Norovirus GII.4 Sydney: origin and global distribution. A systematic review

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Introduction

Noroviruses (NoV) (family Caliciviridae, genus Norovirus) infect a wide range of vertebrate species, including humans. Norovirus gastroenteritis affects all age groups and is accompanied by symptoms of nausea, vomiting, diarrhea, and abdominal cramps 12–24 hours after infection. NoV gastroenteritis outbreaks most often occur in semi-enclosed settings (daycare centers, schools, hospitals, nursing homes, cruise ships), with the primary mode of transmission being fecal-oral. The disease is seasonal, peaking during the cold season. Currently, there are no specific treatments or preventive measures, including registered vaccines [1–4].

Human NoV virions are non-enveloped and have a diameter of 27–38 nm. The capsid consists of 90 dimers of the structural protein VP1 and several copies of the small protein VP2. The human NoV genome is a linear, single-stranded (+)RNA approximately 7.5 kb in length and organized into 3 open reading frames (ORFs) [5]. ORF1 encodes a large polyprotein — the precursor of 6 non-structural proteins NS1/2–NS7 [6]. ORF2 encodes the major capsid structural protein VP1, and ORF3 encodes the minor capsid structural protein VP2 [1, 7]. VP1 consists of a shell (S) domain and a protruding (P) domain, which serves as a target for antibodies and also participates in binding to host receptors (HBGA) [8–10].

At the junction of the reading frames encoding non-structural and structural proteins (the ORF1/ORF2 overlap region), there is a “hotspot” where recombination frequently occurs in the NoV genome. Consequently, a dual nomenclature has been proposed that accounts for NoV genotypes based on two reading frames [6, 11]. According to the currently accepted classification, NoV is divided into 10 genogroups (GI–GX). Based on analysis of the VP1 amino acid sequence, 48 genotypes are distinguished. Based on nucleotide diversity in the RNA-dependent RNA polymerase (RdRp) region, noroviruses are divided into 60 P-types [12].

The aim of this review is to analyze scientific data on the epidemic NoV variant GII.4 Sydney, the identification of its recombinant forms in various countries, and the characteristics of the antigenic regions of the VP1 capsid protein.

A literature search was conducted for the years 2012–2026 in the PubMed, Scopus, and Web of Science databases, as well as in the Russian scientific electronic libraries eLIBRARY.RU and CyberLeninka. The search query was formulated by combining the following keywords: Norovirus, GII.4, Sydney, recombinant, GII.P31, GII.P4 New Orleans, GII.P16 for foreign sources and their Russian-language equivalents for Russian studies. A total of approximately 6,000 publications were identified.

Relevant scientific materials were selected from domestic and international databases based on the following criteria: relevance to the research topic, year of publication, and availability of the full text (approximately 2,000 sources). The exclusion criterion was the absence in the publication under consideration of original research results devoted to the analysis of the GII.4 Sydney norovirus variant. In addition to the main search, an analysis was conducted of publications containing information on other NoV genotypes that were dominant in certain regions during the period of global circulation of the Sydney NoV variant. Based on the search results, approximately 450 articles were selected. Subsequently, a phased selection of publications, which included an analysis of titles, abstracts, and full texts, resulted in a sample of 169 sources. 110 of these sources were included in the final version of the review due to the length constraints of the article, as well as the exclusion of publications that did not cover the subject in detail, limiting themselves to references to materials already used in the review.

The literature analysis was conducted in chronological order to trace the emergence of the Sydney variant and its recombinants, as well as their subsequent spread worldwide.

The emergence of GII.4 epidemic variants

The evolution of NoV occurs through the accumulation of point mutations in the genome, as well as through recombination events involving genomic segments. The emergence of mutations in the gene encoding the major structural protein VP1 (and, consequently, changes in antigenic epitopes) leads to the emergence of new variants. The ability of RNA polymerases to template-switching facilitates the formation of recombinant RNA viruses with altered functional properties and thereby contributes to viral evolution [6, 8].

Despite the diversity of NoV since the early 2000s, the epidemic process of norovirus infection has been primarily associated with the emergence of new epidemic variants of NoV genotype GII.4 every 2–3 years [13, 14]. Viruses of this genotype demonstrate a pronounced capacity for rapid genetic variability, which serves as a key factor in generating genetic diversity and ensures the survival of more competitive variants.

