The number of deaths attributable to rotavirus in the United States is low; however, rotavirus infections still cause significant morbidity, resulting in hospitalizations and outpatient visits.1,2 The annual rotavirus season in the United States typically occurs in the cooler months, starting in the fall in the southwest and ending in the northeast by spring.3
In the United States, 2 rotavirus vaccines, RotaTeq (Merck and Company, Whitehouse Station, NJ) approved in February 2006 and Rotarix (GlaxoSmithKline, Research Triangle Park, NC) approved in April 2008, are recommended for use by the Advisory Committee on Immunization Practices.4 These 2 vaccines were expected to significantly decrease the number and severity of rotavirus infections in the United States. We conducted a retrospective analysis to examine the epidemiology of rotavirus infections and hospitalizations in northeast Florida before and after the introduction of rotavirus vaccines.
We reviewed records at the Wolfson Children's Hospital (WCH), the only regional tertiary care center serving children in northeast Florida. WCH has 192 beds, 8000–9000 annual admissions, and a catchment area of 3 million. The study period was between January 1, 2004, and December 31, 2009, and included patients aged <18 years who were tested for and those who were hospitalized because of rotavirus infection. Prevaccine period was defined as 2004–2006, and vaccine period as 2007–2009. The study was approved by the institutional review board.
Rotavirus vaccine uptake was indirectly estimated by determining the number of vaccine doses distributed in northeast Florida. The number of doses distributed in the public sector was obtained from the Bureau of Immunization, Florida Department of Health, and in the private sector from the 2 vaccine manufacturers. Data of live births for the study period were obtained from the Florida state census online.5
Laboratory records were reviewed to identify patients in whom rotavirus tests were ordered on a stool specimen. Rotavirus tests were done only on bulk stool specimens. It was assumed that clinicians ordered rotavirus tests only on patients who had diarrhea, and patients were assumed to have rotavirus infection if the test result was positive.
Hospitalization because of rotavirus infection was used as a surrogate for the severity of the infection. The number of hospitalizations because of rotavirus was determined and compared with the overall number of hospitalized rotavirus-infected patients and also with all annual hospitalizations at WCH from 2004 to 2009. Children admitted secondary to rotavirus infection were identified using International Classification of Diseases, Ninth Revision, Clinical Modification code 008.61 used only as the primary or secondary discharge diagnosis.
We determined the seasonal pattern (onset, peak, and end) of rotavirus infections in northeast Florida during the prevaccine period (2004–2006) and vaccine (2007–2009) periods. Season onset was defined as >10% positive tests for at least 2 consecutive months, and season end as <10% positive tests for at least 2 consecutive months. Season peak was defined as the month(s) with the highest percentage of positive tests.
Statistical analyses were done comparing prevaccine period (2004–2006) with vaccine period (2007–2009) for rotavirus infections and hospitalizations using SAS 9.1 software (SAS Institute, Cary, NC). Categorical comparisons were done using χ2 tests. Means were compared using 2-sample t tests.
Shown in Table 1 are live births, rotavirus tests done, rotavirus tests positive, total hospitalization, rotavirus hospitalizations, and vaccine doses distributed for each of the 6 study years. Monthly distribution of rotavirus tests ordered and positive results in the study period are depicted in Figure, Supplemental Digital Content 1, http://links.lww.com/INF/A440.
The number of tests performed decreased by 12.1% from an average of 758 in the prevaccine period to an average of 666 in the vaccine period (P = 0.28, t test statistic). There was a 57.8% decline in the absolute number of positive rotavirus tests from an annual average of 207 in the prevaccine period to 87.3 in the vaccine period (P = 0.03, t test statistic). The proportion of positive tests decreased significantly from 27.3% in the prevaccine period to 13.11% in the vaccine period (P < 0.0001, χ2). A positive test was 2.08 times more likely in the prevaccine period compared with the vaccine period.
