Molecular Epidemiological Characteristics of Gastroenteritis Outbreaks Caused by Norovirus GII.4 Sydney [P31] Strains — China, October 2016–December 2020

INTRODUCTION: Human noroviruses are the leading cause of acute viral gastroenteritis (AGE) worldwide in all age groups. GII.4 strains have been the predominant genotype circulating globally over the last 2 decades and since 2012. GII.4 Sydney viruses have emerged and caused the majority of AGE outbreaks worldwide. METHODS: Data from norovirus outbreaks from the laboratory-based surveillance of norovirus outbreaks in China (CaliciNet China) between October 2016–December 2020 were analyzed. RESULTS: During October 2016–December 2020, 1,954 norovirus outbreaks were reported, and positive fecal samples from 1,352 (69.19%) outbreaks were genotyped. GII.4 Sydney [P31] viruses accounted for 2.1% (October 2016–August 2017), 5.5% (September 2017–August 2018), 3.3% (September 2018–August 2018), 26.6% (September 2019–August 2020), and and 1.1% (September 2020–December 2020) of GII outbreaks, respectively. Compared to reference strains of GII.4 Sydney [P31] from 2012 to 2013, 7 amino acid mutations in epitopes[A (297, 372 and 373), B (333), E (414), and H (309 and 310)] and 1 in human histo-blood group antigens binding site at site II 372 were found by analyzing 9 GII.4 Sydney [P31] complete genomic sequences. CONCLUSIONS: This report identified the genomic variation of GII.4 Sydney [P31] from CaliciNet China. Continued surveillance with prompt genotyping and genetic analysis is necessary to monitor the emergence of novel GII.4 variants.