Over the past three decades, since the mid-1990s, epidemic strains forming the evolutionary lineage of genotype GII.4 have successively emerged and spread (Fig. 1).

 

Fig. 1. Variants of the NoV GII.4 genotype.

The phylogenetic tree was constructed using BEAUti v. 1.10.4 and BEAST v. 1.10.4 and visualized in FigTree v. 1.4.4.

 

A likely explanation for the shift in dominant variants is that the established herd immunity exerts strong pressure on the previously dominant variant, while a new variant emerging against this background of immunity faces minimal immune pressure and, consequently, may gain a competitive advantage in terms of transmission [15, 16].

Epidemic variants include Grimsby 1995 (US95-96), Farmington Hills 2002, Hunter 2004, Yerseke 2006a, Den Haag 2006b, New Orleans 2009 (all of which had their own GII.P4 polymerase variant), Sydney 2012 (detected in association with four polymerase types: GII.P31, GII.P4 New Orleans, GII.P16, and GII.P12) [17]. Over the past 10 years, analysis of VP1 properties has identified four new GII.4 variants, which, however, have not achieved epidemic spread: San Francisco 2017 [18, 19], Hong Kong 2019 [19, 20], Allegany 2018 (all had the GII.P31 polymerase), and Wichita 2021 (had the GII.P4 polymerase) [14, 21].

First reports of the new variant

In 2012, epidemiological surveillance and laboratory monitoring systems in various countries recorded an increase in NoV activity. According to available molecular genetic data, this surge was associated with the emergence of a new GII.4 variant. The first NoV of the new variant, for which a whole-genome sequence (JX459908) was deposited in GenBank, was isolated in March 2012 in Sydney (Australia), and the variant was consequently designated Sydney 2012 [22].

In Australia, New Zealand, France, and the Netherlands, increased NoV activity was observed in late 2012 compared to previous seasons [22]. According to information from Denmark, the new variant was detected sporadically between January and September 2012, and became dominant between October and December 2012 [23]. According to data from an Italian research group on enteroviruses, monitoring of NoV circulation indicated the presence of the GII.4 Sydney variant in Italy as early as November 2011, January, and February 2012. It then ceased to be detected until it was found again in November 2012 and quickly became the dominant strain during November–December 2012 and January 2013 [24].

In England, the first case of the new NoV variant was identified in June 2012, and widespread transmission occurred later, during the winter months; however, overall, the incidence of NoV did not exceed the levels observed in previous seasons [25]. In Scotland, the NoV season began slightly earlier than usual (compared to previous seasons) — in October 2012 — followed by an increase in the number of outbreaks in November 2012, when the Sydney 2012 variant became predominant [26]. In the United States, a virus with GII.4 Sydney characteristics (as was later determined) first appeared in September 2011, circulated at low levels until the winter of 2012–2013 [27], and subsequently spread rapidly throughout the United States and the United Kingdom [28].

Analysis of archived samples showed that the ancestral Sydney 2012 strain (pre-Sydney) was circulating in England as early as 2010. This indicates a long interval between the first appearance and the first detection of a strain with Sydney 2012 characteristics [25]. In Alberta (Canada), two strains of the GII.4 Sydney variant were identified, characterized by minor differences in nucleotide sequence. The first strain was identified in September 2011, and the second strain in January 2012; it was the latter that subsequently became the predominant strain in October 2012 [29].

In China, Sydney 2012 was first detected during the 2011–2012 season. According to data from Hong Kong, the new NoV variant caused an increase in the number of cases of acute NoV-related gastroenteritis between July and October 2012, peaking in August, which falls outside the winter season of disease transmission [30]. In Shanghai, the new NoV variant caused an increase in sporadic cases in October–December 2012 [31]. Sydney 2012 was also detected near Shanghai in Jiangsu and Zhejiang provinces (Huzhou) [32]. Subsequently, the new variant circulated as the predominant strain in Beijing (China) during the winter of 2012/2013 [33]. This indicates the rapid spread of this NoV variant among the Chinese population.