There was a decrease in rotavirus hospitalizations relative to total hospitalizations in the vaccine period (0.20%) compared with the prevaccine period (0.71%; P < 0.0001, χ2). Among those who had rotavirus infections, the absolute number of hospitalizations decreased by 72% from an annual average 63.3 in the prevaccine period to 18 in the vaccine period (P = 0.01, t test statistic). The proportion of patients hospitalized because of rotavirus infection decreased significantly from 30.6% in the prevaccine period to 20.6% in the vaccine period (P = 0.0024, χ2).
The rotavirus season in northeast Florida started in January and ended in May or June, with a peak in March or April for the prevaccine years and 2007. However, in 2008, the season started in September, peaked in October, and ended in April 2009 (Fig., Supplemental Digital Content 1, http://links.lww.com/INF/A440). No defined rotavirus season was appreciated in 2009.
We observed a decrease in infections and hospitalizations because of rotavirus and a change in the rotavirus season after the introduction of rotavirus vaccines in 2006. This is similar to the previously reported data.6,7 However, the change in the seasonal pattern of rotavirus infection in northeast Florida after the introduction of the vaccine was different from what has recently been reported from other parts of the United States.6
As has been reported for the southern US,6 our study showed that both the total number of rotavirus tests performed and reported positive decreased. We also observed that during the vaccine period there was a decrease in the proportion of positive tests relative to total rotavirus tests performed.
Our study also showed a decrease in the severity of rotavirus infections as reflected by the decline in hospitalizations. This is similar to what has been reported in a postlicensure evaluation of the effectiveness of the rotavirus vaccine.7
The introduction of rotavirus vaccines not only decreased the number of infections and hospitalizations but also changed the annual seasonality of rotavirus infection. National reports showed that the onset of 2007–2008 rotavirus season was delayed by 11 weeks but the duration was shorter than 2000–2006 season, whereas 2008–2009 season had an earlier season onset but longer duration compared with 2007–2008 season.6 According to our data, the rotavirus season onset in northeast Florida during the prevaccine years and 2007 was in January, with the peak occurring in March or April and the season ending in May or June. The duration of rotavirus season in our region during the prevaccine years and 2007 was 5–6 months. However, in 2008, the duration of the season was 8 months, starting in September, peaking in October, and ending in April 2009. Compared with the prevaccine period, the onset of rotavirus season in 2008 was delayed by 8 months, the peak occurred sooner (within 1 month of season onset), and the duration was 2–3 months longer. No rotavirus season was observed in 2009. Secular variability has been reported after introduction of rotavirus vaccine.8 Prolonged follow-up of season patterns will clarify this issue. We continue to monitor rotavirus infections and hospitalizations in northeast Florida.
There are several limitations to our study. This is a retrospective analysis subject to numerous confounders and biases. Although we believe that there were no changes in admission or testing policies at our institution, changes in clinical practice over the years may have some influences on the changes we observed. Some patients with acute gastroenteritis might not have been tested for rotavirus infection and some may have been tested at outside laboratories, leading to a lower detection rate. Individual patient information was not available before 2008, which may have resulted in counting some positive rotavirus results more than once. However, this was not very likely because we did not observe any duplication of results in 2008 and 2009 when patient-level information became available. Additionally, vaccine uptake in our study was assessed indirectly using vaccine distribution, which may not accurately represent actual vaccine use. However, vaccine uptake has been steadily increasing, with 40%–64% first-dose coverage reported for children aged 3 months.9 In addition, use of International Classification of Diseases, Ninth Revision, Clinical Modification codes often underestimate incidence of rotavirus-related admissions.10 However, the difference in infections and hospitalizations because of rotavirus between the prevaccine and vaccine periods is significant and seems to be temporally related to rotavirus vaccine availability.
In conclusion, our study showed a decrease in infections and hospitalizations because of rotavirus infection temporally associated with the introduction of rotavirus vaccine. In addition, the season seems to be evolving from delayed onset and prolonged duration in 2008 to no appreciable “season” in 2009. Continued surveillance over the next several years will be critical to determine whether the decrease in rotavirus infection and the seasonal differences in northeast Florida are sustained over time.
The authors acknowledge the contributions of Page McKitrick, Diane Halstead, PhD, and Nizar Maraqa, MD.
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