other important preventive health
care services in children.45
Assessing days undervaccinated reveals weaknesses in childhood vaccination
programs. Physicians are in the best position to assess the needs of their
patients to determine reasons for delay. Evidence-based solutions exist for
many of these needs, such as reminder-recall systems, extended office hours,
expanding availability of pediatric care, and education regarding the importance
and safety of vaccinations.46,47 Minimizing
the time spent incompletely protected from vaccine-preventable diseases is
important to the health of individuals and to public health and should be
given greater emphasis by public health programs and vaccination providers.
Corresponding Author: Elizabeth T. Luman,
PhD, National Immunization Program, Centers for Disease Control and Prevention,
1600 Clifton Rd NE, MS E62, Atlanta, GA 30333 (ECL7@cdc.gov).
Author Contributions: Dr Luman had full access
to all of the data in the study and takes responsibility for the integrity
of the data and the accuracy of the data analysis.
Study concept and design: Luman, Buehler, Pickering.
Analysis and interpretation of data: Luman,
Barker, Shaw, McCauley.
Drafting of the manuscript: Luman, Shaw.
Critical revision of the manuscript for important
intellectual content: Luman, Barker, McCauley, Buehler, Pickering.
Statistical analysis: Luman, Barker, Shaw.
Study supervision: Barker.
Financial Disclosure: None reported.
Funding/Support: This research and the National
Immunization Survey were conducted through funding by the Centers for Disease
Control and Prevention, US Department of Health and Human Services.
Role of the Sponsor: This study, including
design and conduct, data collection, analysis and interpretation of the data,
and manuscript preparation, review, and approval was conducted under the auspices
of the Department of Health and Human Services and the Centers for Disease
Control and Prevention. The Centers for Disease Control and Prevention gave
final approval of the manuscript.
Acknowledgment: We thank Karen Broder, MD,
Mark Papania, MD, and Susan Chu, PhD, for their review and suggestions.
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