In South Korea, the NoV GII.4 Sydney variant emerged in May 2012 and spread widely in 2012–2013 (from November 2012 to June 2013, with the highest prevalence during the winter months) [34]. In August 2012, the first whole-genome sequence of NoV isolated from a patient at a hospital in South Korea (strain KM272334) was determined [35].

In Japan, the NoV with GII.4 Sydney characteristics was first detected in a patient from Tokyo in November 2011, and began to spread throughout Japan in mid-2012. The highest incidence of NoV was recorded in December [36]. In Brazil, a study of samples collected from July to September 2012 also showed the presence of the new Sydney 2012 variant in circulation [37]. In South Africa, according to the study results, reports of active detection of GII.4 Sydney were received in 2013, and during the same period it became the dominant strain [38].

Consequently, the availability of a large dataset on the increase in the number of NoV cases reported from various countries during the 2011/2012 and 2012/2013 seasons was consistent with the emergence of a new NoV variant, which was named Sydney 2012 and subsequently acquired epidemic status.

Recombinants of the GII.4 Sydney variant

Both intergenotypic and intragenotypic (intervariant) recombination are widespread in the NoV GII.4 genotype lineage, which expands its genetic repertoire. According to the scientific literature reviewed in this study, the NoV GII.4 Sydney variant contains gene sequences derived from recombination [39].

The ORF1 region (specifically, the GII.P31 polymerase gene, formerly GII.Pe) in the reference strain of the Sydney variant identified in 2012 was derived from the Osaka 2007 variant, which had regional circulation in Southeast Asian countries. The ORF2/ORF3 region is linked to a common ancestor of strains belonging to the Apeldoorn 2008 (in some sources, Apeldoorn 2007) and New Orleans 2009 variants. Since New Orleans 2009 originated from Apeldoorn 2008, the likely progenitor of Sydney 2012 in the ORF2/ORF3 region was an early strain of Apeldoorn 2008 [1, 22, 35, 39, 40].

It is hypothesized that GII.P31 is an obligate recombinant genotype, as no core, non-recombinant capsid sequence has been reported in combination with the GII.P31 polymerase genotype [41, 42]. We should not rule out the possibility of an evolutionary process during which both the polymerase and the capsid underwent changes, since a certain period of time elapsed before the identification of the new Sydney 2012 variant, which had epidemic potential [43]. Accordingly, there is reason to believe that pre-Sydney strains existed that emerged several years before the identification of Sydney 2012 itself, as has been shown for other epidemic variants of NoV GII.4 [44].

According to a study by L.D. Bruggink et al., in 2010 all ORF2 sequences studied corresponded to the pre-Sydney variant, while in 2012, in most cases, ORF2 sequences were identified that corresponded to the Sydney variant. In 2010, an intermediate form of NoV was detected in which the GII.P31 polymerase region in ORF1 corresponded in characteristics to viruses from the 2008–2009 period, and the major capsid protein region in ORF2 corresponded to viruses from 2012. The identification of this intermediate form suggests that ORF1 properties may be crucial for increasing the virulence of the NoV strain, as the virus became widespread only after 2010 due to a change in ORF1 [42]. According to J-S. Eden et al., the likely pre-epidemic variant of Sydney 2012 is the Auckland 2010 variant (KF060124), which was detected in New Zealand in 2010 [45].

Analysis of whole-genome sequences of Canadian strains showed that the Alberta strain, identified in September 2011, belonged to Sydney 2012 based on the ORF2 region and had an ORF1 from New Orleans 2009. It was suggested that it might also have been an intermediate pre-Sydney 2012 strain [46].

A re-analysis of Sydney 2012 variant samples collected in Canada [29], Denmark [23], and Italy [24] in 2011–2013 revealed the circulation of an inter-variant recombinant, specifically Sydney in association with the New Orleans GII.P4 polymerase acquired from the previous epidemic variant, New Orleans 2009.

There is insufficient data on which recombinant appeared first — in association with the New Orleans GII.P31 or GII.P4 polymerase — since many countries used a partial genotyping method, typically based only on the capsid. It is possible that they arose independently of one another. However, overall, this indicates that the co-circulation of the two aforementioned NoV strains created conditions for the emergence of new recombinants with epidemic potential.

