On December 31, 2021, the Chinese Center for Disease Control and Prevention Weekly Published a study on the situation of norovirus infection in China: CaliciNet China reported a total of 1954 norovirus-induced outbreaks from October 2016 to December 2020, with each number ranging from 5 to 391 people, with a median of 15 people.
Schools are the "hardest hit areas" of norovirus outbreaks
Norovirus is the leading cause of acute viral gastroenteritis (AGE) worldwide. Studies mention that the peak season for norovirus infection occurs in November-December and March of the following year. Between October 2016 and December 2020, a total of 1954 norovirus-induced outbreaks were reported by CaliciNet China.
Of the 1887 norovirus outbreaks, 852 (45.15%) occurred in nurseries, followed by 565 in primary schools (29.94%), secondary schools with 223 (11.81%) and other schools with 94 (4.98%). Among them, the transmission pattern of 1833 outbreaks (93.80%) was dominated by human-to-human transmission (1748 cases, 95.36%), followed by 59 cases (3.21%) foodborne transmission and 26 (1.42%) waterborne transmission.
Involving 1244 cases, the most transmitted in China in recent years is the norovirus GII.2 strain
Norovirus can be divided into two genomes: GI and GII. The study genotyped 1352 (69.19%) positive stool samples from norovirus outbreaks in China. Among them, 1244 cases (92.01%) were GII positive, 63 cases (4.66%) were GI positive, and 45 cases (3.33%) were mixed genotypes. A total of 10 GI genotypes and 16 GII genotypes were detected.
The study found that GII.2 was the dominant strain in China between October 2016 and December 2020. In the outbreak of GII strains in China, 69.0% (859/1244 cases) were GII.2, followed by 8.12% (101 cases) for GII.3, 6.59% (82 cases) for GII.6, 6.19% (77 cases) for GII.4 Sydney and 5.87% for (73 cases) GII.17, and other genotypes accounted for less than 5%.
Monthly trends in the genotype of norovirus genotypes of cases of acute gastroenteritis uploaded to the Norovirus Outbreak Surveillance Network from October 2016 to December 2020
In addition, GII.4 Sydney remains today the main strain in sporadic cases in China. The study highlights the importance of monitoring gii.4 Sydney strains for potentially emerging GII.4 variants. Of the more than 30 norovirus genotypes that cause disease in humans, GII.4 norovirus has been reported to cause most norovirus infections worldwide. Every few years, a new GII.4 variant emerges, replacing the previous dominant strain and triggering a new wave of outbreaks.
The GII.4 Sydney [P31] variant appeared at the end of 2012 and became a globally circulating strain; GII.4 Sydney [P16] emerged in 2015 and replaced GII.4 Sydney [P31] as the dominant strain in the United States and European countries during the winter of 2016-2017.