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 unvaccinatedolder 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 theprevaccine 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.