In South Africa, the first GII.4 Sydney[P4 New Orleans] recombinant was detected as early as 2011 [38]. In Australia (Victoria), GII.4 Sydney[P4 New Orleans] was first identified in August 2015 and, starting in June 2016, caused a surge in NoV cases. This period was likely necessary for the recombinant strain to adapt and acquire unique genetic characteristics [47]. In Italy in 2015–2016, the recombinant strain GII.4 Sydney[P4 New Orleans] was of high epidemiological significance, accounting for 40% of all fully typed strains [48]. In France during the 2016–2017 season, GII.4 Sydney[P4 New Orleans] became dominant among other recombinants [49].

According to available data, prior to 2015, of the two Sydney variant recombinants known at that time, GII.4 Sydney[P31] was the most widespread [50]. In 2015, a new recombinant Sydney variant with the GII.P16 polymerase was discovered.

For a long time, it was believed that noroviruses carrying the GII.P16 polymerase gene were quite rare [51]. In 2014, researchers from South Africa suggested that the GII.P16 polymerase possesses an increased capacity for recombination [38]. This conclusion is based on the detection of new recombinant forms of the virus in association with different variants of capsid proteins: GII.2 [48, 52–55], GII.3 [56, 57], GII.4 [48, 54], GII.10 and GII.12 [58], GII.13 [57, 58], GII.17 [38].

Between 2016 and 2017, reports emerged from many countries regarding the detection of two recombinants — GII.2[P16] and GII.4 Sydney[P16] — with epidemic potential (East Asia [54, 55, 59, 60], Europe [48, 49, 61, 62], and the Americas [50, 63, 64]).

The new recombinants shared a common GII.P16 polymerase, as their sequences were virtually identical [8]. According to a study by C. Ruis et al., the ancestor for the new polymerase of the GII.2[P16] and GII.4 Sydney[P16] variants emerged in March 2013 [65]. It was hypothesized that the Sydney variant acquired the new GII.P16 polymerase from the GII.2 genotype [48], since earlier variants of GII.2[P16] had been detected as early as 2008 in Japan [52], and since 2010 in China (Wuhan) [53], and since 2011 in Italy [48].

Researchers from Japan analyzed several GII.2[P16] recombinant lineages isolated in 2009–2010, 2010–2012, and 2012–2014, and proposed that polymerase may contribute to the evolution of the capsid gene, as the analysis revealed a high rate of evolution for GII.2[P16], which could have contributed to changes in the antigenicity of GII.2 [66]. We also obtained similar data in a previous study [67]. In 2016, a new recombinant NoV GII.4 Sydney[P16] was detected in patients with gastroenteritis in Kawasaki, Japan [68].

In South Korea, between March 2015 and May 2016, water samples were analyzed from suburban streams in two provinces where a new GII.4 Sydney[P16] recombinant was first detected; this strain soon displaced the GII.17[P17] strain that had previously dominated in Asian countries [59, 60]. In mid-2015, the GII.4 Sydney[P16] recombinant was detected in Oceania [69].

In Canada, GII.4 Sydney[P16] emerged between July 2015 and June 2016 and became the predominant strain between June 2017 and February 2018 [50]. In the United States, the new recombinant strain GII.4 Sydney[P16] was detected in November 2015 and subsequently replaced the then-dominant GII.4 Sydney[P31] [8, 70]. In Brazil [64] and the United Kingdom [65], GII.4 Sydney[P16] was detected in 2016. In China (Hubei Province), according to an analysis of data from 2017–2019, the GII.4 Sydney[P16] recombinant, first detected in 2016, became the predominant strain by 2019; prior to that, GII.4 Sydney[P31] had been prevalent [71].

At the same time, reports emerged of new recombinants detected in Germany between September and December 2016. There, GII.2[P16] was the predominant strain, followed by GII.4 Sydney[P31], GII.4 Sydney[P16], and GII.4 Sydney[P4 New Orleans] [61]. Subsequent analysis of nucleotide sequences available in the NoroNet database revealed the presence of early variants of GII.4 Sydney[P16] as early as 2014 in Germany and the Netherlands [72].

