Trends in acute viral gastroenteritis among children aged ≤5 years through the national surveillance system in South Korea, 2013–2019

Abstract Acute gastroenteritis is a global public health concern. This study aimed to analyze the trend and characteristics of acute viral gastroenteritis through a national surveillance network. Enteric viruses were detected in 9510 of 31,750 (30.1%) cases assessed from 2013 to 2019 by EnterNet. The most prevalent pathogens were norovirus (15.2%) and group A rotavirus (9.7%); most infections were reported in 2017 (34.0%). Norovirus and rotavirus coinfections were the most common. Norovirus infections were prevalent among 1‐year‐old children (1835 out of 9510 cases) during winter, and group A rotavirus infections were common during spring. Seasonality was not observed among enteric adenovirus, astrovirus, and sapovirus. The prevalent viral genotypes detected included norovirus GII.4, enteric adenovirus F41, astrovirus genotype 1, and sapovirus GI.1. However, changes in enteric virus trends were noted during the study period. Norovirus prevalence extended into spring, and new genotypes of enteric adenovirus, astrovirus, and sapovirus were identified. These surveillance data elucidate enteric virus epidemiological characteristics.

cases of HAdV, HAstV, and SaV infections have been reported at varying levels. [16][17][18] Viral gastroenteritis is prevalent in winter with the cooler temperature and tends to decrease with an increase in temperature. The seasonality may vary depending on the region. 19 In Korea, the nationwide acute diarrhea surveillance system

| Sample preparation
One gram of fecal sample was placed in 9 ml of sterile phosphatebuffered saline (pH 7.4, Sigma) and sufficiently vortexed to prepare a 10% fecal suspension, which was then centrifuged at 4°C and 3000 rpm for 15 min to extract the supernatant containing the viruses. In the antigen detection test, the pretreated fecal supernatant was used immediately. For genetic analysis, nucleic acids were extracted from the pretreated solution using the commercially available nucleic acid extraction kit, that is, NucleoMag 96 virus kit (Macherey-Nagel), in accordance with the manufacturer's instructions, using 200 µl of the pretreatment solution. The final nucleic acid extract was 50 µl and was stored at −70°C until subsequent use.

| Virus detection test
For virus detection, antigen detection and gene detection were performed. RVA and HAdV antigens were detected using an enzymatic immunoassay. Pretreated 10% fecal supernatants were assessed using a RIDASCREEN EIA kit (R-Biopharm). Gene detection was performed for NoV, HAstV, and SaV. Norovirus genotypes GI and GII were simultaneously detected using a commercially available real-time reverse transcription PCR (RT-PCR) assay, that is, AccuPower® Norovirus Real-Time RT-PCR Kit (Bioneer) and PowerCheck™ Norovirus GI/GII Multiplex Real-time PCR Kit (Kogenebiotech). HAstV and SaV were detected through RT-PCR analysis using specially generated primers (Table 1). RT

| Genetic analysis
Genes were re-amplified for positive samples. NoV was subjected to semi-nested RT-PCR ( HAstV and SaV, RT-PCR products secured in the detection process were directly used for sequencing without additional experiments.
The sequence was analyzed in a batch at the analysis center, and the results were submitted to the EnterNet surveillance system to confirm the genotype using the analysis tool in the EnterNet system.
Sequences not analyzed using this analysis tool were analyzed using NCBI BLAST. RVA, whose genotype was not identified as an En-terNet system analysis tool, was excluded.

| Data analysis
Among the data registered in the EnterNet system, the pathogen detection rate was analyzed using the data registered during the study period. Patients for whom data, including age or sample col-   (Figure 1). Furthermore, three viruses were detected simultaneously in seven fecal samples (Figure 2).

| Seasonality
The enteric viral detection rate displayed typical seasonality. NoV, RVA, and HAdV displayed clear seasonality with relatively high detection rates. However, seasonality and periodicity were not observed for HAstV and SaV owing to the low detection rates of these viruses (Figure 3).

NoV presented a high detection rate between November and
February, and the highest levels were in December (36.8% ± 7.9) and January (31.6% ± 5.3). However, in 2018, the detection rate increased from end-November 2018 until early June 2019. RVA displayed a high detection rate between March and May during spring and then decreased during early summer. In particular, the RVA detection rate peaked in March (23.9% ± 6.2). HAdV displayed a relatively high detection rate during summer and autumn; from January to July, its average detection rate was less than 2.0%, but it was high in August (3.4% ± 1.7) and October (4.0% ± 3.0).

| Genotype distribution
Genotypes were confirmed in 5508 samples of four viruses except for RVA (

| DISCUSSION
Viral acute gastroenteritis is a prominent disease in children. 10,[16][17][18]  Rotavirus vaccine has been introduced in Korea and infants and toddlers are becoming exposed to it. Since this surveillance project is not a prospective cohort study on vaccination, the direct effect of vaccines cannot be determined, but it was confirmed that the rate of pathogen detection had continuously decreased. As the detection period of norovirus has recently extended until summer, it was expected that the detection rate of rotavirus would also be delayed, but the correlation between the two pathogens was not confirmed.
However, the yearly decrease in the detection rate of rotavirus may be due to vaccination, but it seems to be related to the report that the outbreak of rotavirus is delayed and moving. [13][14][15]25,26 Around this time, enteric HAdV, HAstV, and SaV tend to have newer genotypes relative to the previous ones. The detection rate of HAdV B and C genotypes was increased in addition to that of the F genotype, which was the predominant genotype, along with the detection rate of HAstV Type 4 and Type 5. Furthermore, the emergence of new genotypes is associated with the prevalence of new pathogens. [27][28][29] The present surveillance data potentially broaden the current