In 2007, the Advisory Committee on Immunization Practices (ACIP) recommended a routine
second dose of varicella vaccine for children at age 4–6 years, in addition to the
first dose given at age 12–15 months (1). One strategy recommended for increasing
varicella vaccination coverage is a school entry requirement of proof of varicella
immunity (1,2). To determine the extent of implementation of the routine 2-dose varicella
vaccination program, the number of states with a 2-dose varicella vaccination elementary
school entry requirement in 2012 was compared with the number in 2007, and 2-dose
varicella vaccination coverage during 2006 was compared with coverage in 2012 among
children aged 7 years, using data from six Immunization Information System (IIS) sentinel
sites. The number of states (including the District of Columbia) with a 2-dose varicella
vaccination elementary school entry requirement increased from four in 2007 to 36
in 2012. Two-dose varicella vaccination coverage levels among children aged 7 years
in the six IIS sentinel sites increased from a range of 3.6%–8.9% in 2006 to a range
of 79.9%–92.0% in 2012 and were approaching the levels of 2-dose measles, mumps, and
rubella (MMR) coverage, which had a range of 81.9%–94.0% in 2012. These increases
suggest substantial progress in implementing the routine 2-dose varicella vaccination
program in the first 6 years since its recommendation by ACIP. Wider adoption of 2-dose
varicella vaccination school entry requirements might help progress toward the Healthy
People 2020 target of 95% of kindergarten students having received 2 doses of varicella
vaccine.
Data on the number of states with 1-dose and 2-dose varicella vaccine elementary school
entry requirements at the start of the school year were obtained from state immunization
websites for 2007 and 2012. Data on varicella vaccination coverage were obtained from
six sentinel IIS sites. IIS, also known as immunization registries, are computerized,
population-based systems that consolidate data from participating vaccine providers
and provide tools for supporting effective immunization strategies at the vaccination
provider and program levels (3). The IIS sentinel site project is a collaboration
between CDC and state- and city-based IIS. To be eligible to compete for CDC sentinel
site funding, ≥85% of vaccination providers must participate in the IIS, ≥85% of children
aged <19 years must have at least two vaccinations recorded in the IIS, and ≥70% of
doses administered must be reported to the IIS within 30 days of administration. The
six IIS sentinel sites funded for the 2013–2017 project period are Michigan, Minnesota,
North Dakota, New York City, and Wisconsin, which include data from the entire jurisdiction,
and Oregon, which includes data from six counties (56% of the state population).
De-identified individual record-level data were received from IIS sentinel sites and
processed in accordance with IIS best practices (4). Children who were designated
in the IIS as permanently inactive (i.e., deceased) or “moved or gone elsewhere” were
excluded from analysis.
Varicella and MMR vaccination coverage were assessed at age 7 years to allow time
for the 2-dose series to be completed. Two-dose varicella vaccination coverage estimates
were calculated for each year of the study period (i.e., January 1, 2006–December
31, 2012) among children aged 7 years (born during January 1, 1999–December 31, 2005).
Intercensal population estimates for 2006–2009 and postcensal estimates for 2010–2012
were used for the denominators (5). Valid doses of varicella vaccine were defined
as dose 1 administered no earlier than 4 days before age 12 months, dose 2 administered
at least 28 days after dose 1, and either dose administered on the same day as or
≥4 weeks after any other live vaccine.* Coverage was calculated by dividing the number
of children aged 7 years with 2 valid doses of varicella vaccine by the U.S. Census
estimate of the total number of same-aged children in the sentinel site population.
To have a single measure of coverage at the six sites that could be compared from
year to year, the unweighted average of the estimates for each of the six sites was
calculated for each year.
Two-dose varicella vaccination coverage estimates derived from IIS data for children
aged 6 years were compared with data from the kindergarten vaccination assessment
for the four sentinel sites (Michigan, Minnesota, North Dakota, and Wisconsin) that
had 2-dose varicella vaccination school entry requirements for the 2012–13 school
year (6). Kindergarten assessments, conducted annually by federal immunization grantees
through a vaccination coverage survey or census of enrolled students to determine
compliance with school vaccination requirements (6), are the only available source
of national data on 2-dose varicella vaccination coverage. Differences between 2-dose
varicella vaccination coverage in 2012 at sites with and without 2-dose school entry
requirements for children aged 6 years were examined and analyzed for statistical
significance using the Wilcoxon-Mann-Whitney test.
The number of states requiring 2 doses of varicella vaccine for school entry increased
rapidly, from four in 2007 to 36 by 2012, and all but one state required 1 or more
doses of varicella vaccine for elementary school entry by the 2012–13 school year
(Figure 1).