In France, two recombinant strains carrying the GII.P16 polymerase were detected during the 2016–2017 winter season: GII.4 Sydney[P16] and GII.2[P16]. GII.4 Sydney[P4 New Orleans] and GII.4 Sydney[P16] were dominant, while GII.4 Sydney[P31] was detected at low levels [49]. In Italy, GII.2 and GII.4 Sydney strains with GII.P16 were detected in April 2016, but were only sporadically reported until October [48, 62].

In 2019, reports emerged of the detection of a new intergenotypic recombinant GII.4 Sydney[P12] in Tokyo (Japan) from September 2017 to August 2018. According to the authors of the article, the new strain may have formed as a result of genetic recombination between the GII.3[P12] and GII.4 Sydney[P31] strains. A comparison of the full-length amino acid sequences of RdRp and VP1 from this new strain with the amino acid sequences of the GII.3[P12] polymerase and the GII.4 Sydney[P31] capsid revealed no specific substitutions [73]. In 2025, L. Barclay et al. reported the detection of a GII.4 Sydney[P12] recombinant in 2022 in the United States [21].

According to national reports, the NoV Sydney variant was first detected in Russia in 2013 in association with the GII.P31 polymerase gene [74, 75]. In November of that same year, the GII.4 Sydney[P31] recombinant was detected in Nizhny Novgorod [76]. In 2015, an inter-variant recombinant was identified in Russia in combination with the GII.P4 New Orleans polymerase gene (geographic origin not specified in the reported data) [74, 75]. In the spring of 2016, a Sydney intergenotypic recombinant with the GII.P16 polymerase was detected in Novosibirsk [51]. In Nizhny Novgorod, NoV GII.P16 variants (GII.4 Sydney[P16] and GII.2[P16]) were detected in the fall of 2016, coinciding with an increase in the frequency of NoV variant detection during that period [67, 77]. In March 2019, an inter-variant recombinant with the GII.P4 New Orleans polymerase was detected in Nizhny Novgorod; it circulated at low levels until 2022 [78], then disappeared and was detected again in September 2025 [79]. The most recent isolation of NoV GII.4 Sydney[P31] in Nizhny Novgorod dates back to September 2021. The only recombinant that has been actively detected in all subsequent epidemic seasons since its emergence was GII.4 Sydney[P16] [80]. In some epidemic seasons (2018–2019, 2020–2021, 2021–2022), the simultaneous circulation of all three recombinants was observed in Nizhny Novgorod.

Thus, the first pre-epidemic forms of the Sydney variant were already detectable as early as 2010. It took some time for them to adapt before eventually becoming the dominant strain. Published data on the detection of GII.4 Sydney in various countries have been compiled and presented in chronological order in the table below (Table).

 

Timeline of the detection of the GII.4 Sydney variant based on the analyzed data

Year of detection	Country	Recombinant	Source
2011 (September)	Canada	GII.4 Sydney[P4 New Orleans] GII.4 Sydney[P31]	[29]
2011 (September)	USA	GII.4 Sydney[P31]	[27]
2011 (November)	Italy	GII.4 Sydney[P4 New Orleans] GII.4 Sydney[P31]	[24]
2011 (November)	Japan	GII.4 Sydney[P31]	[36]
2011 (November)	South Africa	GII.4 Sydney[P4 New Orleans] GII.4 Sydney[P31]	[38]
2012 (January)	Denmark	GII.4 Sydney[P4 New Orleans] GII.4 Sydney[P31]	[23]
2012 (March)	Australia	GII.4 Sydney[P31]	[22]
2012 (May)	South Korea	GII.4 Sydney[P31]	[34]
2012 (June)	English	GII.4 Sydney[P31]	[25]
2012 (July)	China	GII.4 Sydney[P31]	[30]
2012 (July)	Brazil	GII.4 Sydney[P31]	[37]
2012 (October)	Scotland	GII.4 Sydney[P31]	[26]
2013	Russia	GII.4 Sydney[P31]	[75, 76]
2015	Russia	GII.4 Sydney[P4 New Orleans]	[74, 75]
2015 (July)	Canada	GII.4 Sydney[P16]	[50]
2015 (August)	Australia	GII.4 Sydney[P4 New Orleans]	[47]
2015 (November)	USA	GII.4 Sydney[P16]	[8]
2016 (January)	South Korea	GII.4 Sydney[P16]	[59]
2016	Russia	GII.4 Sydney[P16]	[51, 74]
2016 (September)	Japan	GII.4 Sydney[P16]	[68]
2016 (September)	Germany	GII.4 Sydney[P16]	[61]
2016	Italy	GII.4 Sydney[P16]	[62]
2016	France	GII.4 Sydney[P16]	[49]
2016	English	GII.4 Sydney[P16]	[65]
2016	Brazil	GII.4 Sydney[P16]	[64]
2017	Japan	GII.4 Sydney[P12]	[73]
2022	USA	GII.4 Sydney[P12]	[21]

