Norovirus (NoV) is the most common pathogen causing acute viral gastroenteritis (AGE) worldwide in all age groups [1,2]. Although the symptoms of NoV infection are usually mild and self-healing, in young children, elderly individuals, and immunocompromised people, NoV may cause severe symptoms and even death [3,4].

NoV is a highly diverse virus [1,5]. Although various human NoV genotypes have been identified, GII.4 Sydney is the most prevalent norovirus genotype worldwide [6]. In the past two decades, six major GⅡ.4 variants have emerged [7], [8], [9]. Among the six pandemic GⅡ.4 variants, the first four may have developed through antigenic drift within the capsid protrusion (P) domain of previous GⅡ.4 viruses [7]. New Orleans_2009 and Sydney_2012 evolved through antigenic shell mutations at the ORF1/2 overlap and recombination within the GII.4 gene [10]. Of course, studies have shown that differences in antigenicity caused by changes in the blocking epitope are a major reason for the emergence of new dominant strains [11,12].

Point mutations and recombination are the driving forces behind norovirus evolution [3]. The NoV RdRp enzyme is responsible for viral genome transcription and replication [13]. Research indicates that the prevalence of certain genotypes or variants, such as GⅡ.4 and GⅡ.2, is associated with the acquisition of de novo RdRp [14,15]. VP1 sequence variation is critical for the identification and monitoring of NoV because the VP1 protein is believed to be closely related to the infectivity and antigenicity of the strain, and rapid evolution of the VP1 gene has led to large differences in antigenicity [16], [17], [18].

In 2020, COVID-19 became a global epidemic [19,20]. Although China successfully contained the first wave of COVID-19 at the end of March 2020, the country faced the challenge of a comeback due to foreign tourists [21,22]. Since 2020, the domestic epidemic has been sporadic and mainly local. Therefore, China mainly implements NPIs, including social distancing, wearing masks, banning indoor gatherings, and imposing travel restrictions. Subsequently, according to the policy adjustment, from January 8, 2023, in accordance with the Law on the Prevention and Control of Infectious Diseases, COVID-19-infected persons will no longer be isolated, and close contacts will no longer be determined. China is no longer divided into high-risk and low-risk zones. Therefore, we define 2021–2023 as the implementation period of NPIs, and the period after 2023 is defined as the abolition of NPIs.

These measures have been shown to significantly affect the transmission of common respiratory viruses and rotavirus and allergic diseases in children [23], [24], [25]. Some studies have shown [26,27] that the spread of NoV is also inhibited during NPIs. The implementation of NPIs may affect the dynamic balance between viruses and human immunity [24,[28], [29], [30]]. This study investigated the epidemiological and genomic characteristics of NoV during and after the implementation of NPIs by analyzing data from patients aged 0–18 years who underwent NoV RNA detection at the Children's Hospital affiliated with Zhejiang University between January 2021 and October 2023.