*Average calculated as arithmetic mean.
The length of the light shaded bars reflects the average rate during
1990-1997 for a given age- and sex-specific category, and the darker shaded
bars indicate the equivalent 2003 rates. The difference in the bar lengths
reflects the absolute difference in rates between the 2 time periods.
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Wasley A, Samandari T, Bell BP. Incidence of Hepatitis A in the United States in the Era of Vaccination. JAMA. 2005;294(2):194–201. doi:10.1001/jama.294.2.194
Context In the United States, hepatitis A is a frequently reported vaccine-preventable
disease. Vaccination has been recommended for persons at increased risk since
1996. In 1999, it was recommended that children living in 11 states with the
highest incidence of hepatitis A be routinely vaccinated, and that children
living in 6 additional states, with incidence above the national average,
be considered for routine vaccination.
Objective To assess impact of the current vaccination strategy by evaluating trends
in reported cases of hepatitis A since implementation.
Design, Setting, and Cases A longitudinal analysis of characteristics of cases of hepatitis A reported
in the United States since 1990 to the National Notifiable Diseases Surveillance
Main Outcome Measure Incidence rates of reported cases of hepatitis A. Incidence rates in
2003 were compared with those for the prevaccination baseline period (1990-1997)
overall and in the 17 states in which children should be routinely vaccinated
or considered for routine vaccination (vaccinating states). Incidence rates
in vaccinating states were also compared with those in the remaining states
where there is no recommendation for statewide vaccination of children (nonvaccinating
Results Between the baseline period (1990-1997) and 2003, overall hepatitis
A rates declined 76% to 2.6 per 100 000, significantly lower than previous
nadirs in 1983 (9.2/100 000) and 1992 (9.1/100 000). The rate in
vaccinating states declined 88% to 2.5 per 100 000 compared with 53%
elsewhere (to 2.7/100 000). In 2003, cases from vaccinating states accounted
for 33% of the national total vs 65% during the baseline period. Declines
were greater among children aged 2 to 18 years (87%) than among persons older
than age 18 years (69%); the proportion of cases in children dropped from
35% to 19%. Since 2001, rates in adults have been higher than among children,
with the highest rates now among men aged 25 through 39 years.
Conclusions Following implementation of routine hepatitis A vaccination of children,
hepatitis A rates have declined to historic lows, accompanied by substantial
changes in the epidemiologic profile. Greater decreases in the age groups
and regions where routine vaccination of children is recommended likely reflect
the results of implementation of this novel vaccination strategy. Continued
monitoring is needed to verify that implementation continues to proceed and
that low rates are sustained.
In the United States, an average of 26 000 hepatitis A cases were
reported annually to public health agencies during the 1980s and 1990s, representing
an estimated 270 000 infections per year when anicteric disease and asymptomatic
infections are taken into account.1 More than
half of the estimated infections occurred among children. In 1995, highly
effective hepatitis A vaccines became available in the United States for use
among persons aged 2 years or older, providing an opportunity to substantially
reduce hepatitis A incidence.
In 1996, the Advisory Committee on Immunization Practices recommended
targeted hepatitis A vaccination of selected high-risk populations, such as
men who have sex with men, users of illicit drugs, and travelers to endemic
countries.2 Routine vaccination was recommended
for children living in communities with the highest hepatitis A rates, such
as Native American communities. In 1999, the Advisory Committee on Immunization
Practices expanded its recommendations for routine vaccination of children
to include children living in states that had consistently elevated hepatitis
A rates and a distinctive pattern of hepatitis A epidemiology.3 The
average rate during the previous 11-year (1987-1997) baseline period was used
to identify states targeted for childhood vaccination, which included approximately
one third of the US population but from which more than half of hepatitis
A cases were reported.
We present an analysis of data from national hepatitis A surveillance
that describes fundamental changes in the incidence and epidemiology of hepatitis
A in the United States following implementation of recommendations for hepatitis
A vaccination of children.
The incidence of hepatitis A by age, race/ethnicity, state, and year
was determined per 100 000 population using data on hepatitis A cases
reported through the National Notifiable Diseases Surveillance System (NNDSS).4 NNDSS is a passive national surveillance system through
which states voluntarily report more than 50 nationally notifiable diseases.