 

In 2011–2012, two recombinants circulated simultaneously: one was an intervariant recombinant with the GII.P4 New Orleans polymerase, and the other was an intergenotypic recombinant with the GII.P31 polymerase. Until 2015, GII.4 Sydney[P31] predominated, until it was displaced by a new intergenotypic recombinant with the GII.P16 polymerase. In 2017, another intergenotypic Sydney recombinant with the GII.P12 polymerase was identified, which to date has been detected only in Japan (2017) and the United States (2022). However, GII.4 Sydney[P16] remains the predominant variant among all Sydney recombinants.

Trends in the shift of dominant norovirus genotypes amid the global circulation of GII.4 Sydney

Since the emergence of the GII.4 Sydney variant in 2012 and up to the present (2026), it has consistently remained one of the dominant variants in the global NoV population, though it has been overtaken by other genotypes on several occasions [15]. This occurred either in specific geographic regions or during specific seasons.

From 2014 to 2016, NoV GII.17 dominated in several Asian countries, which was associated with the emergence of new genetic clusters that differed from those previously circulating [81]. In 2021, genotype GII.17 was predominant during an outbreak in Romania [82]. In Europe in 2023–2024, a decline in the circulation of GII.4 was observed, coinciding with an increase in the prevalence of GII.17 to 64% of all cases of second genogroup (GII) NoV detection [83]. In the United States, a rise in NoV incidence associated with GII.17 was recorded during 2023–2025. Its share increased from less than 10% in the 2022–2023 season to 75% in the 2024–2025 season [84, 85]. During the 2024–2025 season, many cities in China saw a significant increase in the number of cases of NoV infection caused by genotype GII.17 (62%), while the proportion of genotype GII.4 (27%) decreased [86, 87].

In Nizhny Novgorod, Russia, GII.17 was first detected during the 2014–2015 season, and by the 2015–2016 season, it had become the second most common genotype among NoV strains, following GII.4 [88]. A similar situation was observed during the 2022–2023 season, when GII.17 ranked second, trailing the dominant GII.4 Sydney by a narrow margin in the spectrum of identified genotypes [89]. In the current 2025–2026 epidemic season, the GII.17[P17] genotype has been observed to be prevalent in the Nizhny Novgorod region [90].

According to J. Kendra et al., the GII.2 genotype was the predominant one worldwide in 2017 [13]. This is associated with the emergence in 2016–2017 of a new recombinant norovirus GII.2[P16] [91], as described earlier. Furthermore, in 2023, regional spread of GII.2[P16] was noted in South Korea [92].

In Nizhny Novgorod, from 2018 to 2021, an increase in the detection rate of recombinant NoV GII. 3[P12] from 6.8% in 2018–2019, when it was first detected, to 34.9% in 2020–2021, when it ranked first among the total number of typed isolates, surpassing GII.4 Sydney [93]. These data are also consistent with a study from China, where GII.3[P12] was the dominant strain during NoV outbreaks in Beijing in 2021–2023 [94].

Overall, GII.4 was the most prevalent NoV genotype worldwide from 2020 to 2025 (the Sydney 2012 variant accounted for 47%). During this period, new GII.4 variants (San Francisco, Wichita, and Allegany) were identified in certain regions: in Africa in 2021–2022, in Central and South America in 2022–2023, and in Central America alone in 2023–2024. [14].

