A new wave of resurgence for GII.4 Sydney in Huzhou, particularly GII.4 Sydney[P16], between 2019 and 2023

Norovirus (NoV) infection is a major pathogen causing acute gastroenteritis (AGE) across all age groups worldwide. In the past few years, there were some situations where non-GII.4 genotypes of NoV became predominant in Huzhou region. To understand the latest prevalence of genotypes, we investigated the prevalence and genetic diversity of NoV in sporadic AGE cases from January 2019 to October 2023 in Huzhou City, Zhejiang, China.

Between January 2019 and October 2023, a total of 2846 specimens collected from patients with AGE were tested for NoV in Huzhou. Partial sequences of the RNA-dependent RNA polymerase (RdRp) and capsid gene of the positive samples were amplified by RT-PCR and sequenced. Genotyping of NoV sequences was carried out by the RIVM online NoV Genotyping Tool. Phylogenetic analyses were conducted using MEGA.

In total, 460 (16.16%) specimens were identified as NoV-positive. GII genogroup accounted for most of the NoV-positive specimens (83.70%, 385/460), followed by the GI genogroup (13.26%, 61/460), and dual infection with both GI and GII genogroups (3.04%, 14/460). NoV infection was found in all age groups tested. During this period, at least 20 NoV genotypes were observed, with GII.4 Sydney being the most predominant. Phylogenetic analysis of selected strains revealed that all GII.4 Sydney[P16] strains clustered together and were closely related to strains from Beijing, Shanghai, Hangzhou, Nanjing and the United States, with nucleotide homologies ranging from 96.9 to 99.7%.

We report that during the period from January 2019 to October 2023, the GII.4 Sydney is undergoing a new wave of resurgence, and becoming the main epidemic strain again, particularly GII.4 Sydney[P16] with P16 polymerase.