Hepatitis A has been a nationally notifiable condition since 1966. A reportable
case is defined as an acute illness with discrete onset of symptoms and jaundice
and/or elevated serum aminotransferase levels in a person who tests positive
for IgM antibody to hepatitis A virus or who is a contact of a laboratory-confirmed
case. Hepatitis A is reportable by law to the relevant public health authority
in all jurisdictions that report to NNDSS.
NNDSS case reports are submitted weekly to the Centers for Disease Control
and Prevention (CDC) by state health departments and summarize the results
of case investigations conducted by local and state health department personnel.
Before reporting a case, investigators determine if the criteria of the case
definition have been met. The information to complete these investigations
is collected through communication with the patient and/or the health care
practitioner. Other information routinely collected and reported includes
where the case was reported (state and county) and the basic demographic characteristics
(age, sex, race, and ethnicity) of the patient. In NNDSS, race and ethnicity
are reported using 5 categories of race and 2 categories of ethnicity as recommended
by the White House Office of Management and Budget. Classification of cases
with regard to race and ethnicity is made according to the patient’s
self-report or as reported by the health care clinician in the patient’s
medical record. Race/ethnicity are assessed for viral hepatitis and other
nationally notifiable diseases to identify racial disparities in the incidence
of disease and to develop effective disease prevention strategies.
The data analyzed here are collected as part of the routine disease
control activities of state health departments and have been determined to
be exempt from institutional review board review. Informed consent is not
required for investigation of cases of nationally notifiable diseases.
Population estimates by state for 1966 to 2003 and more detailed state-level
estimates by age, sex, race, and ethnicity for 1990 to 2003 were obtained
from the US Census Bureau.5 To assess how current
rates compare with those observed in previous disease cycles, national and
regional rates in 2003 were compared with those reported during peak and valley
years that have occurred since 1966. These current rates were also compared
with the average (defined as the arithmetic mean) rates during 1987 through
1997, the baseline period used to develop geographically based recommendations
for vaccination.3 It was recommended that routine
vaccination of children be implemented in 11 states with hepatitis A rates
during the 1987 to 1997 baseline period that were 20 or more cases per 100 000
(Alaska, Arizona, California, Idaho, Nevada, New Mexico, Oklahoma, Oregon,
South Dakota, Utah, Washington) and that it be considered in an additional
6 states with rates less than 20 per 100 000 but with rates of 10 or
more per 100 000 during this period (Arkansas, Colorado, Missouri, Montana,
Texas, Wyoming).3 Rates in these 17 states
(vaccinating states) were compared with those in the remaining states where
there is no recommendation for statewide vaccination of children (nonvaccinating
states). Overall hepatitis A rates in 2003 were compared with equivalent rates
during the baseline period (1987-1997) but in evaluating changes in age-,
race- and ethnicity-specific rates, a modified baseline (1990-1997) was used
because detailed demographic information was unavailable for cases reported
Rates in 2003 were compared with appropriate baseline and annual rates
by the calculation of a normal z statistic.6 A P value <.001 was chosen
for assessing the statistical significance of these changes because, as a
result of the large denominators in these comparisons, even small differences
in rates were statistically different at the more typically used cutoff of P = .01. We chose to use a more conservative
definition of statistical significance to better highlight differences of
potential public health importance. To facilitate the interpretation and comparison
of changing rates in different groups, these changes were evaluated not only
as absolute differences in rates over time, but also as percent declines normalized
to the baseline rate. To compare percent declines, a z statistic
comparing the ratio of 1 pair of rates with the other pair was calculated.
Statistical analysis was facilitated by using Microsoft Excel (Microsoft Corp,
During the 35 years from 1966 through 1995, there were 2 cycles of hepatitis
A incidence with peaks occurring in 1971 (59 000 cases; 29/100 000)
and 1989 (36 000 cases; 14/100 000), and corresponding nadirs in
1983 (22 000 cases; 9.2/100 000) and 1992 (23 000 cases; 9.1/100 000)
(Figure 1). Following the nadir in 1992,
rates increased through 1995 (31 582 reported cases; 12.0/100 000),
and then declined steadily. The 2003 rate of 2.6 per 100 000 (7653 reported
cases) is the lowest ever recorded, and is 71% lower than either previously
recorded nadir (P<.001).
The percent difference between peaks and nadirs for the 2 earlier cycles
was 68% (1971-1983) and 40% (1989-1992). From 1995 through 2003, rates declined
by 78% with 90% of that decline occurring since 1997. During 1998 through
2003, rates dropped by an average of 21% per year (with declines exceeding
20% during 4 of those years). Since 1966, there were 2 other years (1974 and
1991) when annual declines relative to the previous year exceeded 20%.