While GII.17 was gradually becoming the dominant strain worldwide in 2023–2024 [95], the Nizhny Novgorod Region saw an increase in the detection rate of the previously rare GII.7 genotype, which accounted for 42.9% of the spectrum of typed isolates, placing it in first place [96].

According to data published in recent years, recombinants of the GII.4 Sydney 2012 variant continue to evolve as they circulate. For example, researchers from Japan, based on the results of a phylogenetic analysis, identified two clusters in GII.4 Sydney[P31], in which early strains (2013–2016) were phylogenetically distinct from later ones (2019–2022) [97]. This is consistent with data from South Africa for 2019–2021 [98]. A similar situation is observed for GII.4 Sydney[P16], which shows a process of diversification within this recombinant, resulting in the formation of two sublineages (strains from 2016 to 2020 and from 2021 to the present), a finding supported by data from the United States [21], China [99, 100], and Russia [101].

Despite occasional declines in the circulation of the GII.4 Sydney variant in certain regions, no other genotype or new GII.4 variant has yet been able to completely displace or replace GII.4 Sydney 2012, as has been the case with other epidemic variants [15].

The presence of such fluctuations among the predominant genotypes in different countries underscores the need for continuous monitoring systems capable of detecting and tracking such shifts. Studying the mechanisms underlying the cyclical spread of NoV genotypes is important for developing prevention and control strategies, as well as for forecasting trends in the development of the NoV epidemic process [104].

Characterization of VP1 antigenic regions in GII.4 Sydney recombinant viruses

Immune selection drives the evolution of antigenic epitopes on the NoV capsid, which, in turn, leads to the emergence of new variants, some of which have the potential to displace dominant variants and take their place [47, 102, 103]. Currently, 9 antigenic sites (A–I) are identified in the protruding P2 domain of the major capsid protein VP1 [21]. Most of the amino acid substitutions among GII.4 variants are localized in at least 6 epitopes (A–G) within the P2 domain of the capsid [45, 70, 102, 104]. Epitope A is immunodominant and strongly influences the antigenic diversification of NoV GII.4 variants [105].

The evolution of variable sites occurring in the Sydney 2012 variant indicates that the virus expands and shifts its antigenic determinants over time [47, 104, 106]. The literature provides data comparing the Sydney 2012 variant with its predecessors, which also had epidemic characteristics (Fig. 2).

 

Fig. 2. Amino acid composition of the VP1 antigenic epitopes of the NoV GII.4 Sydney variant compared to other variants.

 

One of the first publications to compare amino acid residues in antigenic epitopes for the New Orleans 2009 and Sydney 2012 variants is the article by J. Fonager et al. [23]. The authors showed that changes were observed in 3 epitopes (A, D, E). The most substitutions were found in epitope A — S294T, R297R/H, A368E, D372D/N — and one substitution each in epitopes D (S393G/S) and E (T/I413T).

In Japan, according to a study, six amino acid substitutions were identified in the P2 subdomain of VP1 between the New Orleans 2009 and Sydney 2012 variants (P294T, S359A, A368E, T377A, S393G, I413T), four of which were located in antigenic epitopes [36].

Researchers from England conducted a similar analysis and found that the NoV variants of New Orleans 2009 and Sydney 2012 differ at 6 positions in the P2 domain (S294T, S310N, T/S359A, A368E, N373H, and P396H) [25].

P.A. White et al. identified 4 sites in the P2 domain where variant-specific mutations (P/S294T, A368E, N373H, D/E376E) subject to positive selection were observed. Regions 294, 368, and 373 belong to epitope A, while 376 is located in epitope C. Furthermore, a number of substitutions were detected among the capsid amino acids that could potentially affect the antigenic properties of the virus, as well as its ability to interact with cellular receptors. Variability within the Sydney 2012 variant was demonstrated, for example, substitutions at positions V333M/V (epitope B) and S393S/G (epitope D) [1, 45].

