During an outbreak of acute gastroenteritis (AGE) in Norwalk, Ohio, USA in 1968, the norovirus was first identified from the faeces of affected patients (Chen et al., 2024). The viral genome was first characterised in 1990 and subsequently sequenced and classified in 1993 (Winder, Gohar and Muthana, 2022). Since then, it has been prevalent in both developed and developing countries, with a greater disease burden observed in developing regions.
Outbreaks
The UK alone had 3.7 million infections each year according to a 2017 data estimate. A median attack rate of 50% (range, 9-78) was found during an outbreak event, according to systematic reviews of outbreaks occurring in hospitals and nursing homes. In another surveillance study of gastroenteritis outbreaks in three England hospitals, the attack rates of norovirus were confirmed to be 24.5% for staff (95% CI, 17.8-31.2) and 53.2% for patients (95% CI, 41.5-65.0) (Carlson et al., 2024). Norovirus cases had significantly decreased in the UK over the last few years, possibly due to the COVID-19 pandemic. It is likely due to the overburdened healthcare services. However, the reduction in rate of COVID-19 cases has been asssociated with a 48% higher incidence of reported cases of norovirus than expected by the UK Health Security Agency (UKHSA) (Winder, Gohar and Muthana, 2022).
Norovirus outbreaks can vary according to climate. The epidemic of norovirus peaks in the winter due to its ability to thrive in colder temperatures. It has been reported that 63-73% of cases occur in winter between October to March (Omatola et al., 2024).
Molecular Epidemiology
Noroviruses are icosahedral viruses in the family Caliciviridae, with a single-stranded, positive-sense RNA genome (Carlson et al., 2024). Virions are non-enveloped and quite small, with diameter ranging from 27-35 nm. They are highly stable under extremely hostile conditions, including temperatures as high as 60°C and pH 3-7 (Winder, Gohar and Muthana, 2022). The genome is a single RNA segment ~7.5 kilobases long, divided into four open reading frames (ORFs). Noroviruses are genetically diverse and can infect a wide variety of hosts, including humans, dogs, pigs, mice, bats, and sea lions (Carlson et al., 2024).
The genetic diversity of norovirus is currently classified into ten genogroups (GI-GX), along with two unassigned groups. Among these genogroups, GI, GII and GIV are known to infect humans, comprising 9, 27, and 2 genotypes respectively. GII.4 is the most prevalent globally, contributing to an estimated 51-79% of the overall disease burden. Studies over the past decade suggest that fluctuations in norovirus incidence are closely associated with the emergence of new viral variants. For example, in the United States the longest norovirus season was observed in 2014-15 while analyzing an outbreak data from 2009 to 2019, which coincided with the GII.4 Sydney variant, although overall outbreak intensity remained comparable to other years. Similarly, variants such as GII.2 and GII.17 have been reported internationally as the norovirus activity increased. A similar pattern was observed in countries like China, Germany and Japan, where temporal variations in outbreak frequency aligned with the introduction of new genotypes.
Furthermore, clinical evidence suggests that infections caused by GII.4 strains often tend to result in more severe disease outcomes compared to other genotypes, although definitive conclusions remain challenging due to confounding epidemiological factors (Carlson et al., 2024).
Vaccine Development
The development of a norovirus vaccine is accompanied by substantial challenges, such as the virus’s extensive genetic and antigenic diversity, which enables the continual emergence of new variants capable of evading pre-existing immunity. Additionally, protective immunity to norovirus is not fully understood, as individuals are repeatedly exposed to antigenically distinct strains over time, making it difficult to define clear immunological correlates of protection. Furthermore, progress is constrained by the lack of a reliable and reproducible in vitro culture system and the limited availability of suitable animal models for vaccine evaluation, factors that have collectively delayed the successful development of an approved vaccine (Chen et al., 2024).
Another major challenge in norovirus research has been the absence of a robust and sustainable cell culture system. Although early in vitro models allowed researchers to infect cells and study viral behavior, replication could not be maintained beyond a few cycles—“after just a few rounds, norovirus replication would stop,” preventing the development of stable viral stocks. As a result, studies have long depended on virus isolated from patient stool samples, which are limited and inconsistent, thereby constraining large-scale experimentation. Recent advances, however, have begun to overcome this barrier, with researchers identifying key factors that restrict viral replication and developing improved systems that support prolonged viral cultivation. These developments represent a significant step forward, enabling more reliable investigation of viral biology, immune responses, and the development of preventive and therapeutic strategies (Barnes, 2026).
Despite these complexities, progress in vaccine development is underway. Several candidate vaccines are currently in clinical evaluation. These include virus-like particle (VLP)-based platforms and newer mRNA-based approaches designed to induce broad immune responses against multiple genotypes. For example, a VLP-based mRNA vaccine is under investigation in early-phase clinical trials, while additional candidates include bivalent and quadrivalent formulations targeting key genotypes such as GI.1, GII.3, GII.4, and GII.17. These multivalent designs aim to address the extensive genetic diversity of the virus and improve cross-protective efficacy (Carlson et al., 2024).
Global health authorities, such as the World Health Organization (WHO), have emphasized the urgency of advancing norovirus vaccine development. However, the path forward remains complex due to the interplay of virological, immunological, and methodological challenges. Continued improvements in experimental models, alongside a deeper understanding of immune protection, will be essential to achieving an effective and broadly protective vaccine (Omatola et al., 2024).
References
Baylor College of Medicine (2024). Breakthrough in human norovirus research: researchers overcome major obstacle to grow and study the virus. Available at: https://www.bcm.edu/news/breakthrough-in-human-norovirus-research-researchers-overcome-major-obstacle-to-grow-and-study-the-virus
Carlson, K.B., Dilley, A., O’Grady, T., Johnson, J.A., Lopman, B. and Viscidi, E. (2024). A narrative review of norovirus epidemiology, biology, and challenges to vaccine development. npj Vaccines, 9, Article 94.
https://doi.org/10.1038/s41541-024-00884-2
Chen, J., Cheng, Z., Chen, J., Qian, L., Wang, H. and Liu, Y. (2024). Advances in human norovirus research: Vaccines, genotype distribution and antiviral strategies. Virus Research, 350, 199486. https://doi.org/10.1016/j.virusres.2024.199486
Available at: https://www.sciencedirect.com/science/article/pii/S0168170224001795
Omatola, C.A. et al. (2024). Noroviruses: Evolutionary dynamics, epidemiology, pathogenesis, and vaccine advances—A comprehensive review. Vaccines, 12(6), 590. https://doi.org/10.3390/vaccines12060590
https://www.mdpi.com/2076-393X/12/6/590
National Institute for Health and Care Research (NIHR) (2024). UK’s first norovirus mRNA vaccine trial launched. Available at: https://www.nihr.ac.uk/news/uks-first-norovirus-mrna-vaccine-trial-launched
Winder, N., Gohar, S. and Muthana, M. (2022). Norovirus: An overview of virology and preventative measures. Viruses, 14(11), 2397. https://doi.org/10.3390/v14112397
Available at: https://www.mdpi.com/1999-4915/14/11/2397