Relative to the average rate during the modified baseline period (1990-1997),
the rate in 2003 represented a 76% decline overall, with significant declines
in all age groups (Table). By 2003,
rates among children aged 2 to 9 years had declined by 89% and those among
children aged 10 to 18 years by 84% . Although rates among persons 19 years
of age or more also declined significantly by 69%, the percent decline among
children was significantly greater than for adults. The percentage of all
cases occurring among children 2 through 18 years of age, who account for
24% of the US population, declined from 35% to 19%.
Mapping hepatitis A rates by county illustrates how the geographic pattern
of incidence changed from the baseline period (Figure 2). Of 3145 US counties, 694 (22%) reported average rates
of 10 or more cases per 100 000 during baseline (1987-1997); the highest
average county rate was 738 per 100 000. By 2003, there were 119 (3.8%)
counties with average rates of 10 or more cases per 100 000. The highest
rate of 256 reported cases per 100 000 was in a Pennsylvania county where
more than 500 cases associated with a large foodborne outbreak7 were
In the 17 vaccinating states, incidence rates during 2000 through 2003
were 7.2, 3.8, 3.3, and 2.5 per 100 000, respectively (Figure 3). The 2003 rate was 88.2% lower than the baseline rate
of 21.1 (P<.001) (Table). Nine of the 10 states with the greatest declines in rates
from baseline to 2003 were vaccinating states, and the proportion of hepatitis
A cases originating from vaccinating states, the populations of which account
for 33% of the US population, declined from 65% (baseline) to 33% (2003).
At baseline, 560 (81%) of 694 counties with rates above the national average
for that period (10/100 000) were located in these states; in 2003, only
38 (47%) of the 119 counties with rates at or above that level were in these
During the baseline period, the highest age-specific rates in these
17 vaccinating states occurred among children aged 2 to 9 years (Table). Incidence rates subsequently declined
significantly in all age groups (Figure 3)
(Table), but the declines among children,
especially those aged 2 to 9 years, were greater than among older age groups.
In 2003, age-specific incidence rates were significantly lower among children
aged 2 to 9 years, compared with children aged 10 to 18 years or with adults
(Table). A total of 78% of reported
cases occurred among adults in 2003, compared with 60% at baseline.
Relative to the baseline, incidence in males and females declined 89%
and 87%, respectively. Rates among men, historically higher than among women,
were not statistically different in 2003 (2.6 vs 2.4/100 000, respectively)
(Table) (Figure 4).
In vaccinating states, rates declined significantly in all racial/ethnic
groups (Table). During the baseline
period, incidence rates among Native Americans were 6 to 17 times higher than
among other racial/ethnic groups. By 2003, incidence among Native Americans
declined by 98.8% and was not significantly different from that of any other
racial/ethnic group. Cases among Native Americans accounted for 1.1% of all
cases reported in 2003 compared with 8.5% at baseline. The incidence rate
among Hispanics was 3 to 6 times higher at baseline than among all non-Hispanic
racial/ethnic groups except Native Americans. Although the rate in Hispanics
declined significantly, in 2003 it remained significantly higher than any
other racial/ethnic group, including Native Americans. Cases among Hispanics,
which accounted for 30% of all cases in the vaccinating states at baseline,
accounted for 35% of cases in 2003.
At baseline, the overall rate in vaccinating states was almost 4 times
higher than in nonvaccinating states, but by 2003, the rate in vaccinating
states was not significantly different from the rate in nonvaccinating states
(Table). 2001 was the first year since
surveillance for hepatitis A began in 1966 that the overall rate was not significantly
higher in vaccinating states (Figure 1).
Before 2000, there was only 1 year (1970) in which the difference was less
than 50% (vaccinating states, 36/100 000; nonvaccinating states, 25/100 000).
One third of US counties are located in the vaccinating states. In 2003, 32%
of counties with rates above 10 per 100 000 were located in vaccinating
states (Figure 2), significantly lower
(P<.001) compared with 55% (1983) and 62% (1992),
respectively, during the previous 2 nadirs in hepatitis A incidence.
Among adults, baseline rates were more than 3 times higher in vaccinating
than nonvaccinating states but by 2003, the rate among adults in vaccinating
states was not statistically different from that in nonvaccinating states
(2.7 vs 2.9/100 000) (Table). In
2003, the highest age- and sex-specific rate occurred among adult men aged
25 through 29 years in nonvaccinating states (Figure 4).