Subsequent studies analyzed the mutations that arose within the Sydney variant while it was already circulating. In Italy, two lineages of the Sydney variant were identified: one circulated at a low frequency during the 2011–2012 season, while the other became widespread by the 2012–2013 season. Common differences from the 2009 New Orleans variant included the P294T, A368E (epitope A), and I413T (epitope E) substitutions. Variability was noted at positions R297H/R and D372D/N (epitope A), V333M/V (epitope B), T340A/T (epitope C), S393G/S (epitope D), and N412D/N (epitope E) [107].

Several studies compared the full-length amino acid consensus sequences of VP1 GII.4 Sydney 3 in viral recombinants (with GII.P31, GII.P4 New Orleans, and GII.P16 polymerases, respectively) [8, 69, 70]. The authors reached a similar conclusion that the sequences under study did not exhibit radical differences. The greatest number of changes in antigenic epitopes occurred during the early years of the Sydney variant’s circulation, when its adaptation was taking place; therefore, greater sequence variability was observed in NoV containing GII.P31 and GII.P4 New Orleans polymerases.

Recombinants with GII.P12 and GII.P16 did not exhibit any unique amino acid substitutions in the VP1 region compared to previously circulating strains [21, 50, 73, 108]. However, differences were found in the S-domain (119, 145, and 174) of GII.4 Sydney[P16], which represented a return to amino acid residues typical of the previous variant — New Orleans 2009 [69]. In all likelihood, Sydney recombinants with the GII.P16 polymerase gained their advantage as a result of changes occurring outside ORF2 [50].

As evidenced by a review of the literature, these changes affected the non-structural proteins p48, p22, and RdRp, as well as VP2 [51, 103, 108, 109]. In particular, five substitutions were identified for RdRp (D173E, S293T, V332I, K357Q, T360A). Presumably, these mutations in the RNA-dependent RNA polymerase gene could have affected the kinetics or accuracy of the enzyme [51, 108]. When comparing the GII.P31 and GII. P16, the most significant differences were found in the amino acid composition of the RdRp active site, in motifs F (G163A) and C (L337M), as well as in the RNA-binding site (S502N), which could have increased NoV transmissibility [8].

Thus, Sydney variant recombinants are capable of both accumulating new mutations and reverting to the characteristics of previous strains to gain a selective advantage, which accounts for the variant’s long-term persistence in the human population under conditions of changing herd immunity.

Conclusion

An analysis of scientific publications shows that the NoV GII.4 Sydney variant, identified in March 2012, has been epidemiologically successful for over 10 years. Its long-term persistence and widespread distribution worldwide are due to the acquisition of new non-structural protein genes, which may affect the virulence of the virus, as well as mutations in the VP1 gene, resulting in changes in the antigenic epitopes of the major capsid protein that do not extend beyond the variant. Together, these factors may contribute to evasion of the host immune response and an increase in the epidemiological significance of the virus.

In this regard, further monitoring of the circulation of NoV GII.4 Sydney is necessary, as it likely possesses molecular-genetic and antigenic properties that prevent the development of sustained population immunity against it. This will facilitate the development of effective measures for the specific prevention of norovirus infection, given that in 2024, the WHO included NoV in the list of priority pathogens of global importance for vaccine research and development [110].

About the authors

Svetlana V. Oparina

Academician I. N. Blokhina Nizhny Novgorod Scientific Research Institute of Epidemiology and Microbiology

Author for correspondence.
Email: svetlanochka.o@mail.ru
ORCID iD: 0000-0003-1161-1344

junior researcher, Laboratory of molecular epidemiology of viral infections

Russian Federation, Nizhny Novgorod

Natalia V. Epifanova

Academician I. N. Blokhina Nizhny Novgorod Scientific Research Institute of Epidemiology and Microbiology

Email: epifanovanv@mail.ru
ORCID iD: 0000-0001-7679-8029

Cand. Sci. (Biol.), leading researcher, Laboratory of molecular epidemiology of viral infections

Russian Federation, Nizhny Novgorod

Nadezhda A. Novikova

Academician I. N. Blokhina Nizhny Novgorod Scientific Research Institute of Epidemiology and Microbiology

Email: novikova_na@mail.ru
ORCID iD: 0000-0002-3710-6648

Dr. Sci. (Biol.), Professor, Head, Laboratory of molecular epidemiology of viral infections

Russian Federation, Nizhny Novgorod

References

Supplementary files