Rotavirus infection is the leading cause of severe gastroenteritis among infants and
young children worldwide (1,2). Before the introduction of rotavirus vaccine in the
United States in 2006, rotavirus infection caused significant morbidity among U.S.
children, with an estimated 55,000–70,000 hospitalizations and 410,000 clinic visits
annually (3). The disease showed a characteristic winter-spring seasonality and geographic
pattern, with annual seasonal activity beginning in the West during December-January,
extending across the country, and ending in the Northeast during April-May (4). To
characterize changes in rotavirus disease trends and seasonality following introduction
of rotavirus vaccines in the United States, CDC compared data from CDC’s National
Respiratory and Enteric Virus Surveillance System (NREVSS), a passive laboratory reporting
system, for prevaccine (2000–2006) and postvaccine (2007–2014) years. National declines
in rotavirus detection were noted, ranging from 57.8%–89.9% in each of the 7 postvaccine
years compared with all 7 prevaccine years combined. A biennial pattern of rotavirus
activity emerged in the postvaccine era, with years of low activity and highly erratic
seasonality alternating with years of moderately increased activity and seasonality
similar to that seen in the prevaccine era. These results demonstrate the substantial
and sustained effect of rotavirus vaccine in reducing the circulation and changing
the epidemiology of rotavirus among U.S. children.
NREVSS is a national laboratory-based passive reporting system for respiratory and
enteric viruses, including rotavirus. Participating laboratories report weekly data
to CDC, including the total number of stool samples tested for rotavirus by enzyme
immunoassay and the number of specimens that tested positive. Annually, 75 to 90 laboratories
report rotavirus testing data to NREVSS. A reporting year is defined as the period
from July (epidemiologic week 27) to June (epidemiologic week 26) of the following
year, beginning in July 2000. Rotavirus season onset is defined as the first of 2
consecutive weeks where 10% or more of specimens test positive for rotavirus. Similarly,
season offset is defined as the last of 2 consecutive weeks where 10% or more of samples
test positive. Peak season intensity is defined as the week with the highest proportion
of tests positive for rotavirus. For analysis of season duration and peak intensity,
data from all participating laboratories were included. The proportion of samples
that tested positive for rotavirus and the mean decrease from the prevaccine years
are reported for these data. Analyses of trends in disease were restricted to the
23 laboratories that consistently reported at least 26 weeks of data for each reporting
year from July 2000 through June 2014. For this analysis, data are aggregated by week
and reported as a 3-week moving average of total number of tests and rotavirus positive
tests performed for the prevaccine period (2000–2006) and for each prevaccine season.
Data are presented for the United States overall and for each U.S. census region.
Data from all participating NREVSS laboratories showed that with prevaccine seasons
(2000–2006), median season onset was in epidemiologic week 50 (in December), peak
activity was in week 9 (February/March, 43.1% positive samples) and season duration
was 26 weeks. In comparison, these data showed that each of the 7 postvaccine seasons
from 2007–2014 started later (if at all), had lower peak positivity for rotavirus
(10.9%–27.3%), and were shorter in duration (0–18 weeks) (Table 1 and Figure 1). In
the rotavirus reporting years spanning 2009–2010, 2011–2012 and 2013–2014, no seasonal
onset occurred nationally, and the proportion of tests positive for rotavirus during
the peak week was lower than the immediately preceding and following seasons. Examination
of data for each region individually showed slight differences in seasonal onset,
duration, and offset. Notably, in the South, season onset and duration varied, with
some postvaccine years’ season onset and duration comparable with median values from
prevaccine years. This region also had only one reporting year where no season onset
threshold was reached, whereas all other regions had at least two such reporting years.
Regardless of these variations, most seasons within each region showed decreased length
and activity compared with prevaccine years.
Data from 23 consistently reporting laboratories demonstrated a marked decline in
rotavirus testing and positivity in the postvaccine years (Table 2 and Figure 2).
Overall, after vaccine introduction, the number of total tests performed as well as
the number of positive rotavirus tests declined each reporting year compared with
those of the prevaccine years. Furthermore, the proportion of tests that were positive
for rotavirus declined from 57.8%–89.9% in each of the seven postvaccine reporting
years compared with prevaccine years combined, with alternating years of lower and
greater positivity rates. Similar patterns were observed when the data were examined
for each region.