Among children, age-specific rates at baseline were 3.5 to 5.5 times
higher in vaccinating than nonvaccinating states, with the largest difference
among children aged 2 to 9 years (Table)
(Figure 4). The 2003 age-specific rates
among children aged younger than 2 years, 2 to 9 years, and 10 through 18
years reflect significantly larger declines in vaccinating (90.6%-95.6%) than
in nonvaccinating states (59.1%-69.6%). The 2003 rates among these age groups
in vaccinating states were not statistically different from those in nonvaccinating
states (Table) (Figure 4). These shifts were reflected in the distribution of reported
cases. At baseline, 72% of all reported cases in 2- to 18-year-olds occurred
in vaccinating states (although only 35% of the children in this age group
lived in those states); by 2003, this percentage had dropped to 37%.
Baseline rates were 2 to 3 times higher among all racial/ethnic groups
except Asian Pacific Islanders in vaccinating than nonvaccinating states.
Rates among Native Americans and Asian Pacific Islanders declined similarly
in vaccinating and nonvaccinating states (Table), but among non-Hispanic whites and non-Hispanic blacks the decline
in rates was significantly higher in vaccinating states. Among Hispanics,
baseline rates were nearly 4 times higher in vaccinating than nonvaccinating
states, but in 2003 were significantly lower in vaccinating states, reflecting
a larger decline. By 2003, rates among all racial/ethnic groups in vaccinating
states were similar to or lower than those in nonvaccinating states (Table).
This report describes a dramatic decline in hepatitis A incidence rates
in the United States. The 2003 rate was the lowest recorded in 40 years of
surveillance, and the provisional 2004 rate of 1.9 reported cases per 100 000
represents yet a further decrease. This decline coincided with implementation
of recommendations for routine hepatitis A vaccination of children, and was
most striking in the parts of the country and age groups covered by the recommendations.
The greater declines in hepatitis A incidence rates in children have resulted
in a shift in the age profile of reported cases. The highest rates, which
previously were among children, now occur among adults, with the largest proportion
of cases among adults in historically low-rate states.
Fundamental shifts in the epidemiologic patterns of hepatitis A have
accompanied the decline in disease rates. The large community-wide outbreaks
that accounted for the majority8 of cases during
the past several decades, driven primarily by infections among children2,3 and transmission in households and
extended family settings,8 have virtually disappeared.
This is reflected in a shift in the distribution of reported potential sources
of infection, with a declining proportion reporting exposure in child day
Because of high disease rates among Native American populations, routine
hepatitis A vaccination was recommended for Native American communities several
years before state-wide recommendations were developed. Widespread hepatitis
A vaccination of children in these communities, many of which are geographically
and racially well-defined, produced high coverage levels.10 Hepatitis
A rates among Native Americans began declining sharply in 1995 and have been
below the national average since 2000, eliminating a large racial disparity
in hepatitis A–associated disease burden.10
Determining the role that vaccination has played in the observed changes
in hepatitis A epidemiology is complicated by the historical pattern of hepatitis
A incidence, which varies cyclically and peaks approximately every 10 to 15
years.3 Certainly, the observed decline is
not entirely attributable to vaccination. Indeed, mathematical models of hepatitis
A incidence predicted a 4.5% yearly decline among susceptible persons over
7 decades before the availability of vaccines,1 and
that 39% of the potential cases in 2001 were prevented by vaccination.11 With use of any new vaccine, dynamic modeling theory
predicts that the eventual steady state incidence rate, while lower than during
the prevaccination era, will be higher than the nadir that occurs shortly
after vaccine introduction.12
While available data do not allow for quantifying the relative contribution
of vaccination and temporal trends to the observed declines in incidence,
these declines, particularly since 1999, have been unprecedented in magnitude
and greater in areas in which vaccination of children is occurring. In reality,
in view of the number of years from valley to peak in previous cycles, it
might be argued that in the absence of any intervention, the incidence after
1995 would have been expected to continue to rise for several additional years.