Varicella vaccination coverage levels with 2 doses among children aged 7 years increased
greatly at the six IIS sentinel sites, from a range of 3.6%–8.9% in 2006 to a range
of 79.9%–92.0% in 2012, approaching that of 2-dose MMR vaccination coverage, which
ranged from 81.9% to 94.0% in 2012 (Figure 2). Implementation of the 2-dose varicella
vaccination recommendation was rapid, with the average of coverage percentages increasing
to 72.4% by 2009.
Coverage estimates for 2 doses of varicella vaccine among children aged 6 years at
four IIS sites based on IIS data were similar to those reported in the kindergarten
assessment. The IIS estimate was lower than the kindergarten assessment at two of
the sites (percentage-point differences of 0.5 and 15.6) and higher at two sites (percentage-point
differences of 2.0 and 4.4) (Table). Two-dose varicella vaccination coverage in 2012
for children aged 6 years was slightly higher in the four states with 2-dose school
entry requirements (Michigan, Minnesota, North Dakota, and Wisconsin), compared with
sites with only a 1-dose school entry requirement (New York City and Oregon), although
this difference was not statistically significant (p=0.5) (Table).
Editorial Note
During the first 6 years of the 2-dose varicella vaccination program, the number of
states with 2-dose varicella vaccination elementary school entry requirements increased
from four to 36, and 2-dose coverage among children aged 7 years in IIS sentinel sites
increased from 4%–9% to 80%–92%, approaching the level for 2-dose MMR coverage. The
rapid increase in 2-dose coverage after the ACIP recommendation and before 2-dose
school entry requirements were widely adopted suggests extensive implementation of
the recommendation by health-care providers. School entry requirements have been useful
for increasing 1-dose varicella vaccination coverage among children (2). Adoption
of 2-dose varicella vaccination school entry requirements by additional states and
for higher grades might help reach the Healthy People 2020 targets of 95% and 90%
2-dose coverage among kindergarten and adolescent students, respectively.
IIS sentinel sites provide an important source of population-based, provider-verified
vaccination data and can be useful for assessing coverage for vaccines, such as varicella,
for which other mechanisms to estimate coverage nationally are inadequate. Two-dose
varicella vaccination coverage data are available from surveys of kindergarten-aged
children; however, data collection and validation methodologies vary by state, and
data are limited to doses required for school entry. Two-dose varicella vaccination
coverage estimates for the 2012–13 school year based on IIS data were similar to those
obtained from the kindergarten assessment, except for one site (6). Improvements in
kindergarten survey methodology and ongoing adoption of 2-dose varicella school entry
requirements will make it increasingly feasible to estimate 2-dose coverage for varicella
vaccination nationally using data from kindergarten students, as is already done for
MMR coverage.
The findings in this report are subject to at least two limitations. First, census-based
denominators were used, which might have resulted in underestimation of varicella
protection because children with a history of varicella disease are included in the
denominator even though varicella vaccination would not be indicated for them. Second,
the IIS sentinel sites are highly selected and might not be representative of other
cities and states.
The 1-dose varicella vaccination program, implemented in 1996, resulted in 70%–90%
declines in varicella disease incidence, hospitalizations, and mortality (7–9). The
routine 2-dose varicella vaccination program was implemented to further decrease varicella
disease and control outbreaks. Since its implementation in 2007, declines in varicella
incidence and outbreaks ranging from 67% to 76% have been reported (10). Further declines
in varicella incidence and outbreaks might occur as higher 2-dose varicella vaccination
coverage is achieved.
What is already known on this topic?
A second dose of varicella vaccine was recommended for children by the Advisory Committee
on Immunization Practices in 2007, and the recommendation has been followed by decreases
in varicella incidence nationwide. However, estimates of 2-dose varicella vaccination
coverage have not been available previously.
What is added by this report?
The number of states with a 2-dose varicella vaccine elementary school entry requirement
increased from four in 2007 to 36 in 2012. Two-dose varicella vaccination coverage
levels among children aged 7 years in six selected sentinel sites increased from a
range of 3.6%–8.9% in 2006 to a range of 79.9%–92.0% in 2012, approaching the coverage
level for 2 doses of measles, mumps, and rubella vaccine.
What are the implications for public health practice?
Health-care providers have been important to the increase in coverage levels for 2
doses of varicella vaccine. Wider adoption of 2-dose varicella vaccine school entry
requirements in more states and higher grades might help reach the Healthy People
2020 targets of 95% and 90% 2-dose varicella vaccination coverage among kindergarten
and adolescent students, respectively.