GII.4 was the most prevalent norovirus strain in outbreaks during the 2012–2013 seasons, but became a minor strain during the 2014–2015 season. During the earlier season, the GII.4 Sydney variant was emergent, and this information and previous cases have led us to expect that a new GII.4 variant, rather than GII.17, will be the next prominent strain. Interestingly, GII.17 became a major prominent strain in both outbreaks and sporadic cases during the 2014–2015 season. According to a previous report, GII.17 accounted for 76% of the norovirus contamination of surface water in Kenya [
7]. In Korea, there was no investigation reporting groundwater contamination with norovirus GII.17 during 2002–2003 [
8], but there have been reports detecting GII.17 in commercial food processing facilities (2007) and metropolitan Seoul (2008) [
9,
10]. Furthermore, this strain has been detected continuously in food-catering facilities and was identified as a major genotype, in addition to GII.4, in the Han River and Wang-suk River (2010) [
11]. GII.17 was also detected in several outbreaks from 2012 to 2015, but was not identified in sporadic cases. A GII.17 variant strain was recently reported to be involved in outbreaks in Japan and China [
12–
14]. GII.17 is therefore a major contributor in norovirus-related outbreaks and sporadic cases, although the underlying reasons remain unknown [
4]. It is impossible to know the persistence of this phenomenon. However, GII.17 was predominant during 2014–2015 season than GII.4 [
14]. Changes in the patterns of molecular epidemiology are questionable.
In Korea, norovirus genotyping is routinely performed using 312-bp sequences from partial capsid genes. Here, partial sequence genotypes were not determined via comparisons with reference sequences using the MegAlign program and NoroNet typing tool. A BLAST search identified several similar genes that had largely been registered since 2012: Kibera209 (KF916585, 2012), GyungGi88 (KF773972, 2012), and Kawasaki323 (AB983218, 2014). The Kawasaki323 strain was the only registered complete genome and exhibited 96% amino acid similarity with the emerging GII.17 strain in Korea. However, the Kawa-saki323 strain and emerging GII.17 strain in Korea differed from the NoroNet GII.17 reference genes by approximately 10.8% to 13.4%. This difference was near the cut-off value for distinguishing between genotypes [
1] and might represent a limitation of norovirus genotype determination from partial VP1 sequences.
Although we expect that a new GII.4 variant will emerge in Korea, the GII.17 strain is clearly major contributing genotype. Questions remain regarding the rapid genotypic shift from GII.4 to GII.17 in Korea. The following may provide an important clue. First, no dominant genotype was identified among outbreak cases during the 2013–2014 season. Although GII.4 was always the dominant genotype worldwide, it did not induce outbreaks during the 2013–2014 season. Second, although a significant proportion of sporadic cases were attributed to GII.4, the prevalence of GII.17 increased rapidly among both outbreak and sporadic cases since December 2014. Continuous monitoring and further investigations of environmental contamination are needed to answer the above question.