It is possible that targeted vaccination programs initiated in 1996 and involving
children living in communities with the highest rates, especially Native American
communities, might have played a role in the moderate decline in rates that
occurred in 1996 and 1997. Further, the epidemiologic situation we describe
does not represent maintenance of previous characteristics that have been
reset at a lower incidence rate. Rather, declines have been accompanied by
fundamental changes in the epidemiology of hepatitis A, such as shifts in
the relative distribution of cases by age and geographic region, which are
consistent with the expected effects of this intervention and were not observed
during previous nadirs in the natural disease cycle.
Comprehensive information on hepatitis A vaccination coverage could
provide a useful context in which to interpret the key findings presented
in this report. Unfortunately, only very limited data are available. Vaccination
coverage among adults is not assessed systematically. However, available information
indicates that hepatitis A vaccination coverage of adults in high-risk groups,
although recommended since 1996, is low.2,13 The
only nationwide estimate of coverage among children comes from the National
Immunization Survey, a nationwide survey that provides annual estimates of
vaccination coverage among 19- through 35-month-old children in 50 states
and 28 selected urban areas.14 In 2003, when
hepatitis A vaccine was first included in the National Immunization Survey,
first-dose coverage among children 24 through 35 months old was 50% in the
11 states in which routine vaccination is recommended, and 25% in the 6 states
where routine vaccination is to be considered. By comparison, coverage in
this age group in the remaining states was 1%. According to a 2005 written
communication from CDC health scientist Diana Bartlett, MPH, limited coverage
data among somewhat older children, available from vaccine registries of selected
populations in 5 states included in the recommendations, indicated that as
of the second quarter of 2004, 44% to 81% of children aged 3 to 5 years had
received 1 or more doses of hepatitis A vaccine.
The 2003 National Immunization Survey data suggest that at least some
vaccination efforts to date may have been targeted to children at highest
risk. For example, although the overall statewide vaccination coverage level
in Texas was 32%, coverage in counties bordering Mexico with the highest incidence
rates in the prevaccine era was considerably higher, reaching 70% in some
counties.14 In a multivariate model, factors
that were associated with receiving hepatitis A vaccine included residence
in a vaccinating state, living in an urban area, being either Hispanic or
Native American (compared with non-Hispanic white race/ethnicity), and having
a mother with less than a high school education (compared with having graduated
from high school).15 Targeting these groups
might have increased the impact of limited vaccination efforts.
Trends in vaccination coverage are difficult to measure because before
2003, comprehensive vaccination coverage data were not collected in any routine
or systematic way. Records of yearly pediatric doses distributed in the public
sector offer a rough indication. Before 1998, less than 400 000 doses
had been distributed. In 1998, almost 600 000 doses were distributed,
more than doubling in 1999 to almost 1.5 million, and continuing to rise in
2000 to approximately 2.9 million. During 2001 through 2003, approximately
3 to 3.5 million doses were distributed each year, over 95% to vaccinating
This available information on vaccine use indicates that the observed
declines in rates among children appear to have been achieved with modest
levels of vaccination coverage, supporting the hypothesis of a strong herd
immunity effect. Declines in rates among adults in vaccinating states were
larger than in nonvaccinating states, suggesting that vaccination of children
also might have reduced transmission in other age groups through herd immunity.
Additional evidence of such an effect was seen in a demonstration project
where vaccination of children (approximately 66% coverage with at least 1
vaccine dose) resulted in a substantial reduction in disease rates in adults.16 Similar findings have been reported from other countries
in which routine hepatitis A vaccination of infants or children has been implemented,
including Israel17 and parts of Italy and Spain.18,19 A recent article that presented the
results of modeling the relationship between hepatitis A incidence and vaccination
coverage also found a strong herd immunity effect, accounting for more than
one third of the estimated number of cases prevented by vaccination.11
Although vaccination of children can result in fewer adult cases associated
with contact with children, transmission among selected groups of adults can
be sustained in the absence of transmission among children.16 Thus,
cases increasingly are concentrated among adults in identified high-risk groups,
such as international travelers, users of illegal drugs, and men who have
sex with men.9 The relatively high disease
rates among adult men younger than 40 years are a manifestation of this phenomenon.
We compared rates from 2003 with the averages during the 1987 through
1997 baseline period that was the basis of the recommendations for use of
hepatitis A vaccine per the Advisory Committee on Immunization Practices.