Discussion
A marked and sustained decline in rotavirus activity was seen nationally in all seven
rotavirus reporting years from 2007 to 2014 following the implementation of routine
rotavirus vaccination of U.S. children. The decline was accompanied by changes in
the predictable prevaccine seasonal pattern of rotavirus activity. The later onset
and shorter duration of rotavirus seasons in the postvaccine era, including some years
without a defined rotavirus season, could be a result of fewer unvaccinated, susceptible
infants, resulting in reduced intensity and duration of rotavirus transmission (5).
This reduced transmission of rotavirus likely also explains the declines in rates
of rotavirus disease that have been seen in unvaccinated older children and even in
some adult age groups in postvaccine years compared with the prevaccine era, resulting
from the phenomenon known as herd immunity (6).
Biennial peaks in rotavirus activity also emerged in the postvaccine era in contrast
to the annual peaks before vaccine implementation, although even the postvaccine reporting
years with heavier rotavirus burden still demonstrated rotavirus activity levels that
were substantially lower than those of the prevaccine years. This biennial pattern
might be explained by an accumulation of a sufficient number of unvaccinated susceptible
children over two successive reporting years to result in stronger rotavirus seasons
every other year. Though rotavirus vaccine coverage among children aged 19–35 months
has increased nationally since the vaccine was introduced, from 43.9% in 2009 to 72.6%
2013 (7), some children remain unvaccinated. In a low rotavirus reporting year, these
unvaccinated children might not be exposed to wild-type rotavirus and thus remain
susceptible in their second year of life. These susceptible children aged 12–23 months,
together with unvaccinated infants from the next birth cohort, might form a critical
mass of susceptible children sufficient to sustain more intense rotavirus transmission
in alternate years.
The findings in this report are subject to at least four limitations. First, NREVSS
only receives aggregate reports of the number of stool samples tested for rotavirus
and the number of these that test positive, without any information on demographics
or clinical features of individual patients, precluding detailed examination of these
characteristics. Second, participating laboratory locations do not uniformly cover
all areas of the United States, and as such regional biases might exist. Third, because
testing for rotavirus does not alter clinical management of patients, testing practices
might differ and affect comparability of data from site to site and year to year.
Finally, any changes in rotavirus testing practices coinciding with implementation
of the rotavirus vaccination program could affect interpretation of the disease trends,
although the consistency of the declines in rotavirus activity across all regions
and years argues against changes in testing being the main cause of the decline.
The declines in rotavirus activity seen in NREVSS data after vaccine introduction
are supported by other U.S. studies showing declines in laboratory-confirmed rotavirus
hospitalization (4) as well as reductions in outpatient visits, emergency room visits,
acute gastroenteritis, and rotavirus-coded hospitalizations (8). During 2007–2011
more than 176,000 hospitalizations, 242,000 emergency department visits, and 1.1 million
outpatients visits due to diarrhea were averted, resulting in costs savings of $924
million over this 4-year period (9). Given the sustained decline in rotavirus activity
observed in the NREVSS data through 2014, we would expect additional medical visits
due to diarrhea will have been prevented and additional cost savings accrued in the
United States. The findings in this report are consistent with the high field effectiveness
of vaccination observed in post-licensure epidemiologic studies (10). Taken together,
these findings reaffirm the large public health impact of routine rotavirus vaccination
in reducing the circulation of rotavirus among U.S. children.
What is already known on this topic?
Following the introduction of rotavirus vaccine in the United States in 2006, large
declines have been observed in diarrhea and rotavirus hospitalizations among children
aged <5 years, and onset of the rotavirus season has occurred later.
What is added by this report?
Analysis of data from the National Respiratory and Enteric Virus Surveillance System
showed a marked and sustained decline in rotavirus activity nationally and regionally
for the seven rotavirus reporting years from 2007 to 2014 following the implementation
of routine rotavirus vaccination of U.S. children. In addition to rotavirus seasons
with later onset and shorter duration, a biennial pattern of rotavirus activity emerged
in the postvaccine era, with years of low activity and highly erratic seasonality
alternating with years of greater activity and seasonality similar to those in the
prevaccine era.
What are the implications for public health practice?
These findings reaffirm the large public health impact of routine rotavirus vaccination
in reducing the circulation of rotavirus in U.S. children.