Background Studies have found that norovirus (NoV) infectious diarrhea has caused many outbreaks across the world and has become an increasingly serious public health problem.

Objective This study analyzes the epidemiological characteristics of NoV infectious diarrhea in Hongkou District of Shanghai from 2017 to 2019, aiming to provide evidence for making prevention and control measures.

Methods An epidemiological investigation including time, region, population, occurrence site, clinical characteristics, and transmission route was carried out on reported clusters of norovirus in Hongkou, Shanghai from 2017 to 2019. A total of 699 samples (including 208 feces, anal swabs or vomit samples from infection cases, 67 feces, anal swabs or vomit samples from asymptomatic close contacts, 142 anal swab samples from practitioners of food industry, 239 kitchen cutting boards samples, and 43 reserved food samples) were collected. Real-time fluorescence quantitative PCR was used to detect the type of NoV nucleic acid and amplify the gene sequence of capsid protein VP1 and polymerase RdRp. Virus genotypes were genotyped using the online norovirus typing tool (http://www.rivm.nl/mpf/norovirus/typingtool/), and virus sequences were compared and evolutionary trees were constructed.

Results From 2017 to 2019, a total of 37 norovirus infectious diarrhea clusters were reported in Hongkou District of Shanghai, with a total number of 474 cases and an overall incidence rate of 1.95% (474/24 266). In winter and spring, high incidences of norovirus infectious diarrhea were reported in 8 subdistricts in the district, with predominantly kindergarten and primary school students, and the incidence rates were 1.67%-2.94% across different school categories. The cluster epidemics were mainly transmitted by contact, e.g. improper disposal of vomit and excreta. Of the 37 clusters, NoV GⅡwas the dominant virus type, its nucleic acid was found positive in 130 of the 699 samples, and the positive rate was 18.60%. In the selected district, the genotype of VP1 detected in 2017 and 2018 was GⅡ.2, the genotype of RdRp detected in 2019 was GⅡ.P17, and the genotype of VP1 detected in 2019 was GⅡ.17.

Conclusion NoV GⅡ, which contains many genotypes, is the main pathogen of the NoV infectious diarrhea reported in Hongkou District. NoV cluster epidemics most frequently occur in winter and spring, primary schools and childcare facilities, and are transmitted by contact. The dominant genotype keeps changing over epidemic seasons. Epidemiological and etiological surveillance of clustered NoV infectious diarrhea outbreaks should be strengthened in the future.