Because hepatitis A incidence has varied cyclically with periodic nationwide
increases, the years 1987-1997 were chosen as a baseline that reflected neither
a peak nor a valley in the incidence rate and provided a representative picture
of hepatitis A incidence during the prevaccine era. We compared the baseline
rates with those in the most recent year rather than with the averages from
multiple years because implementation of the vaccination recommendations has
occurred in stages and is still expanding, thus, there is no well-defined
postvaccination period. In describing postvaccine era changes, the use of
any group of years since the recommendations were made (rather than the most
recent year) as an end point reflects the average changes during the entire
time period, a time during which vaccine coverage has been consistently increasing
and rates consistently declining. Thus, while the overall conclusions are
the same regardless of whether the end point used is 2003 or a group of recent
years, given the dynamic situation, the analysis comparing the baseline rate
with 2003 more completely and accurately describes the magnitude of the changes
Our analyses used passive surveillance data reported through NNDSS,
which collects information on symptomatic, serologically confirmed hepatitis
A cases, and has a number of previously described limitations.20 These
reports represent only a portion of all infections, because asymptomatic infections
and some symptomatic cases are not reported. Studies have estimated that for
each hepatitis A case reported to NNDSS, there are 2 to 3 that meet the case
definition that are not reported.1,20 Completeness
of the data reported, such as demographic characteristics, also can vary.
However, available information indicates that in general, data quality from
vaccinating states is better than from nonvaccinating states and there has
been no evidence of systematic changes in these reporting patterns since 1990,
the first year in which detailed demographic data by case were reported. For
example, information on age and race/ethnicity is missing from a smaller proportion
of reports from vaccinating states compared with nonvaccinating states (eg,
98.5% of case reports from vaccinating states include age vs 97.5% from nonvaccinating
states). This differential in completeness of reports would have the effect
of underestimating differences between regions. Further, the national trends
reported here were also reflected in the Sentinel Counties Study of Acute
Viral Hepatitis, a long-standing CDC-sponsored study (since the early 1980s)
in 6 US counties where the accuracy and completeness of reporting have been
assessed and are known to be high.21 Hepatitis
A incidence rates in the Sentinel Counties Study declined from an average
of 15.3 per 100 000 in 1987 through 1997 to 1.2 cases per 100 000
in 2003, and the incidence curve closely mirrors national trends. Thus, while
use of NNDSS data may have resulted in imprecision in incidence estimates,
we believe these data accurately reflect trends in incidence, both overall
and among regions.
In summary, we report early apparent impact of implementation of a novel,
and to our knowledge, unique vaccination strategy. This strategy is based
on distinctive features of hepatitis A epidemiology, including the geographic
clustering of areas with consistently elevated rates, the important role of
children in sustaining transmission, and the hypothesized large effect of
herd immunity.22 These features lent themselves
to a geographically focused strategy using a vaccine that could not be readily
integrated into the routine vaccination schedule, and maximized the impact
of limited vaccination. The changes we describe represent a transformation
in hepatitis A epidemiology in the United States. However, because hepatitis
A incidence has historically exhibited a pattern of periodic increases, further
monitoring is needed to determine the extent to which the declines that have
occurred will be sustained and are attributable to vaccination. In addition,
more data on vaccine coverage levels are needed to better describe the relationship
between hepatitis A vaccine usage and disease rates.
Sustaining and further reducing hepatitis A incidence can be achieved
by improving vaccination coverage in groups for which it is currently recommended,
including children living in the historically higher-rate states and children
and adults in high-risk groups.3 Elimination
of hepatitis A virus transmission will require expansion of existing recommendations
to include routine vaccination of all US children.
Corresponding Author: Annemarie Wasley,
ScD, Division of Viral Hepatitis, Centers for Disease Control and Prevention,
1600 Clifton Rd NE, Mailstop G-37, Atlanta, GA 30333 (firstname.lastname@example.org).
Author Contributions: Dr Wasley 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: Wasley, Samandari,
Acquisition of data: Wasley, Samandari.
Analysis and interpretation of data: Wasley,
Drafting of the manuscript: Wasley, Samandari,
Critical revision of the manuscript for important
intellectual content: Wasley, Samandari, Bell.
Statistical analysis: Wasley, Samandari, Bell.
Study supervision: Bell.
Financial Disclosures: None reported.
Acknowledgment: We thank David Culver, PhD,
for providing advice on statistical methodology and Steven Bloom for his assistance
in data analysis. Our thanks also to the staff of state and local health departments
and health care providers in the community, which by reporting cases of hepatitis
A make surveillance for this disease possible.
Funding/Support: Support for the analyses presented
here was provided by the Centers for Disease Control and Prevention.